U.S. patent number RE33,273 [Application Number 07/120,599] was granted by the patent office on 1990-07-24 for materials having improved nonfouling characteristics and method of making same.
This patent grant is currently assigned to Georgia Tech Research Corporation. Invention is credited to Lois M. Speaker.
United States Patent |
RE33,273 |
Speaker |
July 24, 1990 |
Materials having improved nonfouling characteristics and method of
making same
Abstract
A method of improving the separatory properties of membranes by
the deposition of a fluorinated amphiphilic compound in an oriented
Langmuir-Blodgett layer on the membranes surface so as to increase
membrane selectivity and counteract membrane surface properties
leading to fouling during liquid-liquid separations and enhance gas
selectivities of membranes used for gas-gas separations. The use of
a fluorinated long-chain pyridinium bromide is specifically
disclosed.
Inventors: |
Speaker; Lois M. (Atlanta,
GA) |
Assignee: |
Georgia Tech Research
Corporation (Atlanta, GA)
|
Family
ID: |
26818542 |
Appl.
No.: |
07/120,599 |
Filed: |
November 13, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
409097 |
Aug 18, 1982 |
04554076 |
Nov 19, 1985 |
|
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Current U.S.
Class: |
210/639;
210/500.27; 210/500.33 |
Current CPC
Class: |
B01D
67/0088 (20130101); B01D 69/122 (20130101); B05D
1/202 (20130101); B82Y 30/00 (20130101); B82Y
40/00 (20130101) |
Current International
Class: |
B01D
67/00 (20060101); B01D 69/00 (20060101); B01D
69/12 (20060101); B05D 1/20 (20060101); B01D
031/00 () |
Field of
Search: |
;210/639,500.2
;424/85,178 ;427/369,370 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
G D. Rose et al., "Composite Membranes: The Permeation of Gases
Through Deposited Monolayers", Science, 159, 636-637, (1968). .
G. D. Rose et al., "Gas Transport Through Supported
Langmuir-Blodgett Multilayers", J. Colloid & Interface Science,
27(2), pp. 193-207. .
R. A. Wallace et al., "The Electret Effect in Cellulose Acetate
Reverse Osmosis Membranes", Polym. Eng. & Sci., 14, p. 92,
(1974). .
T. Fort, Jr. et al., "Desalination Membranes Foam Built-Up
Multilayer Films", Apr. 1974, PB-232 364, NTIS. .
T. Sata et al., "Modification of Properties of Ion Exchange
Membranes etc.", J. Polymer Sci./Polym. Chem. Ed., 17, pp.
1199-1213, (1979)..
|
Primary Examiner: Spear; Frank
Attorney, Agent or Firm: Oldham & Oldham Co.
Claims
What I claim is:
1. A method of modifying a surface of a separatory material
comprising the step of depositing on said surface .[.a.]. .Iadd.at
least one oriented monomolecular .Iaddend.layer of .[.a.]. .Iadd.an
essentially water-insoluble .Iaddend.fluorinated amphiphilic
molecule .Iadd.which is capable of forming an oriented
monomolecular layer and .Iaddend.which is oriented such that the
fluorinated portion of said molecule extends outwardly from said
surface.
2. A method as claimed in claim 1 wherein said molecule is a
fluorinated aromatic heterocyclic bromide.
3. A method as claimed in claim 1 wherein said molecule is
fluorinated pyridinium bromide.
4. A method as claimed in claim 1 wherein said layer is a
monomolecular Langmuir-Blodgett layer.
5. A method as claimed in claim 1 wherein said molecule is neutral
in charge.
6. A method as claimed in claim 1 wherein said molecule has the
same electrical charge as said membrane surface.
7. A method as claimed in claim 1 wherein said layer of said
molecule is deposited on said surface at a deposition pressure
ranging from 30 to 35 mN M.sup.-1.
8. A method as claimed in claim 1 wherein said layer of said
molecule is deposited on said surface at a deposition temperature
ranging from 1.degree. to 10.degree. C.
9. A method as claimed in claim 1 wherein the molecular weight of
said molecule ranges from 350 to 700.
10. A method as claimed in claim 1 wherein said molecule is
water-insoluble at a deposition pressure ranging from 30 to 35 mN
M.sup.-1 and a deposition temperature of 1.degree. to 10.degree.
C.
11. A method as claimed in claim 1 wherein said separatory material
is a semipermeable membrane.
12. A method as claimed in claim 11 wherein said membrane is a
liquid-liquid separatory membrane.
13. A method as claimed in claim 11 wherein said membrane is a
gas-gas separatory membrane.
14. A method as claimed in claim 1 wherein said separatory material
is a resin particle.
15. A method of treating the surfaces of semipermeable membranes
which separates a component of a fluid, comprising the steps
of:
(a) placing .Iadd.on the surface of a body of water .Iaddend.a film
of molecules of .[.a.]. .Iadd.an essentially water insoluble
.Iaddend.fluorinated amphiphilic compound .[.on the surface of a
body of water.]. .Iadd.which is capable of forming an oriented
monomolecular layer.Iaddend.;
(b) compressing said film into an oriented monomolecular layer;
and
(c) moving said membrane vertically into and out of said film
spread on a water surface a selected number of times whereby said
monolayer is transferred onto an exterior surface of said membrane
such that the fluorinated portion of said compound is directed
toward said fluid.
16. A method as claimed in claim 15 wherein said compound is
fluorinated pyridinium bromide.
17. A method of enhancing the selectivity of a selective
semipermeable membrane for a component of a mixture introduced to a
surface of said membrane, comprising the step of depositing on said
surface .[.an.]. .Iadd.at least one .Iaddend.oriented monolayer of
.[.a.]. .Iadd.an essentially water insoluble .Iaddend.fluorinated
amphiphilic compound .Iadd.which is capable of forming an oriented
monomolecular layer and .Iaddend.which exhibits an affinity for
said component.
18. A method as claimed in claim 17 wherein said compound is
surface-active.
19. A method of modifying the surface of a semipermeable membrane
comprising the step of depositing on said surface .[.an.]. .Iadd.at
least one .Iaddend.oriented monolayer of .[.a.]. .Iadd.an
essentially water insoluble .Iaddend.fluorinated amphiphilic
compound .Iadd.which is capable of forming an oriented
monomolecular layer and .Iaddend.which has the same charge as said
surface.
20. An improvement in a semipermeable membrane of the type having a
surface in contact with a fluid to be separated, the improvement
comprising .[.a.]. .Iadd.an essentially water insoluble
.Iaddend.fluorinated amphiphilic compound .Iadd.capable of forming
an oriented monomolecular layer .Iaddend.being deposited on said
surface in .[.an.]. .Iadd.at least one .Iaddend.oriented
monomolecular Langmuir-Blodgett layer so as to impart non-fouling
characteristics to said membrane.
21. An improvement as claimed in claim 20 wherein said compound is
fluorinated pyridinium bromide.
22. An improvement as claimed in claim 20 wherein said layer is
oriented with the fluorinated portion of said compound directed
toward said fluid to be separated.
23. An improvement as claimed in claim 20 wherein said layer is an
x-layer of fluorinated pyridinium bromide.
24. A method of preventing the fouling of a semipermeable membrane
used for separating dissolved materials from liquids, comprising
the step of depositing on the surface of said membrane .[.an.].
.Iadd.at least one .Iaddend.oriented monololecular layer of
fluorinated pyridinium bromide prior to the use of said surface in
said separating.
25. An improvement in a semipermeable membrane of the type used for
separating dissolved materials from liquids, the improvement
comprising fluorinated pyridinium bromide being deposited in
.[.an.]. .Iadd.at least one .Iaddend.oriented monomolecular layer
on the surface of said membrane that is to be in contact with said
liquid. .Iadd.
26. A method of modifying a solid surface of a material comprising
the step of depositing on said surface at least one oriented
monomolecular layer of an essentially water-insoluble fluorinated
amphiphilic compound which is capable of forming an oriented
monomolecular layer and which is oriented such that the fluorinated
portions of the molecules of said compound extend outwardly from
said surface. .Iaddend. .Iadd.27. A method as claimed in claim 26
wherein each layer of said fluorinated amphiphilic compound is a
monomolecular Langmuir-Blodgett layer. .Iaddend. .Iadd.28. A
material having improved non-fouling characteristics, said material
including a smooth solid surface having thereon at least one
oriented monomolecular layer of an essentially water-insoluble
fluorinated amphiphilic compound capable of forming an oriented
monomolecular layer, said compound imparting improved non-fouling
characteristics to said surface. .Iaddend. .Iadd.29. A material
according to claim 28 wherein said layer is a Langmuir-Blodgett
layer. .Iaddend. .Iadd.30. A method according to claim 26 wherein
said compound is a long chain compound having a fluorinated group
and a polar group at opposite ends of said chain, said compound
being neutral in charge or having the same charge as that of said
surface. .Iaddend. .Iadd.31. A method according to claim 26 wherein
said surface is hydrophilic. .Iaddend. .Iadd.32. A material
according to claim 28 wherein said compound is a long chain
compound having a fluorinated group and a polar group at opposite
ends of said chain, said compound being neutral in charge or having
the same charge as that of said surface. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to liquid purification or separation and,
more particularly, to the deposition of oriented monolayers on the
surface of separation membranes.
2. Description of the Prior Art
The cost and energy effectiveness of membrane separation processes
are seriously compromised by the readiness with which available
membranes undergo fouling by colloidal materials. The
anion-exchange membranes of electrodialysis (ED) stacks and the
incharged membranes of reverse-osmosis (RO) systems are especially
prone to fouling.
The heart of every modern electrodialytic treatment system is an
alternating array of polymer-based ion-exchange membranes. A
serious obstacle to cost-effective operation of electrolytic
desalination plants is the ease with which these membranes undergo
concentration polarization and fouling by humic acids. These end
products of biodegradation are present in most natural waters as
colloidal materials bearing partially ionized acid groups. Their
negative character renders them much more likely to adhere to a
positively charged anion-exchange membrane than to a
cation-exchange membrane, and this adherence has two deleterious
effects: first, the pores of the membrane become physically
occluded by colloidal material; and second, the positive bulk with
a negative fouled surface functions as a bipolar "sandwich
membrane", greatly enhancing its tendency to undergo further
fouling.
During continuous operation, these insoluble impurities occlude the
membrane surfaces at an increasing rate, and the electrical
resistance of a stack is raised to the point where power costs make
further operation uneconomic. The stack must then be disassembled
for stringent cleaning or replacement of membranes. The combined
expenses of down-time, replacement, or cleaning, and power
requirements that rise steadily during operation seriously
compromise the cost effectiveness of this method of water
purification.
Fouling of RO membranes proceeds by a less well known, but related
pattern, in which colloidal materials are occluded on the working
surfaces of the membranes almost immediately after operation has
been initiated. In RO separations, greatly reduced membrane flux is
the negative economic factor.
Considerable evidence indicates that a propensity toward
polarization and fouling is governed by the nature of a membrane
surface. Critical surface characteristics have been shown to
include rugosity (roughness), chemical homogeneity, and
hydrophilicity. Their demonstrated importance indicates that
surface modification may offer a fruitful avenue to a mechanistic
definition of concentration polarization and fouling, and to their
mitigation.
SUMMARY OF THE INVENTION
A membrane surface is modified by coating with individually
oriented layers of amphiphilic molecules, i.e., molecules with one
polar or hydrophilic end and one non-polar or hydrophobic end,
using the classical Blodgett dipping technique. Because the layers
are extremely thin (around 20 .ANG.), they can modify the surface
characteristics of a semipermeable selective membrane which lead to
fouling without affecting its bulk properties, i.e., the separatory
action of the membrane. Membrane surface so treated are physically
smooth (the deposited layers having strong lateral cohesive
forces), chemically homogeneous and hydrophobic. The treatment is
to be carried out after manufacture of the membrane under
nonfouling conditions and before its exposure to fouling
conditions.
The types of amphiphilic molecules which are best utilized in the
present invention include fluorinated compounds (bearing a polar
group at the end of the chain opposite to the fluorinated group). A
fluorinated long-chain pyridinium bromide, hereafter referred to as
R.sub.f PyrBr, was tested extensively as a modifier of the surfaces
of anion-exchange membranes and found to be very effective in
preventing fouling. For maximum benefit, the compounds must be
applied to surfaces as Langmuir-Blodgett layers so that the
resulting layer is monomolecular, free of defects and strictly
oriented with, in the case of fluorinated amphiphilic compounds,
the fluorinated ends facing the feed solution.
Fluorinated polymerizable materials that can be deposited in the
monomer form and polymerized on the surface in such a manner that
the orientation of the molecules is retained may be even more
effective, in that the lifetime of the coating may be increased by
entanglement with the original surfaces.
Most non-fluorinated amphiphilic compounds will not be useful in
the invention. The determining factors for prevention of fouling
are that the compound must exhibit either a neutral charge or a
charge identical to that of the membrane, and that it must be
fluorinated. Presumably, identical charge and the greatest possible
extent of fluorination compatible with the Blodgett transfer
technique are most desirable. The only criterion for matching
membranes with fouling-preventive (fluorinated) amphiphilic
compounds is the avoidance of opposing charges.
Many amphiphilic molecules, nonfluorinated and with charges
opposite to that of the membrane, will lead to greatly enhanced
fouling, as is demonstrated by the experiments set forth herein
with arachidic acid coated membranes. This effect is also
consistent with the current understanding of fouling, which is said
to be irreversibly initiated by the very first molecular layer of
oppositely charged surface-active material that contacts the
membrane.
Molecular weight of the amphiphilic compound is of great
significance in that it governs, to a large extent, the
surface-active character of the compounds to be used. Molecular
weight should probably range between 350 and 700 for fluorinated
compounds, depending on the nature of the polar group and the
atomic weight of a counterion, if there is one. The compounds must
be virtually insoluble when delivered to a water surface (by the
standard Blodgett technique) as a very dilute solution in a
water-immiscible low-boiling solvent. To some extent, slight
solubility can be compensated for by lowering the deposition
temperature, as was done for R.sub.f PyrBr.
In summary, the amphiphilic compounds that are useful in the
present invention are fluorinated, surface-active, neutral or
charged like membrane and sufficiently water-insoluble at the
temperature and pressure of transfer to be amenable to deposition
as Langmuir-Blodgett layers.
The deposition of R.sub.f PyrBr was carried out at the lowest
temperature reading obtainable under the laboratory conditions,
10.5.degree. C. It is probable that a still lower temperature would
further decrease the water solubility of R.sub.f PyrBr, which is
desirable. Therefore, a range of 1.degree. C. to 10.degree. C. is
recommended.
Deposition pressures of 30 mN M.sup.-1 and 35 mN M.sup.-1 were
satisfactory for R.sub.f PyrBr, whereas 25 mN M.sup.-1 led to lower
surface density of the transferred compound and 40 mN M.sup.-1
apparently produced crowding and disorientation of some molecules.
Therefore, a pressure range of 30-35 mN M.sup.-1 is
recommended.
The experimental data demonstrate that a single monomolecular
layer, which is approximately 20 .ANG. thick, is most effective in
preventing fouling. Multiple layering, which would lead to greater
materials costs and much higher processing costs, is also
undersirable from the standpoint of ultimate performance.
A dipping speed of 0.1 cm/sec for deposition of R.sub.f PyrBr was
utilized. This speed was determined by observation of the meniscus,
which is horizontal and smooth at appropriate transfer rates.
The category of liquid-liquid separation membrane types which can
be treated by the present invention includes all those intended for
the separation of ions or ionic, colloidal, crystalline,
particulate, or vaporizable material from liquids. In addition, the
category of membrane types is not limited to polymeric materials
and includes membranes designated as electrodialysis,
cation-exchange, anion-exchange, bipolar, reverse-osmosis,
ultrafiltration, pervaporation, and hemodialysis membranes, but it
does not exclude any selective membranes, known by any other
designation, intended to carry out a process that can be described
as the separation of ions or ionic, colloidal, crystalline,
particulate, or vaporizable matter from liquids.
Experimental observations and data obtained during separation
processes that employed two types of anion-exchange membranes and
two types of reverse-osmosis membranes are set forth. The membranes
were treated by deposition of oriented layers of a fluorinated
long-chain pyridinium bromide. Also, comparable data are disclosed
for control membranes that are identical but untreated.
An alternative embodiment of the concept of the invention, with
specific application to separatory membranes, is that oriented
deposition of appropriate long-chain amphiphilic compounds can be
used to decrease the scaling tendencies of anion- and
cation-exchange electrodialysis membranes. An electrodialysis
membrane tends during operation to build up regions of high pH at
the surface that is not fouled by colloidal materials. As they come
in contact with this region, many inorganic cations commonly
present in water (calcium, magnesium, etc.) form insoluble
materials that precipitate as scale on the membrane surface.
Accumulated scale, like layers of foulant, increases the electrical
resistance and decreases the flux of the membrane.
Surface modification by deposition of an appropriately oriented
monolayer on any membrane face that is prone to scaling should
prevent the attachment of materials such as inorganic oxides and
hydroxides. Thus, although insoluble compounds may continue to
form, they can have no deleterious effect upon the operation of the
membrane.
A further embodiment of the present invention is that oriented
deposition of appropriate long-chain amphiphilic compounds can be
used to enhance the inherent selectivities of several categories of
selective membranes. The categories of selective membrane types
include selective semipermeable membranes intended for the
separation of ions or ionic, colloidal, crystalline, vaporizable or
particulate matter from liquids (e.g., salt from brackish water or
proteins from cheese whey); and selective semipermeable membranes
intended for the separation of one liquid from another liquid
(e.g., ethanol and water).
It has been demonstrated, for example, that oriented deposited
layers of a nonfluorinated long-chain fatty acid will impart
ethanol selectivity to a membrane that is intrinsically
water-selective or increase the ethanol selectivity of an
ethanol-selective membrane. That confirms the hypothesis of the
present invention that deposited oriented layers of an amphiphilic
material exhibiting a strong affinity for one component of a
mixture will confer selectivity for that component upon a
separatory membrane.
The present invention may also be useful for the enhancement of the
gas selectivities of gas-gas separation membranes (e.g., separating
nitrogen or oxygen from the air). Colloidal fouling does not
present difficulties in gas-separation processes, but such
processes are not presently cost-effective because of the
relatively low selectivities of available membranes.
Appropriate selection of amphiphilic materials may lead to the
simultaneous desirable modification of more than one property of a
membrane. For example, deposited oriented layers of a selected
amphiphilic material might simultaneously heighten the water
selectivity and reduce the fouling propensity and scaling tendency
of a given membrane.
As another alternative embodiment closely related to the treatment
of the surfaces of semipermeable membranes, the use of the present
invention mitigates the fouling of anion- and cation-exchange
resins, macroreticular resins, zeolites and similar materials
intended for the separation of one component from a mixture or
solution. This mitigation would be accomplished by deposition of
appropriate oriented amphiphilic layers on the surfaces of the
resin particles, after their preparation under nonfouling
conditions and before their exposure to fouling conditions.
The concept of beneficial surface modification by deposition or
oriented amphiphilic layers has several applications that are
unrelated to the modification of semipermeable membranes. These
include the following: modification of heat-transfer surfaces to
promote dropwise condensation and to mitigate fouling, microbial
growth, and scaling; modification of the surfaces of liners for
solar-energy ponds to mitigate fouling and microbial growth;
modification of marine surfaces to mitigate fouling, microbial
growth, and inorganic scaling; modification of the surfaces of
metals to mitigate their tendencies to corrode when exposed to
certain environments; modification of the surfaces of photochemical
solar converters to protect them from oxide formation and to
enhance their light absorption; modification of metal and polymer
surfaces to heighten or reduce either their adhesive
characteristics or their lubricities; modification of the surfaces
of biomaterials used on prosthetic devices, bioimplants, etc. to
minimize biorejection; modification of the surfaces of dialysis
membranes to minimize both hemolysis and fouling; and modification
of the surfaces of dialysis membranes to increase their
selectivities for blood factors found to be associated with renal
lesions, rheumatoid arthritis, muscular dystrophy, schizophrenia,
and other diseases.
BRIEF DESCRIPTION OF THE FIGURES OF DRAWINGS
FIGS. 1A-C are representative illustrations of the steps leading to
the fouling of an anion-exchange membrane;
FIG. 2 is a series of schematic diagrams of the deposition of
Blodgett "y-layers";
FIG. 3 is series of schematic diagrams of the deposition of
Blodgett "x-layers";
FIG. 4 is a schematic diagram of the manner in which a close-packed
monolayer may bridge over surface roughness as it is deposited on a
solid;
FIG. 5 is a schematic representation of the hydraulic circuit for
fouling evaluations;
FIG. 6 is a representation of the electrical circuit for the
fouling evaluation stack;
FIG. 7 is a schematic diagram of the assembled test electrodialysis
cell;
FIG. 8 is a pressure-area curve for arachidic acid at 23.3.degree.
C.;
FIG. 9 is a .pi.-A curve of R.sub.f PyrBr;
FIGS. 10A-D are schematic representations of contact-angle decay at
membrane surfaces;
FIG. 11 is a contact angle-time surve for contact-angle decay on
membranes coated with three layers of R.sub.f PyrBr at 25.degree.
C. and 25 mN M.sup.-1 ;
FIG. 12 is a contact angle-time curve for contact-angle decay on
three layers of R.sub.f PyrBr at 25.degree. C. and 30 mN M.sup.-1 ;
and
FIG. 13 is a contact angle-time curve for contact-angle decay on
membranes coated with three layers (Sample 5) and ten layers
(Sample 6) of R.sub.f PyrBr at 10.5.degree. C. and 40 mN
M.sup.-1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
I. AN ANALYSIS OF CONCENTRATION POLARIZATION AND FOULING
A. Mechanism of Polarization and Fouling
The limiting current (i.sub.lim) of an electrodialysis stack is
generally agreed to be that current density at which the
boundary-layer concentration of salt ions approaches zero. Current
continues to pass only because water dissociates to form hydrogen
and hydroxyl ions, a process that requires a high energy input. As
a rule, industrial desalination operations avoid this region of
inefficiency by operating at 70% of the limiting current, where the
current-voltage relationship is linear.
Some degree of concentration polarization, even at current
densities far below the limiting current density, occurs during
electrolysis with all types of ion-exchange membranes. Hydrogen and
hydroxyl ions resulting from the "splitting" of water are involved
in the conduction process, with hydrogen ions accumulating on the
depleting surfaces of an ion-exchange membrane while hydroxyl ions
are transported through the membrane structure. Organic anions are
converted, as they reach the resulting localized region of low pH,
to sparingly soluble organic acids, which then deposit on a
positively charged anion-exchange membrane. Once this occurs, the
bipolar membrane produced gives rise to even faster production of
hydrogen ions and the rate of deposition of organic material (i.e.,
fouling) increases.
A basic separation unit comprises a cation-exchange and an
anion-exchange membrane mounted between two electrodes, with
electrolyte solution flowing through the enclosed compartments. The
flow ensures good mixing in the center of the compartment, but its
effect diminishes as the surfaces of the membranes are approached.
In the static boundary layers immediately in front of and behind
the membranes, ions are transported only by electrolytic transfer
and diffusion; in the mixed zone, ion transport is a function of
electrolytic transfer, diffusion, and physical mixing.
With the passage of an electrolytic current through the system,
anions migrate toward the anode and cations transfer toward the
cathode. If the electrolyte is KCl, cations and anions share
equally in the passage of current through the solution bulk, and
the transference number is 0.50 for both. The membranes, however,
are selective; the transference number for potassium is essentially
1.0 in the cation-exchange membrane and 0.0 in the anion-exchange
membrane. Similarly, the transference number for chloride ion is
1.0 in the anion-exchange membrane and 0.0 in the cation-exchange
membrane. Chloride ions carry only 50% of the electrical current in
solution, but 100% of the current through the anion-exchange
membrane. These differences in transference numbers between
solution and membrane are the source of the depletion and
concentration effects that make electrodialysis a valuable
separation procedure. The same transference-number differences also
lead to difficulties like concentration polarization.
If one faraday of electricity is passed through the above-described
electrodialysis cell, 0.5 g eq of chloride will be transferred to
or away from the membrane surface, and 1.0 g eq of chloride will be
transferred through the membrane. There will be a concentration of
chloride ions at the rear surface of the anion-exchange membrane,
but a depletion of chloride ions at its front surface. At steady
state, chloride ions that are not electrolytically transported to
the front surface must be supplied by diffusion through the static
boundary layers, and concentration gradients are established. This
steady-state condition can be described by Equation 1: ##EQU1##
where: i=current density, coulomb sec.sup.-1 cm.sup.-2
F=faraday, 96,500 coulomb eq.sup.-1
t.sub.s.sup.- =transference number of anion in solution
D=diffusion coefficient of ion
C.sub.b =concentration of ion in the bulk
C.sub.1 =concentration of ion in the boundary layer
.delta.=thickness of boundary layer
t.sub.m.sup.- =transference number of anion in membrane
This equation can be rearranged to Equation 2: ##EQU2## which shows
that, as current density increases, the boundary-layer anion
concentration approaches zero. Although some hydroxyl ions are
transported through the membrane and some hydrogen ions accumulate
at the membrane surface even at very low current densities, the
fraction of current carried by hydroxyl ions is insignificant until
the limiting-current density is reached. At this point, continued
passage of current requires that hydroxyl ions take the place of
the no-longer-available anions, and hydroxyl ions can be furnished
only by the ionization of water. At higher current densities,
therefore, hydrogen and hydroxyl ion concentrations become larger
relative to the concentrations of other ions. Hydroxyl ions are
transferred through the anion-exchange membrane, leaving an excess
of hydrogen ions on its surface (FIG. 1A), which markedly lowers
the pH at that surface.
Most naturally occurring organic contaminants bear negative
charges, and their acid forms are insoluble. When the supply of
hydrogen ions at the depleting surface of an anion-exchange
membrane is sufficiently great, colloidal organic substances are
partially neutralized (FIG. 1B) and precipitation is initiated
(FIG. 1C). The autocatalytic nature of this colloidal fouling, its
energetic consequences, and some of the membrane characteristics
contributing to it can be summarized as follows:
Whenever current is passed, salt ions are depleted near membrane
surfaces because of transference number differences between
solutions and membrane, and concentration polarization occurs.
Further passage of current requires the furnishing of hydrogen and
hydroxyl ions through the continuous ionization of water.
Hydrogen ions accumulate at the depleting surfaces of
anion-exchange membranes whenever concentration polarization causes
water molecules to ionize.
Organic anions are driven toward the surfaces of anion-exchange
membranes by the electric field.
In the zone in which hydrogen ions accumulate, organic anions are
converted to sparingly soluble acids, which deposit on the
membrane.
Precipitation of negatively charged material at the surface of a
membrane bearing fixed positive charges effectively produces a
"bilayer membrane". At the interface of the oppositely charged
ion-exchange materials, negative ions migrate through the
anion-exchange membrane and positive ions migrate through the
negatively charged colloidal layers. The net result is salt
depletion in the interfacial region, which further potentiates
water ionization, hydrogen ion production, and colloidal
precipitation (FIG. 1C).
Once deposition of organic material occurs, the deposited material
is tightly held by van der Waals forces and is difficult to
completely remove.
The energy required for continuous ionization of the water that
diffuses to the interfacial region leads to an increase in the
apparent membrane resistance.
The resistance of the interfacial layer of solution also becomes
higher as it is depleted of electrolyte, raising the total
resistance of the system.
The fouling behaviors of a given membrane type and composition can
vary widely between samples; "glossy" surfaces appear to be related
to fouling resistance. Grossman, G. and Sonin, A., Office of Saline
Water Research and Development Progress Report, 742 (1971).
The degree of microscopic surface homogeneity is important.
Membranes containing reinforcing materials, or membranes with
micro-heterogeneous surfaces show a greater tendency to foul
rapidly. Korngold, E.; de Korosy, F.; Rahav, R.; and Taboch, M.,
Desalination 8 (1970), 195.
The tendency to foul is the same in electrodialysis stacks
containing only anion-exchange membranes as in those comprised of
alternating cation-exchange and anion-exchange membranes.
Anion-exchange membranes have much greater fouling propensities
than cation-exchange membranes, because most organic contaminants
are negatively charged.
These observations cumulatively support the hypothesis that
membrane polarization leading to fouling is primarily a
surface-controlled phenomenon. Therefore, a means of permanently
modifying an operating membrane surface without changing the bulk
electrical properties offers the best hope of producing
desalination membranes with long operating lifetimes.
B. A Mathematical Model for Fouling
Adopting the physical model of fouling that evolved from the work
of Cooke and Korngold, et al., Electrochem. Acta 4 (1960), 1979;
and Desalination 8 (1970), 195, Grossman and Sonin derived an
expression for the amount of fouling in terms of the resulting
reduction in limiting current. Office of Saline Water Research and
Development Progress Report 813 (1972); Desalination 10 (1972),
157; Desalination 12 (1973), 107. They concluded that a fouling
film with the same charge as the substrate would not affect the
limiting current. An oppositely charged, extremely thin film can
cause marked limiting-current reduction, the thinness required for
effective fouling being an inverse function of the concentration of
fixed charge. A neutral film can reduce the limiting current but,
to do so, it must be many times thicker than an oppositely charged
film.
When a Blodgett layer is deposited on a substrate, its thickness,
surface concentration, and charge density are both known and
controllable. This fact can provide an excellent basis for detailed
experimental testing of the Grossman-Sonin model for membranes
fouled by known thicknesses of oppositely charged or neutral
molecules.
II. BLODGETT MULTILAYERS
A. The Monolayer Assembly Technique
If a solution of amphiphilic molecules in a hydrocarbon solvent is
gently dropped on a water surface, the drops will fan out as the
solvent evaporates. Simultaneously, the amphiphilic molecules
become oriented into a solid monolayer exactly one molecule thick,
with their polar "heads" at the water interface and their non-polar
"tails" at the air interface. Langmuir, I., Science 87 (1938), 493.
This monolayer lowers the surface tension of the water by an amount
equal to its own "surface pressure".
At a given surface pressure, each type of monolayer film occupies a
characteristic surface area. If the molecules are pushed together
by a moving barrier, regions characterized by different
compressibilities appear until the film "collapses", usually at
around 20 .ANG..sup.2 /molecule for fatty acid monolayers.
If a monomolecular film is in the so-called "condensed" region that
corresponds to high surface pressures, it will readily transfer to
a solid that is passed vertically through it. Blodgett, K. B., J.
Am. Chem. Soc., 57 (1935), 1007. The Blodgett-Kuhn dipping
apparatus provides a moveable polyethylene float activated by a
suspended weight, so that constant surface pressure and constant
molecular area of the spread film enclosed by the barrier are
maintained throughout the dipping procedure. As a monolayer of film
transfers from the liquid surface to the solid, the floating
barrier moves forward so that the area of the spread film decreases
by the area of both sides of the dipped solid. The "transfer
ratio", i.e., the areal coverage on the solid relative to the areal
coverage on the water is virtually unity for each layer, and
orientation is conserved during transfer.
Repetitive dipping of the solid to be treated results in the pickup
of successive monolayers, oriented in a y (head-head, tail-tail)
(FIG. 2) or an x (head-tail, head-tail) (FIG. 3) pattern. This
technique results in multilayers of molecules oriented
perpendicular to the surface on which they are deposited. The
number of deposited layers is found by actual counting of the
number of forward movements of the barrier during successive dips.
Layering patterns, as well as the total number of layers that can
be deposited, are dictated by the chemical and steric nature of the
amphiphilic molecules. After a given number of layers has been
attached, an assembly becomes "autophobic", rejecting the addition
of more layers of any substance, including itself.
Whether or not a substance will transfer as a multilayer to a solid
substrate depends on several factors, including the attraction of
the molecules for the water surface, the cohesive attraction
between the molecules and the attraction of the molecules to the
solid substrate.
B. The Properties of Monolayer Assemblies
Monolayers attached in the proposed manner have unique properties
for two reasons: each layer is reproducibly one molecule thick; and
the polar-nonpolar portions of the molecules in each layer are
strictly oriented with regard to the substrate. These features are
found in the membranes of biological cells and are believed to be
critical to natural phenomena like activated transport.
Oriented monolayers adhere to their substrates with extraordinary
strength. In the case of a stearate film on ordinary glass, for
example, penetration of the carboxylate groups of the fatty acid
salt into the glass surface is so extensive that the salt can be
removed only by sandblasting, which, of course, also destroys the
involved region of the substrate. Some substances can form
monolayer assemblies up to 4,000 layers thick and many of these
assemblies, also called Blodgett multilayers, are indefinitely
stable. For example, the nonreflecting glass used in optical
equipment is prepared by monolayer deposition techniques.
Although assembled monolayers are stable, they are also
"penetrable", a term suggested by Sobotka to emphasize that passage
through multilayers is possible because they are in continuous
thermal motion. J. Colloid Sci. 11 (1956), 435.
The application of monolayers to the surfaces of membranes should
have little influence on their ion-exchange properties, even if the
monolayers are charged. This is true because of the exceedingly
small number of charged groups in even a completely ionized
monolayer. The van der Waals' forces responsible for hydrophilicity
decay with the sixth power of the distance and therefore operate
over an exceedingly small range, whereas ion-exchange depends on
electrostatic forces, which decreases only with the square of the
distance. Thus, deposited multilayers will probably allow
sufficiently close approach of ions so that they can be attracted
to the fixed charge groups on the membrane.
1. The alteration of surface hydrophilicity by attachment of
monolayer assemblies
Langmuir was the first to observe that a metal surface coated by
stearate multilayers became non-wettable by water and also by many
hydrocarbons. J. Franklin Inst. 218 (1934), 143. The coated surface
had become both hydrophobic and oleophobic. Orientation of the
attached multilayers had apparently produced a smooth surface
consisting of closely packed and strictly aligned methyl groups,
i.e., a very "low-energy" surface. Wettability of a solid is thus
dependent only upon its outermost atomic layers. Shafrin, E. G. and
Zisman, W. A., J. Phys. Chem. 64 (1960), 519. In general, factors
that increase the polarity of a surface, including unsaturation of
hydrocarbon groups, increase wettability. Hydrogenation decreases
wettability, and fluorination has a still more marked effect.
Zisman found that surfaces coated by multilayers of perfluorolauric
acid exhibited the lowest surface energy of any surface yet
prepared. Shulman, F. and Zisman, W. A., J. Colloid Sci. 7 (1952),
465. These surfaces are both hydrophobic and oleophobic, repelling
even alkanes with great efficiency.
The implications are that an appropriate multilayer coating will
eliminate the water wettability of a membrane surface by increasing
its hydrophobicity, and it will simultaneously reduce adhesive
attractions between the membrane surface and organic materials in
the raw waters by increasing the surface oleophobicity.
2. Reduction of surface heterogeneity by deposited multilayers
It has been found that crevices and pores in a surface give rise to
variabilities in wetting and de-wetting behavior, implying that the
wettabilities of rough surfaces and smooth surfaces are very
different. Bikerman demonstrated the integrity of deposited
multilayer films by attaching films of barium stearate to wire
gauze with apertures about 0.53-mm across. Proc. Roy. Soc. A. 170
(1939), 130. The transfer ratio based on the gross area of the
gauze was nearly unity, exactly the same as it would have been if
the object coated had been a solid without holes or subdivisions.
Thus, the oriented layers attached themselves to the gauze at the
available points and possessed sufficient lateral cohesiveness to
bridge the relatively large intervening gaps as long as they were
kept wet. They thereby conferred a homogeneous character upon a
highly heterogeneous surface. (See FIG. 4).
Day and Ringsdorf have carried out experiments in which diacetylene
monocarbonic acids were polymerized at a water surface and
transferred as bilayers to porous substrates. J. Polym. Sci. Polym.
Lett. 16 (1978), 205. These coatings were 60-.ANG. thick and could
be made to bridge pores up to 0.5 mm in diameter on the solid
substrates. These experiments demonstrate that deposited
multilayers can lend a homogeneous character even to solids with
gross heterogeneities. The effect of successive monolayers upo the
fouling propensities of a desalination membrane with a roughened
surface should give a ready evaluation of the importance of
roughness and heterogeneity.
3. Conditioning of Langmuir-Blodgett multilayers
The properties of deposited monolayer assemblies can be altered to
fit specific needs. For instance, a hydrophobic film can be
rendered hydrophilic by post-treatment with dilute solutions of
polyvalent cations like thorium nitrate. If the assemblies are
mixtures, either of a monomer and its polymers, or of an acid and
its salt, one component can be removed by a suitable solvent,
leaving a "skeletonized" film of regular structure. The degree of
polymerization, or the initial relative concentrations of soap and
acid, provide methods of controlling the final coverage on the
substrate.
The "depth" of coverage at the membrane surface can be controlled
through the number of monolayers applied. The percentage of surface
that is covered to this depth can be determined by the proportions
of, for example, polymerizable and nonpolymerizable monomers in the
deposited layers. Fort et al. showed that post-polymerization at
the surface, followed by solvent leaching, effectively removed
ethyl stearate from polyvinyl stearate. J. Coll. Interface Sci. 47
(1974), 705. The attachment of polyvinyl stearate to the membrane
and the structural integrity of the polymer remained unchanged. The
apertures of these skeletonized films can be "refilled" by water or
hydrocarbons. Sobotka, H., Proceedings of the Conference on
Biochemical Problems of Lipids, Butterworths, London, 1956, p.
108.
4. Gas permeation through membranes modified by monolayer
deposition
Two types of membrane modifications by monolayer deposition have
been carried out by Quinn. Science 159 (1968), 636; J. Coll.
Interf. Sci. 27 (1968), 193; and Biophys. J. 12 (1972), 990. In
one, several gas-permeable membranes were coated with assembled
monolayers, and the effects on the permeation rates of different
gases were evaluated as functions of the nature and number of the
multilayers. Permeabilities of the substrate polymers were markedly
decreased by attachment of stearic acid or 3.beta.-cholestanol, but
the effect of oleic acid was much smaller. Presumably, steric
influences on the packing of the multilayers can explain these
differences. These workers also studied the effect of pore sizes on
gas permeation rates, modifying the reproducibly-sized pores of
track-etched mica membranes by depositing stearate multilayers. As
the layers dried, they migrated into the pores by surface
diffusion. Pore radii determined by Knudsen gas flow showed
excellent correlation with the modified radii predicted from the
number of deposited multilayers.
5. Blodgett deposition on reverse-osmosis membranes
Langmuir-Blodgett layers have been deposited and polymerized on
porous polysulfone backing materials to produce asymmetric
reverse-osmosis membranes. Fort, T., Jr. and Lando, J., Office of
Saline Water Research and Development Progress Report, 74-944
(1974). High salt-rejection samples could be prepared with coatings
comprising 18 layers of cellulose acetate, but many technical
difficulties were encountered during the deposition procedures, and
cracks leading to leakage through the multilayers were
frequent.
Such defects would be less deleterious to the successful
utilization of this invention than to the process described by Fort
and Lando. Amphiphilic molecule deposition was designed in this
invention to moderate the polarization and occlusion tendencies of
working membranes, whereas the amphiphilic molecule layers that
they deposited on porous supports were intended to become the
working parts of reverse-osmosis membranes. Any defects thus led to
losses of basic function, while defects in the coatings of the
present invention would lead only to a lower percentage of
modification of undesirable properties. As discussed infra,
Langmuir-Blodgett layers may also be used to enhance the fouling
resistance of standard reverse-osmosis membranes.
6. "Electrets" as fouling preventives in reverse-osmosis
experiments
Wallace and Gable compared fouling behavior of unmodified cellulose
acetate reverse osmosis membranes with that of identical membranes
that had been made into "electrets", Polym. Eng. and Sci. 14,
(1974), 92. These are essentially solid-phase condensers, with
negative charges aligned along the "skinned" surface, and are
electroformed by charging in a five-layer capacitor. Low-humidity
measurements of the net surface charge showed a rapid decay rate
during the first 24 hours, after which detectable charge persisted
for more than 70 days, the total span of observation. Immersion in
distilled water after 20 days brought about extremely rapid charge
dissipation.
When electret membranes were used in reverse osmosis systems, both
the amount and the adherence of foulant deposits were reduced. In
addition to repelling colloidal tannic acid, the electrets absorbed
less colloidal iron oxide. Salt rejection was unchanged.
No data were given on decay behavior of in-service electret
membranes or on the length of time between the "electroforming" and
initiation of the reverse osmosis testing described. It is logical
to assume that electrets would decay rapidly in water that contains
ions, since randomization of the aligned polar portions of the
cellulose acetate will be encouraged by a randomly charged
environment and by water permeation. However, it is apparent that
the presence of negative charge on the electret surface minimized
difficulties with fouling, even though this charge was weak and
shortlived.
Deposition of assembled monolayers to form a sheath may produce the
same protective effect as "electret" production, with the
additional advantage of long-term stability.
7. Electrodeposited polyelectrolytes on cation-exchange
membranes
Sata and Mizutani have reported treatments of commercial
cation-exchange membranes by surface coatings of various cationic
polyelectrolytes. J. Polym. Sci. Polym. Chem. Ed. 17 (1979), 1199.
The polyelectrolytes were applied either by electrodeposition or by
adsorption from solution, and would therefore not exhibit the
strict molecular orientation and layering behavior of Blodgett
layers. However, the properties of electrodeposited layers may
approach those of monolayer assemblies, thus indicating the
direction and degree of modification that can be expected. The
coatings affected current efficiencies, electrical resistances and
selectivities between univalent and divalent cations.
In all cases, the electrodeposited layers produced greater changes
in membrane properties, and were more compact and thicker than
adsorbed layers. The electrodeposited layers effectively prevented
fouling by ionic surface-active agents, so that the membrane
resistance remained constant during electrodialysis of solutions of
these agents.
Monolayer assembling should confer the same tenacity of attachment
that electrodeposition did in Sata's work, with the added benefits
of molecular orientation and the ability to use minimal, precisely
controlled, coating thicknesses. The anti-fouling effect should be
the same in a desalination environment as in a polyelectrolyte
dialysis system.
III. SPECIAL APPARATUS
A. The Wilhemy Balance for Surface Pressure-Area Measurements
An apparatus was constructed for measuring the surface pressure
(.pi.) of an oriented monomolecular film on water as a function of
available molecular area (A). The Wilhemy balance comprises a
shallow film trough of solid Teflon on a heavy aluminum base
equipped with leveling feet. A stainless-steel rod, piercing a
gasket at one end of the enclosing Lucite box, controls molecular
area by manipulation of a spring-loaded Teflon barrier straddling
the trough. Two-dimensional pressure changes are calculated from
differences in the apparent weight of a 3-cm square of Schleicher
and Schull No. 589 filter paper. This piece of paper (a "Wilhemy
plate", chosen because it is completely wetted and no contact-angle
correction need be included in calculations) hangs by a silver
chain from the beam arm of a modified Troemner Model 5100
specific-gravity balance. The observed surface pressure, .pi., is
equivalent to the change in surface tension due to the monolayer
film and is found from Equation 3: ##EQU3## here g=gravitational
constant=980.7 cm sec.sup.-2
.DELTA.G=change in apparent weight of "plate" relative to weight in
water without a monolayer
w=width of plate=3.0 cm
t=thickness of plate=0.005 cm
For our system, the constants can be lumped together to give
Equation 4:
The validities of our measured values for surface pressure and area
were checked by reproducing the well-known curve for arachidic
acid. (See FIG. 8).
B. Blodgett-Kuhn Dipping Trough
A dipping trough was constructed so that the water or salt
hypophase (liquid supporting the monolayer) and any monomolecular
film spread upon it are in contact only with thoroughly cleaned
Teflon or Pyrex glass. The windlass is machined to move clamped
membranes smoothly up and down during deposition, and is
hand-controlled for individual monitoring of each trip.
The polyethylene float confirming the spread monolayer is free to
move forward within the confines of parallel Teflon bars. Its
position at a given moment reflects a balance between the surface
pressure of the spread film and the force exerted by an aluminum
weight that hangs freely from the front of the apparatus and is
attached to the float by a nylon thread. This apparatus is enclosed
by a protective Lucite box. In shakedown runs, arachidic acid
multilayers were deposited on glass slides, for comparison with
literature reports.
C. Contact-Angle Apparatus
A highly sophisticated contact-angle goniometer (Rame-Hart, Inc.,
Model 100-00) was modified and refined from a design originated at
the Naval Research Laboratories. A microsyringe is used to deliver
calibrated drops of liquid to the surface being evaluated. The
apparatus is mounted on the trunnions of a tilting base so that the
alignment of optics and specimen is held constant during
measurement of advancing and receding angles. Special film clamps
are used to secure membrane strips flat on the specimen stage. The
entire apparatus is enclosed in a protective Lucite box.
D. The Laboratory Stack for Fouling Evaluations
1. Stack construction
a. The separators
Three-inch square separators were designed to hold twelve membranes
rigid in each electrodialysis cell. They were built individually on
wooden frames, with Lucite side pieces and evenly spaced Tygon
"spaghetti" tubing potted into Silastic cement serving as end
pieces. After the Silastic cured, the separators were removed from
the frames, and the Tygon tubes were severed at the inside surfaces
of the end pieces. Their other ends were potted into large-diameter
acrylic tubes for attachment to the hydraulic system. Entry and
exit of salt solutions through the resulting multiple ports ensured
thorough, well-distributed flushing of all membrane surfaces, with
a good approximation to laminar flow.
b. The hydraulic circuit
As FIG. 5 illustrates, streams of solution circulate from separate
reservoirs through the electrodialysis cell. Potassium chloride is
the electrolyte of choice in the test compartments, potassium
acetate (KOAC) is the electrolyte for the electrode compartments. A
Cole-Parmer Model WZ1R057 Masterflex pump with one add-on head
drives both solutions through silicone rubber tubing at flow rates
up to 2 L/min. The valves of the system are adjusted during
operation to equalize the flow between test compartments and
electrode compartments.
c. The electrical circuits
An Epsco Model D-612T power supply establishes selected potentials
between a platinized-titanium anode and a stainless-steel cathode,
which are sealed into the two Micarta end blocks that forms the
ends of the electrodialysis cell. Although the power supply has
readouts for both voltage and amperage, a milliammeter and a
voltmeter were included in the circuit for additional precision
(FIG. 6).
d. Stack assembly
The cell is clamped together as diagrammed in FIG. 7.
e. Shakedown fouling runs
The fouling test stack was assembled with eight test membranes in
the central positions, treated sides facing the cathode. Untreated
AMF A-63 anion-exchange membranes were used as electrode membranes
and as isolating membranes between the KCl and KOAc streams.
Potassium acetate (KOAc) was selected as the electrolyte for the
electrode compartments to prevent chlorine evolution, which might
obscure or otherwise interfere with the fouling process. Potassium
chloride (KCl) was used as the electrolyte in the test compartments
because potassium and chloride ions have approximately equal
transference numbers (0.50) in aqueous solution. The test solution
also contained 0.1% of sodium humate (Aldrich Chemical Company,
Milwaukee, Wisc.), which makes it more concentrated than natural
waters by a factor of about 10.sup.4.
All test membranes and isolating membranes were equilibrated in KCl
solution prior to insertion in the cell, and the two
electrode-compartment membranes were equilibrated in KOAc. They
were arranged in the cell in order, with Sample 1 in the cathode
compartment, Number 2 isolating the cathode compartment from the
test compartment, Numbers 3 through 6 in the test compartments,
Number 7 separating the anode and test compartments, and Number 8
in the anode compartment (see FIG. 7). In cases where the test
membranes were coated on only one side, the treated side faced the
cathode. Three 13.5-mil gaskets between the membranes and
separators have good sealing with free solution flow.
IV. EXPERIMENTAL DETAILS
A. Chemicals
1. Chemicals for fouling tests
Sodium humate (technical grade) was purchased from Aldrich Chemical
Co., Milwaukee, Wisc.
2. Chemicals for Blodgett deposition
Samples of surface-active compounds bearing perfluorinated carbons
at the end of their chains opposite to various functional groups
were furnished by the Commercial Chemicals Division/3M Center, St.
Paul, Minn. These compounds are laboratory prototypes, and 3M
policy precludes revealing their molecular weights or any
information other than that shown in Table I, below.
Other fluorinated, non-fluorinated and polymerizable compounds were
purchased from commercial suppliers. Although it is not certain
that all of these are sufficiently surface-active to properly
undergo oriented deposition, fluorinated molecules are usually
surface active at a much lower molecular weight or shorter chain
length than their hydrogenated homologues.
TABLE I ______________________________________ CHEMICALS FOR
BLODGETT DEPOSITION Molec- Compound Name ular Catalog or Formula
Source Weight No. ______________________________________
Fluorinated Compounds R.sub.fCOOH 3M L-1058 R.sub.fSO.sub.2 K.sup.+
3M L-1159 R.sub.f NHMe 3M L-2338 R.sub.fPO (OH).sub.2 3M L-4317
##STR1## 3M L-4745 Hexadecalfluoro- Gallard- 432 F-4530 1-nonanol
Schlesinger Perfluorotributylamine Gallard- 671 F-6220 Schlesinger
Perfluorodecanoic acid PCR 514 10614-6 11-H-Eicosafluorounde- PCR
546 13174-8 canoic acid Non-fluorinated compounds Cetylpyridinium
bromide Sigma 384 C5881 Hexadecyltrimethyl- Sigma 364 H5882
ammonium bromide Dodecyltetramethyl- Sigma 308 D8638 ammonium
bromide Tetradectyltrimethyl- Sigma 336 T4762 ammonium bromide
Polymerizable monomers Hexafluoroisopropyl Polysciences 236 2401
methacrylate Hexafluoroispropyl Polysciences 222 2400 acrylate
______________________________________
B. Anion-exchange Membranes
1. AMF A-63
The anion-exchange membrane, AMF A-63, is lightly crosslinked
polystyrene imbibed into polyethylene film, chlorinated, and
quaternized with dimethylethanolamine. Korngold focused most of the
experiments in his detailed study of fouling of anionselective
membranes on this material, providing extensive data on fouling of
A-63 as a function of time, current density, salt concentration,
feed solution velocity and buffered pH. Desalination 8 (1972),
195.
A-63 is not commercially available at present, but Dr. Richard N.
Smith of Southern Research Institute, Birmingham, Ala., kindly
donated a large supply, which he personally prepared while employed
by AMF Corporation.
2. SORI A568-007
Kressman and Tye suggested many years ago that membranes exposed to
tap water during their manufacture were effectively pre-fouled
before any exposure to the colloidal content of natural waters. J.
Electrochem. Soc. 116 (1969), 25. Therefore, the critical
initiating step that catalyzes fouling had already taken place.
For this reason, and because there was difficulty in evaluating
limiting currents to characterize the AMF A-63 membranes, novel,
low-resistance anion-exchange membranes were prepared under
carefully controlled non-fouling conditions. No water is used
during actual preparation, and the membranes were exposed only to
reagent-grade chemicals, with solutions in Milli-Q water used for
equilibration to saline conditions. Because oils and surfactants
are omnipresent on skin, gloves were worn during the handling, and
the samples were protected from other sources of contamination.
C. Pressure-Area Curves of Selected Compounds
A fluorinated aromatic heterocyclic bromide
Initially, a 1:1 chloroform-methanol mixture was used as a
spreading solvent for R.sub.f PyrBr, a fluorinated pyridinium
bromide furnished by Dr. Kenneth D. Goebel of the 3M Corporation.
This compound is a laboratory prototype, and its molecular weight
and precise composition are proprietary. It is believed to be a
long hydrocarbon chain which links the pyridinium bromide group
with a fluorinated end group.
Because of its compatibility with water, 1:1 chloroform methanol is
not an ideal spreading solvent, and thirteen possible alternatives
were screened. This study showed that the first attempts at
dissolving and spreading R.sub.f PyrBr had pinpointed an optimal
solvent; in fact, this compound was unable to be dissolved in
chloroform-methanol mixtures containing less than 50% methanol.
With sufficient care, this system produces reliable monolayers,
especially at reduced temperatures, as shown by the .pi.-A curves
in FIG. 9. Because the molecular weight is not known, units of
.ANG..sup.2 /.mu.g.times.10.sup.16 were used instead of .ANG..sup.2
/molecule for these plots.
The behavior in high pressure ranges described by these curves is
very different from that observed for non-fluorinated compounds.
Quite ordinary compressibility changes up to a surface pressure of
20 mN M.sup.-1 were observed. Most monolayers exhibit sharply
decreased compressibilities above this pressure, causing the curve
to become nearly vertical until the collapse pressure is reached
between 30 and 50 mN M.sup.-1. (See FIG. 8 for arachidic acid). In
contrast, R.sub.f PyrBr monolayers are highly compressible up to 35
mN M.sup.-1, with smooth transitions between several
compressibility ranges. At 55 mN M.sup.-1, the film does not
collapse, but it exhibits a constant surface pressure. Inducement
of collapse in R.sub.f PyrBr monolayers was never successful.
A definitive interpretation of this behavior is impossible without
knowledge of the area available to each molecule at given surface
pressures. Nevertheless, the observed high compressibility at
surface pressures above 30 mN M.sup.-1 would correspond with
Gaines' statement that " . . . the fluorinated compounds occupy
considerably larger areas in monolayers on water than their
hydrocarbon analogs . . . " Insoluble Monolayers at Liquid-Gas
Interfaces, Interscience, New York, 1966. There is an impliction
that, at least in such an oriented states, strong repulsions exist
between the fluorinated molecules. Increases of surface pressure
would be utilized in overcoming these repulsions, up to the point
where solution in the water hypophase becomes energetically
preferable to further compression or collapse. Thus, at 56 mN
M.sup.-1 of surface pressure, R.sub.f PyrBr may be dissolving at a
rate exactly balanced by the rate of film compression.
Two facts critical to attaining the objectives of this research
emerged from examination of the .pi.-A plots for R.sub.f PyrBr
(FIG. 9). First, at 30 mN M.sup.-1, the highest deposition pressure
heretofore used, this film cannot be considered to be highly
condensed. For effective film transfer to a substrate, with a
transfer ratio close to 1.00 and rigid orientation throughout the
film, the monolayer must be compressed quite close to the collapse
point.
Second, as suspected, R.sub.f PyrBr has a nontrivial solubility in
water. It is apparent from the parallel but offset .pi.-A curves at
different temperatures that the solubility is highly temperature
dependent. To deposit R.sub.f PyrBr films of optimal compactness
and coherence, the work must be done at low temperatures and
surface pressures of at least 35 mN M.sup.-1.
D. Multilayer Deposition in the Blodgett-Kuhn Trough
1. Sample preparation
a. Membrane cleaning
A procedure was devised to free the surfaces of AMF A-63
anion-exchange membrane from grease, surfactants, and other
contaminants. Although it is probable that commercial
anion-exchange membranes are somewhat fouled during the
manufacturing process itself, samples carefully cleansed of
removable materials gave the most reliable baseline for
evaluations.
In every step of the cleaning protocol, the operator wore clean
gloves, and only Milli-Q water was allowed to contact the membrane
samples. Use of this high-purity water, which is extremely low in
both salt and organic content, prevented further contamination and
reversed, if possible, prior contamination.
The treatment included the following steps: brushscrub both sides
of each membrane with a Milli-Q solution of Oxford Laboratory
Cleaner; rinse in hot Milli-Q water; treat overnight with Milli-Q
water at 70.degree. C. in an ultrasonic cleaner; dry in an oven at
50.degree. C.; and store in a desiccator.
b. Coding of samples
Completed samples are marked by a hole puncher with a code
designating which side is treated, the type of treatment, and the
position of the membrane during dipping. It is then possible to
differentiate, for example, between the sample that faced the left
front of the dipping trough and the one that faced the right rear.
Such differences in position may lead to significant variations in
properties, as observed by Fort and Lando for multilayered
reverse-osmosis membranes. Office of Saline Water Research and
Development Progress Report 74-944 (1974).
c. Multilayer preparation
(1) Arachidic acid multilayers
Arachidic acid monolayers at 25 mN M.sup.-1 and 25.degree. C. were
spread on Milli-Q water from a 10.sup.-4 M chloroform solution.
Float movements during membrane dipping cycles were reproducible
for clean, flat membranes. Sets of membranes coated with 1, 2, 3
and 10 y-layers (head-to-head, tail-to-tail) of arachidic acid were
prepared. These samples provided sufficient material for a
fouling-evaluation run as well as for examination of wetting
behavior, microscopic surface structure, electrical resistance, and
transference number. Immediately after preparation, the samples
were immersed in Milli-Q water and stored in water until
evaluation.
(2) Fluorinated pyridinium bromide multilayers
R.sub.f PyrBr proved to be almost insoluble in chloroform, which is
a preferred spreading solvent. It was highly soluble in methanol
and was spread from a 1:1 methanol-chloroform solution, the highest
chloroform concentration in which it would dissolve. This mixture
is far too compatible with water to be a good spreading solvent,
and the utmost care was required to prevent drops of solution from
piercing the water surface and dissolving in the hypophase.
Although monolayers were formed, it is probable that the pyridinium
bromide was also able to swim out of the bulk hypophase onto the
exposed water surface at the back edge of the float. This can
create a competitive lowering of the surface tension at that edge,
and the force exerted on the enclosed film may be erratic.
Pressure-area curves indicated that a low temperature is desirable
to ensure formation of stable films and minimize solution of
R.sub.f PyrBr. By packing the dipping trough in an ice-brine
slurry, its temperature was maintained between 9.0.degree. and
11.5.degree. C. Films were spread from a 10.sup.-4 M solution in
1:1 chloroform-methanol.
On the basis of the .pi.-A curves for R.sub.f PyrBr, it was also
decided to work at higher surface pressures than those used in the
exploratory phase. Sets of membranes were prepared coated with 1,
2, 3, and 10 layers of R.sub.f PyrBr, at deposition pressures of 30
mN M.sup.-1 and 40 mN M.sup.-1.
The differences in float behavior between the 30-mN M.sup.-1 and
40-mN M.sup.-1 dipping runs were striking. At 30 mN M.sup.-1, float
movement (by which the amount of material deposited is measured)
was always somewhat erratic during immersion. Although the first
several layers of R.sub.f PyrBr transferred as x-layers
(head-tail-head . . . ), float movement on the third to fifth
emersion signalled the deposition of a y-layer (tail-tail-head-head
. . . ). Additional y-layers were deposited at random between
x-layer sequences on the 10-layer samples. Considerable randomness
thus occurred in the multilayer structures deposited at 30 mN
M.sup.-1, although examination by SEM (to be discussed later)
portrays a remarkably coherent, frictionless and homogeneous
surface.
On initiating R.sub.f PyrBr depositions at 40 mN M.sup.-1, it was
decided to "vacuum" remaining monolayer from the hypophase surface
between each immersion and emersion. This procedure ensures
x-layering throughout the entire structure and eliminates any
randomness in the multilayer pattern. All emersions under these
conditions produced very clean, smooth minisci at the emerging
membrane surfaces. This is a good indicator that the surface is
sub-microscopically smooth. Membrane samples dipped singly, so that
both sides were coated with pyridinium bromide, swelled and buckled
to some extent, but not as severely as similar samples dipped
through arachidic acid. The buckling problem was virtually resolved
when samples were sealed together for asymmetric coating. All of
the asymmetrically coated samples curled toward their coated sides
after separation from the sandwiches and drying, implying that the
two sides are indeed different.
Through careful monitoring of the behavior of the meniscus at the
vertical membrane surface, it was found that maximum deposition was
achieved when the rate of dipping was adjusted to ensure a smooth
meniscus at all times. For several emersions, maintaining a smooth,
intact meniscus required a withdrawal rate as slow at 0.13
cm/min.
Another source of randomness was eliminated by the increase in
surface pressure. Float movements on emersion were about 40% larger
than at 30 mN M.sup.-1, and they were very reproducible. Thus, more
material was deposited with each layer, and the amount was
identical from layer to layer at the higher pressure. Because the
molecular weight of R.sub.f PyrBr is not known, the surface
coverage cannot be estimated. It is probable, however, that by
transferring the monolayer films at surface pressures about 30 mN
M.sup.-1, still greater film coherence would be achieved, which
should give rise to a distinctive increase in hydrophobicity.
Immediately after preparation, all samples were immersed in Milli-Q
water and stored in water until evaluated.
E. Contact Angle Determination
All ion-exchange membranes sorb water, making true measurement of
the contact angle displayed by a sessile drop a practical
impossibility. Soon after application, the profile of a standing
water drop becomes lower while the membrane surface undergoes a
simultaneous localized rise. The phenomena responsible for the
resulting continuous changes in baseline and profile include at
least the following: localized absorption of the water; diffusion
of water into surrounding membrane material; and swelling of the
polymeric membrane as it sorbs the water. There may also be a
degree of actual polymer solution, such as Stamm has documented for
wood and other cellulosic materials. Wood Sci. Technol. 3 (1969),
301.
Exploratory measurements showed that all of the present series of
membrane samples, treated and untreated, were so hydrophilic that
measurement of advancing and receding contact angles was
impossible. This corresponds to the experience of Lloyd et al. with
sulfonated polysulfone membranes. Annual Report to Office of Water
Research and Technology, April 1980. Therefore, it was decided to
note initial contact angles and also to follow the change in
contact angle as a function of time.
No possibility exists for obtaining an "equilibrium-contact-angle"
value in these systems. An "instantaneous contact angle" was
obtained as a function of time, anticipating that the slopes of
these experimental curves will furnish bases for both comparison
and interpretation.
So that a water drop would experience some of the same environment
that it would see if the membrane were part of an operating
electrodialysis stack, its contact angle has measured on a "damp"
sample. This procedure would be more reliable if it were carried
out in a chamber with 100% relative humidity, which is possible
when an environmental chamber surrounds the contact-angle
goniometer.
The sample was removed from Milli-Q storage and briefly patted
between paper towels. A standard drop of Milli-Q water was applied
from a syringe fixed above the stage of the goniometer. The contact
angle was read as quickly as possible, and at regular intervals
thereafter.
Changes visible at the drop-membrane junction follow the pattern
diagrammed in Steps A through C of FIG. 10. Step A represents the
appearance of the system immediately after the water drop is
applied. Within 30 seconds, a haze appears at the junction. Later
events indicate that the boundary of this haze (represented by a
dotted line in FIG. 10) is indeed the surface of the membrane,
which is swelling as it sorbs water but is still too "dilute" to
appear dark in the telescope.
At Stage C, contact-angle decay has reached a point where an angle
is barely measurable, and the drop is virtually completely sorbed.
The curved surface of the swelling membrane appears dark in the
telescope.
1. Untreated controls
Water drops at the surfaces of untreated AMF A-63 anion-exchange
membranes were completely sorbed in about 60 min.
2. Membranes coated with arachidic acid
Three (3) y-layers of arachidic acid, deposited at 25.degree. C.
and 25 mN M.sup.-1, markedly enhanced the sorption process. A
standard water drop was sorbed within 36 min.
3. Membranes coated with R.sub.f PyrBr
FIGS. 11-13 illustrate the unusual behavior of water drops at the
surfaces of membranes treated with R.sub.f PyrBr. In every case,
the time required for drop disappearance was lengthened beyond the
period observed for untreated controls. Furthermore, in the cases
of samples covered by three multilayers, the prolongation was
roughly proportional to the increase in the surface pressure at
which the layers were deposited. These observations are in line
with the hypotheses on which the present invention was based.
The slope changes shown in FIGS. 11-13 have been observed with
membranes modified by R.sub.f PyrBr. They do not, in one sense,
represent the actual progress of the wetting process, which was
schematically illustrated in FIG. 10, A through D. It is evident
that the amount of non-sbsorbed water is greater in 10C than in
10D, but the "apparent contact angle" is larger in 10D. The
swelling membrane has reached a plateau, bringing the baseline, to
which the contact angle is referred, nearly back to the horizontal.
At the same time, it was routinely found necessary to relocate the
water drop in the telescope of the contact-angle goniometer. The
drop's shift in position is accompanied by an apparent coalescence,
which reduces its area of contact with the swollen substrate. The
baseline plateau and reduced contact area of the drop both
contribute to an abrupt increase in apparent contact angle, and an
inflection point on the decay curve.
The wetting behavior of Sample 5 (FIG. 13), which was coated with
three x-layers of R.sub.f PyrBr at 40 mN M.sup.-1 and 10.5.degree.
C., differed from the other R.sub.f PyrBr systems. Sudden decline
from an initially high contact angle was followed by stabilization
at an angle of about 65.degree.. This can be termed a pseudo
steady-state condition because the drop, in reality, underwent
several incidents of coordinated baseline and contact area shifts
such as described above. Thus, wetting, absorption, and drop
disappearance were in fact occurring, but are reflected in FIG. 13
only as small variations around an average steady-state angle. In
this instance, interpretation of gross data responses without
appreciation of the more subtle evidences of change in the system
could lead to serious error.
After a span of 75 min. with only slight decreases in apparent
contact angle, the water drop disappeared rapidly into the
membrane. This was a single experiment with this type of
membrane.
Surprisingly, but in line with the SEM observations, infra, the
10-layer R.sub.f PyrBr coating did not lead to further enhanced
hydrophobicity (FIG. 13, Sample 6). The SEM of this sample
indicated a high degree of disorder, which is consistent with more
rapid wetting than observed with the membrane coated by three
layers of R.sub.f PyrBr.
F. Scanning Electron Microscope Examination
1. Sample preparation
The earliest SEM studies were made on samples that had been imbibed
with a glycerol-water mixture, vacuum dried, and sputtered with
gold and platinum. The imbibition step was incorporated as a means
of keeping pores open, and metallic sputtering ensured sufficient
surface conductance to give a clear picture.
It was found, however, that both procedures added experimental
artifacts that obscured rather than enhanced the significant
characteristics of the modified membrane surfaces. Vacuum drying
caused the glycerol mixture to attempt escape, forming blisters and
pockets. Sputtering with metals blanketed the much thinner
deposited multilayers. If it had been mandatory to sputter to
obtain clear micrographs, this disadvantage would have to have been
accepted and allow for it in the interpretation. It was found,
however, that the membranes themselves have sufficient charge
density to yield excellent SEMs, and that sputtering is totally
unnecessary. Indeed, the best pictures were obtained when the SEM
potential was reduced from 10.0 KV to 2.5 KV.
In the refined sample preparation, Milli-Q water was vacuum dried
from the membrane. Vacuum drying itself may be inducing some
collapse within the multilayer structure, but this part of the
procedure is integral to SEM examination. The micrographs give
little or no indication of collapse. This possibility must,
however, be kept in mind in comparing these scans with the results
of fouling evaluations for membranes that have been kept wet since
their preparation.
2. Controls
An untreated sample of AMF A-63 anion-exchange membrane was used as
the control. Considerable debris was obvious on the surface, and
the surface composition itself appeared to be highly inhomogeneous.
No cracks or pores were visible, which was also the case for the
samples that were imbibed with glycerol before examining.
G. Tests of ED-Membrane Fouling Propensities
1. Shakedown runs
Several accelerated fouling runs were carried out that lasted from
146 to 186 hours, and others that were terminated after 20 hours.
Except in one instance, which can be reasonably explained, the
stack reached a steady-state current density before 20 hours had
elapsed. No additional information could be obtained by extending
the experiments, and all runs were limited to a standard 20-hour
period.
Because most of the fouling experiments were carried out before the
limiting current of the stack could be evaluated, an arbitrary
choice was made of operating potential. It was found that 2.0
volts, the constant potential used throughout these tests, was
considerably below the potentials required to produce limiting
currents in this system. For ready comparison with large-scale
industrial desalination conditions, operating potentials giving
rise to 70% of the limiting current would have been preferable.
Nevertheless, these runs are valid for demonstrating the effects of
different modifications upon fouling-induced resistance
increases.
The humate solutions were more concentrated than natural waters by
a factor of 10.sup.4. Therefore, a 20-hour run exposed the
membranes to amounts of humic acid many times greater than would
ever be encountered during successive normal lifetimes in an
electrodialysis stack. Of course, there are many factors in
membrane deterioration other than exposure to humates, and
service-lifetime studies should be included in future
investigations of these membrane modifications.
The first set of shakedown runs was carried out at a constant
potential of 2.0 V, linear stream velocities of 0.68 cm/sec, and
solution concentrations of 0.001N, with 0.1% (w/w) sodium humate
added to the KCl stream. When untreated AMF A-63 control membranes
were mounted in the test positions, the operating current densities
fell from 17.4.times.10.sup.31 3 milliamps/cm.sup.2 to 11.6
mA/cm.sup.2 over the first hour and to 6.8.times.10.sup.-3
overnight. For similar periods, the cell containing test membranes
coated on one side by three layers of R.sub.f PyrBr at 25.degree.
C. and 25 mN M.sup.-1, exhibited currents of
13.6.times.10.3.sup.-3, 10.7.times.10.sup.-3, and
8.7.times.10.sup.-3 mA/cm.sup.2. When membranes asymmetrically
coated by three layers of arachidic acid were installed, the
initial current density, at the constant potential of two volts,
were 7.8.times.10.sup.-3 mA/cm.sup.2 much lower than we observed
with the other two membrane types. This indicates that the three
(3) arachidic acid layers, with a total thickness of only 60 .ANG.,
produced a large resistance increase.
2. Membranes modified by fluorinated pyridinium bromide
Six fouling runs were carried with membranes modified by Blodgett
multilayers of R.sub.f PryBr. Results are shown below in Table III.
Except for two pairs, these membranes were modified under
conditions differing in too many variables to yield truly reliable
progressions.
TABLE III
__________________________________________________________________________
FOULING OF AMF A-63 MEMBRANES MODIFIED BY FLUORINATED PYRIDINIUM
BROMIDE LAYERS Type of Number Number .sup..pi. depo- .sup..tau.
depo- Condition .sup.a R. steady- R. steady- .DELTA.R. Period
treat- of sides of layers sition, sition, at R. initial, state
state kilo- of test, ment treated applied dynes/cm .degree.C.
deposition kilo-ohms kilo-ohms R. initial ohms hours
__________________________________________________________________________
Control -- -- -- -- -- 17.8 45.7 2.6 27.9 17 Control -- -- -- -- --
20.0 52.6 2.6 32.6 186 R.sub..intg. PyrBr 1 1 40 10.5 dry 14.5 17.8
1.2 3.3 183 R.sub..intg. PyrBr 1 .sup. 2X.sup.b 40 10.5 dry 26.7
40.0 1.5 13.3 25 R.sub..intg. PyrBr 2 3X 35 10.5 dry 26.7 40.0 1.5
13.3 146 R.sub..intg. PyrBr.sup.c 1 .sup. 3Y.sup.d 25 25.0 dry 22.9
35.6 1.6 12.7 20 R.sub..intg. PyrBr 2 3X 35 10.5 wet 22.7 57.1 2.5
34.4 186 R.sub..intg. PyrBr 1 10X 40 10.5 dry 25.0 40.0 1.6 15.0 30
__________________________________________________________________________
.sup.a Total resistance of stack assembly with all test membranes
.sup.b Blodgett Xmultilayers, headtail-head-tail pattern .sup.c
This set of membranes has been stored 2 months at room temperature
.sup.d Blodgett Ymultilayers, headtail-tail-head-pattern
With the exception of one membrane set modified while wet, all
R.sub.f PyrBr-coated membranes, compared to untreated controls
after both types were fouled, exhibited lower resistances. One
layer of R.sub.f PyrBr, deposited on a dry membrane at 40 dynes/cm
and 10.5.degree. C. (the third sample in Table III) had two effects
on resistance: it reduced initial membrane resistance by almost
20%, and it reduced the relative resistance rise on fouling to 46%
of the increase for untreated controls. The actual resistance
change due to fouling was reduced from 30 to 3.3 kilo-ohms. The
fouling test samples were always kept wet, affording the modifying
layers optimal conditions for retention of the integrity of freshly
prepared samples.
By comparison, 2x-layers of R.sub.f PyrBr (the fourth sample in
Table III) and 3x-layers of R.sub.f PyrBr (the fifth sample in
Table III), deposited at 40 dynes/cm and 35 dynes/cm, respectively,
raised the initial resistance by 33%, but also reduced the
resistance rise on fouling to 58% of that experienced by untreated
controls. However, the steady-state resistance reading was 81% of
the steady-state resistance of untreated membranes, more than twice
the steady-state resistance of the sample coated by one
monomolecular layer of R.sub.f PyrBr.
X-layers were formed on these samples by vacuuming excess R.sub.f
PyrBr film from the water surface while the dipped membranes,
coated by prior layers, were still submerged. Emersion then
occurred through a clean surface, and a fresh R.sub.f PyrBr film
was spread before the next immersion. Each immersion resulted in
highly reproducible float movement in the dipping trough, which
indicated the transfer of a highly regular film well registered
with the substrate. This arrangement of layers is apparently the
preferred configuration for R.sub.f PyrBr, giving the best
multilayer packing.
It is therefore surprising that 3 y-layers of R.sub.f PyrBr,
deposited at 25.degree. C. and 25 dynes/cm (the seventh sample in
Table III) were almost as successful as 1, 2, 3, and 10 x-layers in
reducing resistance increases during fouling. It was observed
earlier that the .pi.-A curves of R.sub.f PyrBr showed that the
film at a surface pressure of 25 dynes/cm is in only a slightly
condensed state, which might hinder transfer to a substrate.
However, an SEM, at magnification 1000X, of a membrane coated with
R.sub.f PyrBr at 25 dyne/cm exhibited a distinctive appearance
relative to an untreated surface. The SEM makes it obvious that the
previously inhomogeneous membrane surface has been covered by a
film that is coherent except for pores that can be ascribed to
layer collapse during vacuum drying.
A likely explanation of the reduction in resistance caused by these
y-layers lies in the fact that the seventh sample of membranes in
Table III had been coated with R.sub.f PyrBr two months before the
fouling test, and they had been stored at room temperature.
Materials deposited in an arrangement different from the preferred
pattern have a tendency to rearrange to the preferred pattern over
a period of time, while retaining the multilayered configuration.
Fort et al. used X-ray diffraction to discern this behavior in
aging multilayers of ethyl stearate. J. Polym. Sci. Part A-1 10
(1972), 1061. It is possible that the y-layers originally deposited
rearranged to x-layers during the extended interval between
preparation and fouling. Their influence on electrical resistance
would then be similar to the effect of multilayers originally
deposited in the x-pattern.
The seventh test run was the only evaluation during which the test
cell experienced a further resistance increase after 20 hours. In
that case, the resistance was steady at 57.1 kilo-ohms (the value
reported in Table III) for 100 hours or longer and then underwent a
rise toward 65 kilo-ohms at 185 hours. This result is interpreted
as due to partial coverage of the substrate membrane by poorly
attached R.sub.f PyrBr monolayers. It was speculated that, on
undergoing a 20% change in dimensions as they sorbed water,
membranes that were coated while dry might disturb the continuity
of the deposited surface layers. If so, attachment of layers to a
prewetted membrane would improve the coherence of the coating.
During application of successive monolayers to the water-swollen
membranes, it was noted that float movement in the Blodgett-Kuhn
trough was extremely erratic. Total movement during one immersion
was only a fraction of that undergone during normal dipping of a
dry substrate. These observations made both coverage and attachment
of the modifying films questionable. The results of the fouling
tests confirmed the suspicion that preswollen membranes cannot
satisfactorily accept Blodgett layers.
3. Membranes modified by arachidic acid
In all cases of membranes modified by layers of arachidic acid,
resistances doubled during the course of fouling-evaluation runs.
Furthermore, the initial resistance exhibited by an ED stack
containing membrane samples coated with 1, 2, 3 or 10 y-layers were
identically twice that of a stack with untreated control membranes.
It appears that one layer, although it is only 20 .ANG. thick, is
sufficient to provide "pre-fouling" that the addition of more
arachidic acid layers does not supplement.
Resistence increases for membranes coated with arachidic acid were
considerably greater than for membranes coated by R.sub.f PyrBr,
and also larger than exhibited by untreated controls. The SEM
observations imply that, once fouled by an oriented layer, the
membranes gained surface homogeneity that should somewhat reduce
their propensity for continued fouling. It is evident, however,
that the charge disparity between the oriented layer and the
substrate, which Korngold et al. dubbed the "sandwich effect," is
of overwhelming importance in determining fouling propensities.
4. Light transmission of fouled membranes
On visual examination of the fouled membranes, no differences in
coloration could be detected between their modified and unmodified
sides. The unmodified sides, however, displayed a dull patina,
whereas the treated sides were glossy. These differences in
appearance indicate that humic acids have occluded the untreated
sides, but not the treated sides. Humic acid coloration was
homogeneous throughout the samples, but was less intense overall in
the membranes coated with R.sub.f PyrBr. Because, for accelerated
fouling tests, the treatment solution contained much more sodium
humate than would ever be found in natural waters, significant
amounts of humic acid may adhere to the untreated sides of the
membranes by concentration-driven adsorption.
Observations confirming this were made on membranes modified by
R.sub.f PyrBr and by arachidic acid. One membrane of each pair
confronted the humate solution with a multilayered surface, while
the other confronted it with the untreated surface. For both types
of treatment (R.sub.f PyrBr and arachidic acid), the membrane with
a treated surface facing humates retained much less coloration than
its mirror image. The control membranes exhibited about the same
humic acid pickup in both positions.
All fouled membranes, treated and untreated, were stained brown,
and the depth of color varied with the position of the membranes in
the electrodialysis cell. The observations indicate that treatment
with R.sub.f PyrBr had no effect on the total amount of humates
that adhere to the membrane, but that, at least in this case,
multilayer coating prevented the humates from being irreversibly
adsorbed by the substrate material.
V. SUMMARY OF RESULTS
It is possible to construct negatively charged or positively
charged multilayer assemblies on the surfaces of anion-exchange
membranes, thereby modifying the surfaces.
These layers, when assembled at optimal temperature and pressure
conditions, confer a marked degree of microscopic homogeneity on
the surfaces.
The wetting characteristics of the membranes can be altered by
addition of oriented multilayers. In the case of layers with a
charge opposite to that of the substrate, the sorption time for a
standard drop of water is shortened; if the layers are fluorinated
and have the same charge as the membrane, the sorption time is
prolonged.
Membranes modified by oppositely charged multilayers wet more
quickly than untreated controls.
Membranes modified by R.sub.f PyrBr layers wet twice as slowly as
untreated controls.
There is a correlation, in the case of three layers of fluorinated
material with the same charge as the substrate, between the
tightness of packing in the layers and the lengthening of the
wetting time.
Both types of multilayer treatment raised the Cowan-method limiting
current of anion-exchange membranes, relative to i.sub.lim of
untreated controls.
However, the effects upon power requirements of the membranes when
electrolysis was carried out in the operating range (70% i.sub.lim)
of an ultra-clean system were dramatically different. Power
requirements for membranes coated by oppositely charged layers were
multiplied by 9. Power requirements for membranes coated by
fluorinated layers with like charges were multiplied by 3.
The initial resistance of membranes asymmetrically coated by
oppositely charged layers was high relative to that of the
controls. It became still higher during operation of an
electrodialysis system loaded with 10.sup.4 times a natural level
of humates. This behavior implies that the first few molecular
layers added to a substrate during fouling have the most drastic
effect upon its electrodialytic properties.
The actual resistance increase during fouling tests is much greater
for membranes treated by oppositely charged monolayer assemblies
than for untreated controls; the percentage increase over the
initial value is lower.
Membranes modified by oppositely charged monolayer assemblies sorb
more humate color during a fouling run than untreated controls if
the outermost layer is nominally polar; they sorb approximately the
same amount if the outer layer is nominally nonpolar.
The alterations caused by oppositely charged monolayer assemblies
in both wetting and fouling behavior indicate that homogeneity of
the surface exerts an influence on these phenomena that is
negligible relative to the influences of charge disparity and
hydrophobicity.
The examination of pairs of membranes from the fouling stack
demonstrates that treatment by multilayering with either similarly
or oppositely charged materials interferes with the deposition of
humates at a membrane surface.
Judging from visual examination, all of the membranes, including
the asymmetrically modified samples, adsorbed significant amounts
of humates during the fouling evaluations. Thus, although the
modifying layers may have prevented precipitation of partially
neutralized humates at the sides of the test samples that faced the
cathode, they did not prevent entry of unneutralized colloidal
material from the sides facing the anode.
Membranes modified by fluorinated similarly charged monolayers
exhibited an electrical resistance prior to humate fouling that is
almost the same as the resistance of fouled control samples.
Actual electrical resistance increases for membranes modified by
R.sub.f PyrBr x-layers were small compared to those of untreated
controls; percentage increases of fouled over initial resistance
were halved when these layers were present. R.sub.f PyrBr X-layers
cause membranes to sorb much more humate color during a fouling run
than was sorbed by untreated controls.
Treatment with one layer of R.sub.f PyrBr at 40 mN M.sup.-1 and
10.5.degree. C. was the most successful anti-fouling preventive
tested, cutting actual resistance increase from 30 to 3.3 kilo-ohms
and the ratio of final and initial resistances from 2.6 to 1.2.
Correlation of Results and Theory
The experiments have borne out the hypotheses that even a single
deposited oriented monolayer, with a thickness of 20 .ANG.,
strikingly modified both the microscopic appearance and the
electrical and fouling behaviors of anion-exchange membranes. While
the effects upon appearance were similar, the effects upon wetting
behavior and resistance changes during accelerated fouling tests
were opposite for layers charged like and unlike the substrate
membrane. Therefore, surface roughness and inhomogeneity would
appear to be minor factors in the fouling process.
A single monolayer of a fluorinated pyridinium bromide cut
resistance rises due to fouling by a factor of 9, demonstrating
that this modification holds promise of greatly improving the
economics of electrodialytic desalination.
VI. REVERSE OSMOSIS MEMBRANES COATED WITH FLUORINATED PYRIDINIUM
BROMIDE
Oriented deposition of 1 Blodgett layer of R.sub.f PyrBr was used
to modify the surfaces of two types of commercial cellulose acetate
reverse-osmosis (RO) membranes. The dipping pressure was 40 mN
M.sup.-1, and the temperature of the system was maintained at
10.5.degree. C. One type of substrate membrane was obtained from
Hydranautics, Inc., the other from Fluid Systems, Inc.
By the following table, it can be seen that one layer of R.sub.f
PyrBr reduced the throughput of the Fluid Systems membrane almost
to zero, but it had very little effect on the throughput of the
Hydranautics sample. It is probable that the high initial flux of
the Fluid Systems membrane is due to the cracks that can be
detected in its SEM. These cracks, which would also lead to
undesirably low salt rejection, were sealed by application of one
monolayer of R.sub.f PyrBr.
TABLE II ______________________________________ EFFECT OF
MONOLAYERING ON THROUGHPUT OF RO MEMBRANES.sup.a Test Period.
Throughput. Membrane Modification hours gfd.sup.b
______________________________________ Hydranautics None 16 2.71
Hydranautics 1 layer R.sub..intg. PyrBr 18 2.24 Fluid Systems None
10 4.29 Fluid Systems 1 layer R.sub..intg. PyrBr 19 0.06
______________________________________ .sup.a Exposed to 0.1 molar
NaCl containing 2 ppm sodium humate at 160 psi, 25.degree. C.
.sup.b Gal/ft.sup.2 .multidot. day
Thus, monolayering treatment apparently reversed a surface
characteristic of the Fluid Systems membrane that would be a source
of undesirable operating properties; the monolayer simultaneously
reduced transmembrane flux. The Hydranautics membrane, which
exhibited no cracks in its unmodified state, experienced only a
minor reduction in flux.
Long-term fouling experiments are needed to compare flux reduction
by monolayering with flux reduction by foulant buildup. The
preliminary observations indicate that a membrane with high salt
rejection and reasonable transmembrane flux will retain these
properties after monolayering, but will also tend to repel
foulants.
* * * * *