Method of modifying membrane surface with oriented monolayers of amphiphilic compounds

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 membrane 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.

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 uncharged 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 surfaces 
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, hereinafter 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 the 
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 undesirable 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, microfiltration, 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 of 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.

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 electrodialysis 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 modificaiton 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 APATUS 
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 Wilhelmy 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 
"Wilhelmy 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 S100 
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: 
EQU .pi.=-163.16 (.DELTA.G)mN M.sup.-1 (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 confining 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 form 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, WI. 
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, MN. 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.f .about.COOH 
3M L-1058 
R.sub.f .about.SO.sub.3 K.sup.+ 
3M L-1159 
R.sub.f .multidot.NHMe 
3M L-2338 
R.sub.f .about.PO (OH).sub.2 
3M L-4317 
##STR1## 3M L-4745 
Hexadecafluoro- Gallard- 432 F-4530 
1-nonanol Schlesinger 
Perfluorotributylamine 
Gallard- 671 F-6220 
Schlesinger 
Perfluorodecanoic acid 
PCR 514 10614-6 
11-HEicosafluorounde- 
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 
Tetradecyltrimethyl- 
Sigma 336 T4762 
ammonium bromide 
Polymerizable monomers 
Hexafluoroisopropyl 
Polysciences 
236 2401 
methacrylate 
Hexfluoroisopropyl 
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: brush-scrub 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 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 "equilium-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 apears 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 
multi-layer 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 later 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 PyrBr. 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.T 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.f PyrBr 
1 1 40 10.5 
dry 14.5 17.8 1.2 3.3 
183 
R.sub.f PyrBr 
1 .sup. 2X.sup.b 
40 10.5 
dry 26.7 40.0 1.5 13.3 
25 
R.sub.f PyrBr 
2 3X 35 10.5 
dry 26.7 40.0 1.5 13.3 
146 
.sup. R.sub.f PyrBr.sup.c 
1 .sup. 3Y.sup.d 
25 25.0 
dry 22.9 35.6 1.6 12.7 
20 
R.sub.f PyrBr 
2 3X 35 10.5 
wet 22.7 57.1 2.5 34.4 
186 
R.sub.f 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 had 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 conrols. 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. 
Resistance 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 caused 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 inrease 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.f PyrBr 
18 2.24 
Fluid Systems 
None 10 4.29 
Fluid Systems 
1 layer R.sub.f 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.