Mechanism of Membrane Formation. VI. Convective Flows and Large Void Formation during Membrane Precipitation Moshe A. Frommer* and Rami M. Messalem Hydronautics, Inc.,Laurel, M d . 20810
The factors governing the formation of voids and large cavities in a wide variety of membranes made from different polymers cast from different solvents and precipitated in various nonsolvents have been studied. It has been shown that the formation of large voids in membranes can b e eliminated by ( 1 ) lowering the tendency of the nonsolvent to penetrate into the casting solution or (2) increasing the viscosity of the cast solution or creating a thick gel layer on top of this cast solution. It is suggested that the formation of large finger-like cavities in membranes originates from convective flows formed within the cast (fluid) polymer solution upon its immersion in the bath of nonsolvent for final precipitation. It is also shown that the driving forces leading to the formation of these convective flows are not density gradients.
I
he application of membrane techniques to filtration, concentration, and separation processes is continuously increasing. An outstanding example is desalination by reverse osmosis. Severtheless, most of the procedures developed for manufacturing membranes have been formulated empirically, and the understanding of the factors governing membrane structure and characteristics is still poor. One of the most severe problems associated with the practical application of high-pressure filtration techniques is the elimination of membrane defects. The formation of voids and large cavities in many types of desalination and ultrafiltration membranes has been observed and reported by many investigators (Frommer and Lancet, 1972; King, et al., 1970; Gollan, 1970; Model and Lee, 1972; Rozelle, et al., 1971; Nata, 1972), but little is known on the mechanism of their formation. I n a recent article Frommer and Lancet (1972) suggested (among other things) the following points. 1. The formation of finger-like cavities in cellulose acetate (CAI)membranes is generally associated with a high rate of polymer precipitation. The rate of precipitation upon immersing the cast polymer film in the bath of nonsolvent (water) will increase with a n increase in (a) the rate of solvent flow out of the cast polymer solution and (b) the rate of water flow into it. 2. Manjikian-modified Leob-Sourirajan membranes (cast from a C-I-acetone-formamide solution) are apparently composed of two layers. The thickness of the “upper” and denser layer varies from -15 Fm for a 5-10 sec acetone-evaporated membrane to practically the total thickness of the membrane (>lo0 pm) for a 5 min “dried” one. The salt-selective portion of the membrane (the “skin”) is only part (the uppermost portion) of this upper layer and can be removed by a slight abrasion of the membrane surface. 3. The formation of membranes consisting of two abovementioned layers results from the existence of two layers in the cast CX solution prior to its iinmersion in water: a n upper,
* To whom coriespondence should be addressed at Hydronautics-Israel Ltd., S e s s Ziona, Israel. 328
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solidified gel supported by a lower layer of fluid solution. These two layers are formed in the CI-acetone-formamide solution due to a sol-gel transition induced by acetone evaporation. Finger-like cavities can grow only in the &id lower layer of the cast solution. Since the thickness of this fluid layer decreases sharply with a n increase in the acetoneevaporation period prior to leaching, the formation of cavities in tubular desalination membranes can be diminished by increasing this “drying” period. I n the following discussion we shall propose a general mechanism which suggests that the formation of large fingerlike cavities in membranes originates from convective flows formed within the cast fluid solution upon immersion in the bath of nonsolvent for final precipitation. The formation of these convective flows is associated with a high tendency of the nonsolvent to penetrate and mix with the polymer solvent (and they can therefore be described as “mixing currents”). We shall also show that with this mechanism we can explain the structures of a large variety of membranes made from different polymers (e.g., cellulose acetate, a fully aromatic polyamide, polyurethane, and others) cast from different solvents and precipitated in various nonsolvents. The term “convective flows” used in this article describes currents of low molecular weight fluids (which, in our case, are flowing in finger-like cavities formed within the film of the viscous polymer solution). The term “convective flo~vs” should not be confused with motion due to density gradients resulting from temperature changes. The possibility that the formation of convective flows in our system results from the existence of density gradients is actually specifically excluded later on. Experimental Section
Materials. Cellulose acetate (CA) was Eastman Type E-398-10. Polyurethane was a PTLIG (poly(tetramethy1ene ether glycol)), AID1 (4,4’-diphenylmethane diisocyanate)-based resin, manufactured (and kindly supplied) by Uniroyal and coded E-9. It was chosen for this study because polyurethanes
were shown Vieth, et al., 1968) to have promise as reverseosmosis membranes. The aromatic polyamide was a poly(m-phenyleneisophthalamide), prepared by a low-temperature solution polycondensation of m-phenylenediamine and isophthaloyl chloride (Richter and Hoehn, 1971; Preston and Dobinson, 1964; Preston, 1966; Preston, et al., 1967). Acetone, dioxane, acetic acid, tetrahydrofuran (THF), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and the lithium chloride (LiCl) were of analytical grade purity. Formamide, dimethylacetamide @MAC), and triethyl phosphate (TEP) were of practical grade. Membrane Preparation. All membranes were prepared by the “casting-leaching” method, namely, casting a thin (0.3-mm) film of the polymeric solution on a glass plate and, after allowing the solvent to evaporate for a predetermined period a t the desired pressure and temperature conditions, immersing it into a bath of nonsolvent (water, aqueous electrolyte solution, methanol, etc.) for final precipitation. Cellulose acetate membranes were cast from a binary polymer solution containing 30, 20, or 7% of dried CA and one of the following organic solvents: acetone, dioxane, acetic acid, TEP, DMF, or DMSO. (AI1 percentages and ratios given refer to weight per cent and weight ratios.) The films were precipitated immediately after their casting in a n ice-water bath. All CA membranes were annealed in a water bath a t 80’ for 30 min and stored in water. The ternary CA solution employed for observing the rate and the nature of precipitation of the polymer under the microscope consisted a CA: formamide :acetone ratio of 25: 30: 45. This Manjikian-modified LoebSourirajan solution (Sourirajan, 1970) is abbreviated in this paper as the Loeb solution. The polyurethane membranes were cast from 20% solutions of the resin in tetrahydrofuran (THF) or from a 1 :1 mixture of T H F and dimethylformamide (DMF). The cast films were allowed to evaporate a t ambient conditions for 5 min before immersion in an ice-water bath for precipitation. The fully aromatic polyamide membranes were cast either from the reaction solution in which they were prepared (solvent being 5% LiCl in DMAc) or from a DMAc solution of l0-20% of the precipitated and dried polymer. The cast aromatic polyamide films were precipitated in an ice-water bath or in methanol either immediately after their casting or after evaporating the solvent under vacuum a t elevated temperatures (up to -20 hr of evacuation in a vacuum oven a t -SO0). Optical microscope studies of the processes taking place rhiring polymer precipitation were performed, employing the technique developed by Matz (1970). I n this technique, a drop of the casting solution is placed on a microscope slide and covered with a aover slide to form a thin circular film. Drops of the nonsolvent (water, methanol, etc.) are placed in contact with the periphery of the cover slide. These drops are immediately drawn by capillary forces in between the microscopic slides to form a circular water-polymer solution i n t e r face. By utilizing dark-field microscopy, it i s possible to observe and follow the existence and formation of lightscattering zones such as interfaces between homogeneous phases and turbid areas. Scanning electron micrographs (SEM) were taken of cross sections of membranes (initially stored in water) which were allowed to dry a t ambient conditions after immersion in a 66:30:4 water-glycerol-Triton X-100 solution (a solution of isooetylphenoxypoly(etho.uyethanol), produced by Rohm and Haas Co.). Cross sections were obtained from fresh
Figure 1. Scanning electron micrographs of cross sections of membranes cost from 20% CA in six different binary solutions and precipitated immediately in on ice-water bath. The upper surface of the cross sections corresponds to the air-solution interface of the cost solution
edges prepared by fracturing the membranes a t liquid nitrogen temperatures. These membranes were mounted obliquely on the specimen mount (so that the cross section w&sexposed to the incident electron beam) and coated with gold. The axis about which the plane of the cross section was tilted relative to the incident electron beam runs horizontally (left to right) in all the micrographs. The scales indicated refer to sample magnification along that direction. The actual dimensions of the cross sections in the vertical direction are larger by I& 30% than indicated by the scale, depending on the tilting. I n the previous publication in this series (Frommer and Lancet, 1972) it has been shown that the thicknesses of the dried cross sections used for scanning electron microscopy are not representative of the thicknesses of the wet membranes: It has been shown that the differences in the thicknesses of the SEM cross sections result primarily from different extents of shrinkage which the wet membranes undergo during their drying and evacuation. Quantitative information about the extent of shrinkage of the various membrane samples has also been given in that article. Results and Discussion
Factors Responsible for the Formation of Large Voids in CA Membranes. Scanning electron micrographs (SEM) (taken from Frommer and Lancet, 1972) of cross sections of cellulose acetate membranes cast from 20% solutions in six differentsolventsareshown in Figure 1. Membranes cast from acetic acid or from TEP are transparent and their SEM cross sections appear uniform and homogeneous, while membranes cast from DMF, DMSO, or dioxane are white and opaque and contain cavities or voids as large as their thickness. Ind, Ens. Chem. Prod. Res. Develop., Vol. 12. No. 4, 1973 329
of CA membranes cast from DMSO or from TEP solutions and precipitated immediately in on ice-woter bath
Frommer and Lancet (1972) have shown that the formation of large finger-like cavities in CA membranes is associated with (1) a high rate of precipitation of the polymer from the cast polymer solution during its immersion in the bath of nonsolvent (water) and (2) a high tendency (affinity) of the pure solvent (with no polymer) and of the nonsolvent (water) to dissolve mutually in each other. The parameter describing the tendency of the water and of the solvent to dissolve mutually in each other is the free energy of mixing. There are, however, very few data concerning free energies of mixing. Another parameter which may be indicative of the tendency of mutual mixing is the heat of mixing. It has been shown (Frommer and Lancet, 1972) that the larger the heat of mixing of an organic solvent with water (increasing mixing tendency), the greater is the tendency for large-cavity formation. However, while the tendency of the pure liquid components to dissolve mutually in each other is an important factor in determining the structure of the membrane and especially in determining the probability of large voids in the membrane matrix, it is obvious that this is not the only factor. As can clearly be seen in Figure 2, one can obtain both uniform and highly nonuniform membranes when using the same polymersolvent-nonsolvent system. Thus, when the concentration of cellulose acetate in TEP as casting solvent is reduced from 20 to 795, the resulting membrane changes from transparent and highly uniform to opaque and highly nonuniform. Similarly, when the concentration of cellulose acetate in DMSO is increased from 20 to 30%, the large finger-like cavities in the matrix of the membrane are eliminated and the resulting membrane is transparent and uniform. The elimination of finger-like cavities from the matrix of membranes by increasing the concentration of the polymer in the casting solution suggests that cavity formation is dependent upon the fluidity of the casting solution. More fluid polymeric solutions are more likely to contain large 330 Ind. Eng.
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Figure 3. Photomicrographs showing the influence of the air exposure period on the precipitation of CA from a Loeb solution. f i s the period between formation of the thin circular drop and its contact with water. Pictures were taken 1 min after contact with water
voids in their structure when precipitated. The idea that finger formation is dependent on fluidity is strongly supported by the observations shown in Figure 3 (and taken from Frommer and Lancet, 1972) of the influence of the air exposure period on the precipitation of cellulose acetate from a modified Loeb solution. It can be seen that in the three situatiom where acetone was allowed to evaporate from the casting solution, two types of precipitated layer are formed, the “outer” one being transparent and uniform (and appearing dark in the dark-field microscopy pictures shown) and the “inner” one appearing turbid and containing finger-like cavities. It can also he noticed that the thickness of the “outer” transparent layer increases with an increase in the air exposure period prior to contact with water. It has been shown (Frommer and Lancet, 1972) that a slight decrease in the acetone content of the CA-formamide-acetone solution is associated with a considerable increase in viscosity and with a sol-gel transformation. An increase in the acetone evaporation period leads, therefore, to an increase in the thickness of the “outer” solidified gel enveloping the inner fluid solution. The pictures shown in Figure 3 strongly suggest that the formation of finger-like cavities takes place only in the (“inner”)fEuid solution. Membranes from Noncellulosic Polymers. The conclusions suggested in the previous section were found to be applicable, not only for regulating the structures of cellulose acetate membranes but also for controlling the structures of polyurethane membranes, fully aromatic polyamide membranes, and many other polymeric membranes cast from different solvents and precipitated in various nonsolvents. A few specific examples are given below. Scanning electron micrographs of cross sections of polyurethane membranes cast from 20% polymer in (a) T H F or (b) a 1:1 mixture of THF-DMF, exposed 5 min to ambient atmosphere, and then precipitated in an ic&water bath are
Figure 4. Scanning electron micrographs of cross sections of polyurethane membranes cast from polymerin (a) THF Or (b) THF-DMF (exposed min to ambient phere and then precipitated in an ice-water bath)
”%
shown in Figure 4. While the cross section of the membrane (a) does not contain large voids, that of the membrane (b) cast from the 1:1 THF-DMF mixture contains large fingerlike cavities similar to those found in various CA membranes and shown in Figures I and 2. The reasons for the differences in the structures of the Polyurethane membranes shown in Figure 4 can be explRjned along lines similar to those suggested previously for CA membranes. The boiling point of T H F is 65O, and it is, therefore, highly volatile a t room temperature. As a result, during the 5 min of exposure of the c a t 20% polyurethane solution in T H F to the ambient atmosphere, the concentration of polymer solution increases and i t becomes viscous and gellike. Since upon immersion of this film into the water bath, most of it was already gelled, the polyurethane precipitates uniformly throughout its thickness, and no finger-like intrusions are formed. However, upon substitution of part of the T H F with DMF, a less volatile solvent has been added to the casting solution (the boiling point of D M F is 153”). Therefore, the amount of solvent evaporating from the solution is considerably smaller, and the cast film is much more fluid than that cast from T H F a t the time of its immersion in the water bath. Furthermore, it has heen shown (Frommer and Lancet, 1972) that D M F bas a high tendency to mix with water and, as indicated in the previous section, such a tendency is associated with the formation of finger-like cavities in membranes. The same principles were found to he applicable for regulating the structures of membranes prepared from poly(mphenyleneisophthalamide). Thus, optical microscope precipitation experiments similar to those shown in Figure 3 indicate that the rate of penetration of the precipitation front
Figure 5. Scanning electron micrographs of cross sections of poly(m-phenyleneisophthalamide) membranes cast from the reaction solution in which this polymer was synthesized (DMAc contuining 5% LiCI) and precipitated in woter either immediately after casting (a) or after 1 hr of evacuation a t 80’ (b). The upper surface of the cross section corresponds to the air-solution interface of the cast solution
(associated with the formation of finger-like intrusions in the polymeric solution) from the periphery of a circular droplet of a solution of the aromatic polyamide in DMAc toward its center is considerably reduced when water is substituted with methanol or ethanol as the precipitating agent. This is in accordance with the fact that the tendency of DMAc to mix with water is greater than its tendency to mix with methanol or ethanol (Drinkard and Kivelson, 1958; Erva, 1955). Scanning electron micrographs of cross sections of poly(mphenyleneisophthalamide) membranes cast from the reaction solution in which the polymer was synthesized (DMAc containing 5% LiCl) are shown in Figure 5 . It can be seen that membranes precipitated in an ice-water bath immediately after casting contain many voids and large cavities (Figure 5a). However, when part of the DMAo was evaporated from the casting solution (by evacuation a t elevated temperatures) prior to its immersion in the bath of nonsolvent (water) for final precipitation, the extent of formation of large voids in the resulting membrane is substantially lowered (Figure 5h) until finally (evaporation in a vacnnm oven a t -80’ for -20 hr) the resulting film is dense, transparent, and uniform all along its thickness. It may also he noticed that in the “immediately leached” membrane (Figure 5a) the cavities “protrude” into the membrane starting from the “upper” (air solution) interface, whereas in the vacuum-evaporated membrane (Fignre 5b) air bubbles appear only in the lower part of the membrane, the region which could have been still fluid a t the time of immersion in the water bath. It is apparent from the above-mentioned examples that the mechanism accounting for the formation of large voids in membranes must be very general and applicable to many polymeric systems and to a variety of membrane preparation Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 4, 1973 331
Table 1. Proposed Actions Which M a y Be Employed for Eliminating the Formation of large Voids in Membranes Prepared by the “CastingLeaching” Method
(1) Lowering the tendency of the nonsolvent to penetrate into the cast solution a. Lowering the temperature of precipitation by nonsolvent b. Addition of salt to the nonsolvent, thus lowering its activity c. Substituting the nonsolvent with one showing lower tendency to mix with the solvent (e.g., substituting water with methanol) d. Substituting the solvent with one showing lower tendency to absorb “water” (e.g., substituting DMSO with acetic acid as solvent for CA) (2) Increasing the viscosity of the cast solution or creating a thick gel layer on top of it a. Lowering the temperature of casting and of precipitation b. Increasing the concentration of the polymer in the cast solution c. Increasing the period or the temperature of evaporation of the solvent before immersion in nonsolvent
procedures. Table I summarizes various specific actions which may be utilized for eliminating the formation of large voids in membranes. Mechanism of Formation of Large Voids in Membranes. Let us address ourselves now to the more fundamental questions, namely, why is the formation of large finger-like cavities in membranes associated with a high rate of precipitation and why is their formation dependent on the fluidity of the cast polymer solution? Investigations of mass-transfer phenomena through liquid interfaces suggest that, in many cases, it is associated with the formation of intense convective flows near the interface (Sternling and Scriven, 1959; Pearson, 1958; Sherwood and Wei, 1957; Olander and Reddy, 1964; Ruckenstein and Berbente, 1964; Berg and Morig, 1969). Several of the requirements which were listed by Sternling and Scriven (1959) as necessary for promoting “interfacial turbulence” match exactly with the description of the membrane precipitation process. Thus, is has been suggested (Sternling and Scriven, 1959) that “interfacial turbulence is usually promoted by (1) solute transfer out of the phase of higher viscosity, (2) solute transfer out of the phase in which its diffusivity is lower, (3) large differences in kinematic viscosity and solute diffusivity between the two phases, and (4) steep concentration gradients near the interface.” Visual observations of the precipitation profiles of the polymer from the casting solution (by the dark-field microscopy technique discussed in detail by Frommer and Lancet, 1972) suggest that convective flows of the mixing low molecular JTeight liquid components are taking place within the growing “finger-like” patterns. Very convincing evidence for the argument that bulk liquid flows and not diffusional phenomena are responsible for the formation of large voids in membranes has been provided by R. Llatz in his film entitled “The Formation of Aqueous Occlusion Cells during the Gelation of Cellulose -1cetate Nembranes” which has been shown at the 3rd International Symposium on Fresh Water from the Sea. It seems to us, therefore, that the formation of large finger-like cavities 332
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is associat.ed with a high rate of precipitation because both phenomena may result from one common reason, namely, from convective flows induced in the cast polymer solution upon its immersion in the bath of iionsolvent for final precipit’ation. The formation of these convective f i o ~ sis associated with a high tendency of the low molecular weight solvent and the nonsolvent to mix with each other. If the tendency of mixing of the two liquid components is lowered by the various means summarized earlier (see Table I), then both the rate of precipitation and the extent of cavity formation will be lowered. If the viscosity of the cast polymer solution is increased, the viscous resistance to the formation of these convective flows will increase and the extent of their formation will be lowered. If a thick gel layer is created above the fluid polymer solution by partial evaporation of the solvent, then the rate of diffusion of water or any other nonsolvent through this layer may not be large enough to assist the formation of convection cells within the fluid layer and therefore the resulting membrane may be uniform. Having proposed convective flows as the cause for the formation of large voids within membranes, the question arises as to the origin of such flows. Driving forces ivhicli were shown to account for the initiation and formation of convective flows near interfaces are surface tension gradients and/or deiisit,y gradients (Sternling and Scriven, 1959; Pearson, 1958; Sherwood and Wei, 1957; Olander and Redd>-,1964; Ruckenstein and Berbente, 1964; Berg and Morig, 1969). The possibility that density gradients are the driving force behind the formation of these convective flows has been investigated by us, and experiments were performed in which the glass plate on which the membrane has been cast was immersed in different ways in the water bath for final precipitation. The fact that the structures of membranes are practically unaffected by the way the polymer film (cast’on a glass pla.te) is immersed in the bath of nonsolvent (Le., polymer layer being either above or below the glass plate) and finger-like cavities are formed irrespective of t,he way the water penetrates into the cast solution, either downward or upward, suggests that the forces leading to the formation of convective flom are riot density gradients. One is therefore left with surface tension gradients as possible driving forces for initiating convective flows near the surface. Once initiated, these f l o ~ smay be enhanced or damped by other factors affected by these flows such as localized changes in concent,ration,temperature, viscosity, etc. Recently, Matz (1972) has suggested that the convective flows responsible for large-cavity formation in membranes are initiated by irregularities induced on the surface of the cast solution by the casting knife, aiid t,hus by allom-ing the cast layer to “relax” some t,ime before being immersed in the leaching bath, these irregularities can be damped down. thereby eliminating finger formation. This suggestion has been proved to be incorrect by bhe fact that finger-like cavities did form in CA membranes cast from 20% polymer in DMSO solution (see Figure 1) irrespective of the time elapsed between casting and precipitation. It may also be noted in Figure 4 that large cavities did form in a polyurethane membrane which was allowed to “relax” 5 min before its immersion in the ice-water bath. As mentioned earlier, surface tension gradients along the water-solution interface may be responsible for the initiation of convective flows within the polymer film but as yet there is not positive proof to this possibility. A quantit,ative theory of this phenomenon must take into account the details of the membrane precipitation process. This process involves (1) large viscosity changes, ( 2 ) dissolu-
tion reactions among a t least three, and in many cases more, chemical components (polymer, solvent, nonsolvent) and the resulting temperature and specific volume changes associated with these reactions, and (3) phase separation and continuous changes in the location and the shape of the interfaces between the various phases. d complex system like this has never been quantitatively or even semiquantitatively studied. In all the quantitative or semiquantitative studies on convective flows associated with mass transfer through a n interface (see, e.g., Sternling and Scriven, 1959; Pearson, 1958; Sherwood and Wei, 1957; Olander and Reddy, 1964; Ruckenstein and Berbente, 1964; Berg and Morig, 1969; and references listed in these articles), the location of the interface was well defined, no phase separation was involved, and changes in various physical properties (e.g., temperature, density, etc.) were carefully controlled. I n spite of the complexity of the process of membrane formation, it seems t o us that a n extensive, more quantitative study of the parameters governing the structures of membranes is highly necessary and may contribute considerably to better mastering of the “art” of membrane manufacturing.
Literature Cited
Berg, J. C., Morig, C. R., Chem. Eng. Sei., 24,937 (1969). Drinkard, W., Kivelson, D., J . Phys. Chem., 62,1494 (1968). Erva, J., Suom. KemistilehtzB, 28, 131 (1955). Frommer, 11.A., Lancet, D., in “Reverse Osmosis Membrane Research,” pp 85-110, H. Lonsdale and 11. E. Podall, Ed., Plenum Press, New York, E.Y., 1972; also in U . S . Of.Salzne Water, Res. Develop. Progr. Rep., No. 774 (1972). Gollan, A., Hydronautics, Inc., Quarterly Progress Report 7007-3, to the Office of Saline Water, July 1970. King, W. M., Cantor, P. A,, Schoellenbach, L. W., Cannon, C. R., Appl. Polym. Symp., 13,17 (1970). Matz, R., Hydronautics, Inc., Quarterly Progress Report 7003-4, t o the Office of Saline Water, April 1970. Matz, R., C . S . Of.Salzne Water, Res. Develop. Progr. Rep., No. 774 (1972). Modelj F. S . , Lee, L. A., Amer. Chenz. SOC.,Dzv. Org. Coatings Plast. Chem., Pap., 32,387 (1972). Olander, D. R., Reddy, L. B., Chem. Eng.Sci., 19,67 (1964). Pearson, J. R. A,, J . Fluid dfech.,4,489 (1958). Preston, J., Dobinson, F., J . Polym. Sci., Part B, 2, 1171 (1964). Preston, J., J . Polym. Scz., F a r t A - I , 4,529 (1966). Preston, J., Smith, R. W., Stehmon, C. J., J . Polym. Sei., Part C, No. 19,7 (1967). Richter, J. W., Hoehn, H. H., U. S. Patent 3,567,632 (March ,n-, \
I Y I I).
Rozelle, L. T., Cadotte, J. E., Senechal, A. J., Xelson, B. R., Polym. Prepr., Amer. Chem. Soe., Diu. Polym. Chem., 12, 261 ( 1 971 ). \ - -
- I
Acknowledgment
Ruckenstein, E., Berbente, C., Chem. Eng. Sci., 19,329 (1964). Sherwood, T. K., Wei, J. C., Ind. Eng. Chem., 49, 1030
This work was carried out at the Institute for Materials Research of the n’ational Bureau of Standards as part of the Research Associate Program of the NBS. The authors acknowledge gratefully the cooperation and many helpful discussions with R. R. Stromberg, F. Khoury, L. Smith, B. Morrissey, J. Flynn, and other members of the Polymer Division of the Xational Bureau of Standards. The stimulating discussions and valuable comments of S. Reed from Hydronautics, Inc., and J. Berg from the University of Washington are also gratefully acknowledged.
Sourirajan, S., “Reverse Osmosis,” pp 58-63, 112-116, Logos Press, London, 1970. Sternling, C. V., Scriven, L. E., AIChE J . , 5,514 (1959). Vieth, W., Douglas, A. S., Bloch, R., C . S . 08. Saline Water, Res. Develop. Progr. Rep., No. 352 (1968). RECEIVED for review January 18, 1973 ACCLPTLD September 4, 1973 Work supported by Research Grant So. 14-30-2529 from the Office of Saline Water, U. S. Department of the Interior. Parts of this work have been presented at the 1 6 k d National Meeting of the American Chemical Society, Washington, I).C., Sept 1971, and at the 3rd Office of Saline Water Reverse Osmosis Conference, Las Vegas, Nev., May 1972.
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