branes. Other types of cellulose acetate membranes, made by different methods and having different surface and permeability characteristics, may be expected to behave similarly, b u t not identically. A study of the separation and permeability characteristics of different types of membranes using different solution systems may make a significant contribution to the understanding of the controlling parameters of this membrane separation process and the development of quality control techniques in the process of casting superior membranes for a wide variety of industrial applications.
Acknowledgment
T h e author is grateful to I. E. Puddington and G. L. Osberg for their encouragement of this project; to L V . S. Peterson, A. E. McIlhinney, and Teil H. Scheel for their help in building the equipment; and to Lucien Pageau and A. G. Baxter for their valuable assistance in the progress of these investigations.
literature Cited (1) Ambard, L., Trautmann, S., “Ultrafiltration,” p. 23, Charles
C Thomas, Springfield, Ill., 1960. (2) Debye, P., Cleland, R. J., J . A p p l . Phys. 30, 843 (1959). (3) Levich, V. G., “Physicochemical Hydrodynamics,” p. 390, Prentice-Hall, Englewood Cliffs, N. J., 1962. (4) Loeb, S., Sourirajan, S., Advan. Chem. Ser., No. 38, 117 (1963). (5) Loeb, S., Sourirajan, S., “Sea Water Demineralization by Means of a Semipermeable Membrane,” p. 22, Department of Engineering, University of California, Los Angeles, Rept. 60-60 (1961). (6) Riley, R., Gardner, J. O., Merten, U., Science 143, 801 (1964). (7) Sourirajan, S., IKD.ENG.CHEM.FUNDAMENTALS 2, 51 (1963). (8) Ibtd., 3, 206 (1964). (9) Sourirajan, S., J . .4ppI. Chem. (London) 14, 506 (1964). (10) Sourirajan, S., Nature 203, No. 4952, 1348 (1964). (11) Yuster, S. T., Sourirajan, S., Bernstein, K., “Sea Water Demineralization bv the Surface-Skimming Process.” Deuartment of Engineeridg, University of Califgrnia, Los Angeles, Rept. 58-26 (1958). RECEIVED for review January 18, 1965 ACCEPTED June 7, 1965 Issued as N. R. C. No. 8609.
EVAPORATION SUPPRESSION B Y MONO= LAYERS ON AQUEOUS SALINE SOLUTIONS FOREST A. CHEEVES,’ RUSSELL G. DRESSLER, AND W I L L I A M C. M c G A V O C K Chemistry Dppartment, Trinity L’niiersity, S a n Antonio, Tex.
Water evaporation rate tests, with and without monomolecular fatty alcohol film covering, were conducted using laboratory-scale, precisely developed equipment. The results of tests on NaCI, CaC12, AIC13, and Alz(SO4)a aqueous solutions of 0 to 50 weight strength showed almost identical behavior, showing an evaporation maximum, with film coverage, at about 3.5y0concentration. Evaporation savings are sufficient to justify application of monomolecular films to inland brackish or saline water bodies as a means of water conservation. Water bodies of 5 to 20% salinity have potential of about 20% saving of normal evaporation loss, compared with 20% to 50% potential for water bodies of 0 to 5% salinity.
7 0
interest in water conservation has prompted many of the evaporation-suppressant possibilities of monomolecular films of hydrophylic substances on surfaces of water. Workers have investigated the action of monomolecular films on fresh water in the laboratory a n d on outdoor reservoirs and have observed characteristics of optimum film compositions, film pressures, \vind and other meteorological effects, etc. ( 7 , 3, 5, 8,7 7 , 72, 7 4 75). Although many such investigations have been made on fresh waters, the authors could find no references to study of possible effects on brackish or saline \raters. A practical process for treating large reservoirs of fresh water Recently, certain has been described in detail (4-6). questions have been asked. Would the presence of the ions from dissolved salts interfere with the formation of a continuous monomolecular film on a \rater surface? Can brackish or saline water bodies be treated effectively by methods proposed for fresh water reservoirs? T h e present investigation was designed to answer these questions. A mixed film formed from a n octadecanol-hexadecanol mixture was selected for use: since its evaporation-suppressant properties had long been established ( 7 , 3, 5, 8). I t was a commercial product consisting of 80% octadecanol (C18), 8% hexadecanol (Cle), 8% eicosanol ( C ~ O2% ) , tetradecanol URRENT
C studies
Present address, Chemistry Department, University of Florida, Gainesville, Fla. 206
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
(C14),and 2% docosanol ( ( 2 2 2 ) . This particular mixture has been used as a comparison standard over past years by the authors and others, and normally exhibits a 50 to 527’ water evaporation saving when used o n fresh water in laboratorycontrolled tests. Experimental
T h e tests were conducted in battery jars 9 inches in diameter, representing a 0.158-sq. meter circular surface area. Four such jars, partially submerged in a constant-temperature water bath, were operated, side by side, which allowed duplicate tests and duplicate controls to be run simultaneously. T h e
Table 1.
Data and Results of Typical laboratory Test
(Distilled water, pH 7 ) N o Film W i t h Film Water Water Hook gage loss, Hook gage loss, reading, inches reading, inches Hour X Hour inches X IOF3 inches n 1.847 0 0 1.846 0 14 1 1 839 7 1 1 833 15 2 1 833 6 2 1 818 3 1 804 14 3 1 826 7 14 4 1 818 8 4 1 790 57 28 Totals M’ater savings (calculated on basis of hook gage readings), 52y0
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procedure was identical ivith that developed by Dressler (8). A micro hook gage !vas mounted above the water surface, permitting measurements of the level to 1 0 . 0 0 1 inch. Figure 1 gives a diagram of the apparatus, and Figure 2 a detail of the hook gage and method of mounting. A monomolecular film was formed on the water surfaces by the addition of 10 mg. of finely divided, dry octadecanol mixture to each 0.158-sq. meter surface area. This represented a maximum “film pressure” (equilibrium surface pressure), since more than twice the amount of material to cover the surface adequately was used. All evaporation tests were conducted under controlled conditions to eliminate all variables except the salt concentration, and for convenience \\’ere conducted a t an average water surface temperature of 30’ C. A constant flow of air: predried to less than 5Tc relative humidity, a t controlled temperatures of 30’ i 0.5’C . was diffused over the surface area of water in the test jar. This air rate was approximately the equivalent of a one-mile-perhour ivind.
slight precipitation a t 3 7 , concentration or above, but this was not believed to interfere with the validity of the results. T h e reproducibility of results is apparent on comparing hour by hour readings of water evaporation losses sho\\n in Table I. Although the hourly results show a degree of constancy, a greater degree of constancy is obtained by using figures representing at least 4 hours of test. as shown by Jones ( 8 ) . I n Table I , the water savings are calculated from the differential of micro (hook) gage readings of the ”no film” and ‘ w i t h This represents film” tests and average approximately 527, the maximum water evaporation savings that could be expected with this fatty alcohol composition on outdoor reservoirs a t 30’ water temperature.
Of the other methods of application considered, the most interesting involved the use of a n organic solvent to apply the film material. This method was eliminated in vie\\, of La hler and Robbins’ conclusions concerning interfcrences of the solvent ( 7 I ) . Although Dresslrr’s suspension process had been shoivn to be the most effective for applications to large outdoor bodies of water (3..5). it was not considered as convenient for laboratory applications as the use of a weighed dry, poxvdered fatty alcohol. T h e aqueous soliltions tested contained NaC1, CaC12, AlC13, and A1,(S04)B, a series presenting increasing values of the van‘t Hoff factor. I n addition, a synthetic solution approximating sea water in composition was tested. T h e AlCls salts gave a
Figures 3, 4, 5, and 6 sho\v the results using 0 to 207, solutions of NaC1. CaCl:, 41C13. and A12(S04)3. T h e four curves are so nearly alike that they may be superimposed one over the others. T h e synthetic sea \ v a t u sample gave results duplicating those of the NaC1 solution a t the 3.jYGpoint. I n each case the curves Xvithout film sho\v a decreasing evaporation \\-ith salt concentration. Interference of the dissolved salt ions on evaporation appears to increase progressively: retarding the effectiveness of the film between 0 and the maximum point of about 3.57, salt concentration. Above 3.57,%this interference remains fairly constant, and beti\.een 3.5 and 207, the film curve parallels the nonfilm curve in a do\vn\vard trend of evaporation rate. This is
Results and Discussion
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CONCENTRATION (%by weight)
Evaporation rate vs. calcium chloride concentraNo pH control
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SEPTEMBER
1965
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Evaporation rate vs. aluminum chloride concen-
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Evaporation rate vs. aluminum sulfate concentraN o pH control
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assumed to be due to the lesser vapor pressures of more concentrated solutions. T h e various data and plots d o not fully explain the initial increase. Evaporation tests Ivith p H control compared with the non-pH-controlled tests furnished no information. This obviously rules out any anion exchange with the hydrophylic end of the octadecanol to disrupt its water evaporation-suppressant properties. Such a n anion exchange with monomolecular films of octadecanoic acid (stearic acid) as contrasted with octadecanol (stearyl alcohol) on water was reported by \+‘heat, Satore. and Pauley (77). T h e pH of both water and the salt solutions underwent very little change during evaporation ivhen a n octadecanol film covering was present. O n one occasion, evaporation tests were conducted for 8 hours without a p H change and some 24 hours later, the solution had become more acid by about 0.5 to 0.7 p H unit within a 4-hour evaporation test. Surface tension measurements provided no information related to the initial positive slope of the rate-concentration curve. I n fact, the surface tension of the salt solutions both with and without a n octadecanol monomolecular film covering increased in going from zero concentration to the highest concentration tested. T h e occurrence of the maximum in the curves with film in Figures 3. 4, 5, and 6 a t the same \\.eight per cent of salts is not explained. Plots of the rate us. log p [ p being the Lewis and Randall (9) function, termed ionic strength a n d designed to eliminate variations in mass and charge] revealed curves of the same general type but Lvith maxima a t steadily increasing values of p . Not the ionic strength but mass seems to be the controlling factor. Figures 7 and 8 give typical plots for NaCl and A12(SOJ3. I t may be concluded that the dissolved salts in solution interfere ivith the effectiveness of the monomolecular film in retarding \vater evaporation to a n extent proportioned to the formula weight. According to hfcBain (70) and Harkins (7), the molecules of fatty alcohol type compounds orient themselves in one direction and vertically in the interface betlveen lvater and air. O n e might suppose that the ions physically interfere ivith the true orientation-i.e., canting of the vertically aligned molecules-of the monolayer molecules and thus leave openings through which water molecules may escape. Ries suggested a model for film collapse under maximum surface pressure conditions ( 7 3 ) . Using electron micrographs and film balance studies, he postulated that monolayer films, 208
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PRODUCT RESEARCH A N D DEVELOPMENT
0 0
Without octadecanol film With octadecanol film
when under maximum compression, collapsed in four stages: Some of the film material molecules are forced out of the Ivater surface by the compression; as the compression is increased, a tightly packed layer, two molecules thick, rises from the surface; this layer of film material bends and breaks from the material on the surface; and the collapsed fragment rests on the surface of the other film material. Under the conditions of maximum film pressure, the presence of the ions in the substrate solution may someholv aid this proccss, as postulated. If so, the holes so produced would encourage the energized water molecule to escape into the atmosphere rather than attempt the tortuous climb through the closely packed molecules of the octadecanol film. 7-hese film collapses may not occur in layers. Since the film pressure is equal on all sides, it Ivould appear that when the initial molecules are forced out of the water, they tend to form a conetype rupture. If so, such a cone could form a volcanic peak structure, thus allowing many lvater molecules to escape in its finite lifetime. These possibilities are contingent upon the influence of the ions in rapidly increasing their occurrence as the concentration increases until a maximum is reached. From a practical viewpoint, the evaporation savings are sufficient to justify the application of monomolecular films to inland brackish or saline water bodies as a means of \\ater conservation. Water bodies of 0 to 57, salinity with monomolecular film coverage show a potential of about 20Yc to 50y0 saving of the normal loss by evaporation. Thus potable waters (up to 1000 p , p . m , salinity or 0.1 weight %) exhibit approximately the same evaporation rates with film coverage as do fresh waters and potentially can be treated to reduce evaporation by 50y0. Water bodies of from 5 to 207, salinity have a potential of about 20% saving of the normal loss by evaporation. Tests with Synthetic Sea Water
Evaporation tests conducted using a synthetic sea water, with and without film, produced results comparable with those for about the same concentration (3.5% by weight) of sodium chloride. Further ivork with a solution approximating the composition of sea water showed that in the presence of a n octadecanol film the rate corresponded to the maximum of the curve of Flgure 3. Conversely, the maximum read from Figure 3 corresponds to a salt concentration of approximately 0.5936 equivalent per liter (3.5% by weight), the value cited for the concentration of sea water (76). T h e near coincidence between the maximum in the curve of Figure 3 and the average
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Evaporation rate vs. log p for sodium chloride
concentration of sea \vater is noted. T h e inference would require that the presence of monomolecular films govern the salt concentration of the ocean and tend to stabilize it near 3.5y6. S o positive information on the possible presence of such films over \vide areas of the ocean is available, although recently such a film from biological sources has been suggested. Neither is it possible 10 state ivhether the salt content of the ocean? has been constant over lengthy periods: since the first general analyses ivere made by Dittmar a n d his collaborators of the H..Zi.S. Chnilenger in 1884 (2). Literature Cited (1) Archer. 11. J., La Mer, V. K., J . Phys. Chem. 59, 200 (1955). (2) Dittinar. \V.. “Report on Researches into the Composition of Sea \Vater, Collected by H..ZI..S. Chnlleng~r,” Challenger Reports, Physics and Chem., Vol. I, pp. 1-251, 1884. ( 3 ) Dressler. I
