A STUDY O F THE ADSORPTION AT T H E BENZENE-SODIUM OLEATE SOLUTION INTERFACE KEITH M. SEYMOUR,l H. V. TARTAR, AND KENNETH A. WRIGHT Department of Chemistry, University of Washington, Seattle, Washington Received September 6, 1983
Since Langmuir (11, 12) and Harkins (4) first suggested that an oriented monomolecular layer of adsorbed material existed at the interface of emulsions, considerable experimental work has been done to test the validity of their suggestion. The results obtained are conflicting; some shorn adsorption equivalent to a monomolecular film, while others give evidence of films of greater thickness. Griffin ( 2 ) attempted to confirm the theory with a study of emulsions of kerosene with solutions of sodium oleate and stearate. He found approximately a monomolecular layer. Van der Meulen and Riemann (17, 18) reported similar results with toluene emulsified with solutions of sodium ricinoleate and sodium oleate. In the latter case phenol was added to the toluene used in preparing the emulsions. Undoubtedly phenol or sodium phenolate was adsorbed a t the interface in addition to the sodium oleate. Since the interfacial areas were calculated statistically by these workers from relatively small amounts of material the results are somewhat uncertain. Studies by McBain and his coworkers (13, 14) on the adsorption of such substances as p-toluidine, amyl alcohol, nonylic acid, phenol, etc., a t the interface between the aqueous solution and nitrogen gas have repeatedly shown the formation of films more than one molecule thick. With sodium oleate solutions and nitrogen bubbles, Laing, McBain, and Harrison (10) found sufficient adsorption for a dimolecular layer, Nonaka (20), using a Donnan pipet, studied the adsorption of sodium oleate and palmitate on droplets of toluene and found greater than monomolecular adsorption. Harkins and Beeman (3) made measurements on emulsions containing a much greater number of droplets than those of Griffin and of van der NIeulen and Riemann. Using emulsions of various oleates and octane they failed to find adsorption more than sufficient to form monomolecular films. Harkins and Fischer ( 5 ) , in examining sodium oleate-paraffin oil emulsions, measured the size of 1000 to 3000 droplets in each emulsion 1
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studied and computed from these results the distribution of size and the total interfacisl area involved. The adsorption found was only sufficient to form a monomolecular film. All the studies, except those of Nonaka, on the liquid-liquid interface in emulsions of oils with dilute soap solutions have been made by preparing an emulsim in the usual manner and then attempting to estimate the total interfacial area by statistical studies of the size distribution of the droplets. The distribution of the fatty acid between the two phases was permitted to take place during and after emulsification. The measurements on such systems involve approximations which necessitate considerable experimental error. The writers surmise, too, that with the close packing of the oil droplets in such an emulsion, some of the adsorbed material might be squeezed out into the excess of dispersion medium. There is much need for work that is more quantitative and affords more uniform conditions a t the interface. The work presented in this paper is a study of the adsorption of sodium oleate at a water-benzene interface, using droplets of benzene of uniform size. The experimental set-up was similar to that used by McBain and his coworkers (10,13,14) in their study of adsorption at a nitrogen gas-aqueous solution interface. This type of apparatus has not been used before with systems having liquid-liquid interfaces. The number and size of the droplets were known, which permitted a quite accurate estimate of the area of the interface. Benzene and sodium oleate solutions were chosen as the two phases, because this system is easily reproducible and has been widely used, thus permitting a greater comparison with the work of other investigators. The experiments differ further from previous work in that the two phases were brought to equilibrium prior to making the adsorption measurements in all but one experiment. In such a system the fatty acid formed by the hydrolysis of‘ the sodium oleate distributes itself between the two phases. EXPERIMEXTAL
Materials The benzene used was of “c. P. analyzed” quality. It was redistilled and only the middle portion boiling over a one-degree range was used. Eimer and Amend’s “c. P. linolic acid-free” oleic acid, also used by Harkins and Fischer ( 5 ) , was found to be a very pure product. This acid was used throughout and proved to be very uniform in quality. The soap was prepared in two ways. That used in the first two experiments was prepared from sodium ethylate and oleic acid in the method developed by Harkins and Beemsn ( 3 ) . Solutions prepared from this soap tended to become turbid, probably owing to the presencebf acid soap. In
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order to avoid this difficulty and any possible contamination from carbonate, the soap solution used in the subsequent experiments was made directly in the 5-liter flask in which the two phases were later allowed to come to equilibrium. The carbon dioxide-free base used in preparing the solution was made (7) by dissolving stick sodium hydroxide, “c. P. from alcohol,” in distilled water to form a saturated solution. This was centrifuged and the supernatant liquid used to prepare a concentrated stock solution. It was stored in a paraffined bottle and standardized for later use. In preparing the soap solution the required amount of the stock sodium hydroxide solution was carefully pipetted into the 5-liter flask. Oleic acid in slightly less amount than that equivalent to the base was then added to the solution and the mixture heated under a reflux condenser until saponification was complete. After cooling, the soap solution was diluted with freshly boiled distilled water until the solution weighed approximately 2250 g. A solution of a weighed quantity of oleic acid in approximately 2500 cc. of benzene was next prepared. The amount of oleic acid was such that if it were added to an equal volume of the soap solution the concentration of the excess sodium hydroxide would still be a t least 0.001 N . After the foam which formed on the soap solution during the dilution had disappeared, the benzene solution was added. By running the benzene in with extreme care, the emulsion formed a t the interface occupied only a few per cent of the total surface. The weight of the benzene solution added was such that the two phases had an equal volume. The flask was then placed in a thermostat a t 25°C. and the two phases allowed to come to equilibrium. Placing a portion of the oleic acid in the benzene phase greatly shortened the time required to reach equilibrium, since the upward diffusion, into the benzene, of the oleic acid formed by hydrolysis of neutral sodium oleate is very slow. The downward diffmion was hastened by cautiously rotating a bent stirring rod in one phase and then in the other. Preliminary tests showed that equilibrium was attained in ten days to two weeks as a maximum. This was determined by removing, at intervals, a sample of the benzene phase and determining the concentration of oleic acid. When no further decrease in the acid content could be detected after five days standing, it was assumed that the solutions were a t equilibrium. To provide for rapid attainment of equilibrium in the first two experiments, in which pure solid soap was used to prepare the solutions, a small quantity of oleic acid was added to the benzene and slightly more than an equivalent amount of carbon dioxide-free sodium hydroxide was added to the soap solution. Although the alkali added in preparing the soap solution used in experi-
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ment No. 1 was in excess of the oleic acid added to the benzene phase, the solution never became clear. The solid soap must have contained some free fatty acid or, more probably, some acid soap. Undoubtedly this solution, which showed considerable turbidity, contained a larger amount of colloidal material than would have been found in a clear solution of this dilution. When it was desired to use the solutions, they were removed from the flask by siphoning into separate bottles. The soap phase was placed in a Pyrex bottle. Care was taken a t all times to avoid contamination by carbon dioxide. The bottles were stored in the thermostat and both phases were analyzed as described below. As a check on the other experiments, experiment KO.7 was made with a soap solution which was not at equilibrium with the benzene phase. The soap solution was prepared to duplicate the materials used in experiment No. 6 as closely as possible. That is, since pure benzene was used, the soap solution was made not only so that it contained very nearly the ,same normality of sodium as the solution used in experiment KO.6, but also so that it had approximately the oleic acid content which would have resulted if all the oleic acid in a given volume of the benzene used in N o . 6 were transferred to an equal volume of the soap solution for No. 6.
Analytical Owing to the large size of the benzene droplets used in the experiments and the resulting lorn specific interfacial area (total area in square centimeters / total volume in cubic centimeters of the benzene), the amount of material adsorbsd, if a monomolecular layer were formed, was too small to obtain sufficiently accurate results by the volumetric determination of the oleic acid. It was thought that the sodium could be determined with sufficient accuracy by titration. The solutions used in the first two experiments were analyzed in this manner, but satisfactory results were difficult to obtain and in the later experiments the analyses were made entirely by gravimetric methods as described below. Analysis of the equilibrium benzene solution. Approximately 10 cc. of the benzene solution was weighed into a tared weighing bottle. The benzene was then evaporated under reduced pressure in an oven at 50-55°C. After cooling, the oleic acid was weighed on a Kuhlmann microbalance. Weights were rounded off to five decimal places in grams. Two blanks were run with each set of analyses. A quantity of pure oleic acid approximately equal to that in the sample being analyzed was weighed into tared bottles and 10 cc. of pure benzene were added. The evaporation of the benzene, subsequent cooling, and weighing of the blanks and samples were carried out simultaneously. The weighed samples of oleic acid, used as blanks, usually showed an increase of a few hundredths of a milligram,
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probably due to a slight oxidation of the acid while in the oven, although little air came into contact with the bottles during evaporation, since the benzene vapor was carried off by the suction. This increase in weight was used as a correction in the determination of the oleic acid in the sample. From the weight of the sample and the weight of the oleic acid, the concentration in milligrams of oleic per gram of benzene was calculated. The weighings in all analytical work reported in this paper were corrected to weight in vacuo. Checks on carefully prepared standard solutions of oleic acid in benzene showed that the estimation of the acid by microgravimetric means was far more accurate than by titration with standard aqueous or alcoholic sodium hydroxide. In fact, considerable time was lost in unsuccessful attempts to titrate the oleic acid according to the methods used by previous investigators. A n a l y s i s of equilibrium soap solutions. For this analysis special tubes of Pyrex glass were prepared by sealing a piece of tubing 1 cm. in diameter and 8 cm. in length to a test tube 2 cm. in diameter and 22 cm. in length. A sample of the equilibrium soap solution mas placed in the tared tube and weighed. An excess of dilute sulfuric acid solution was then added, weighed, and finally 10 to 12 cc. of pure benzene saturated with water was added and weighed. The tube was sealed off and placed in a horizontal position in a slow rocker. After several hours the oleic acid was completely extracted by the benzene, both phases becoming clear. The neck of the tube was next cut off and a sample of the benzene solution was pipetted into a tared bottle and weighed. The benzene was evaporated as described above and the oleic acid weighed on the microbalance. Weighing bottles containing the proper weight of oleic acid were used as blanks. This procedure afforded the necessary data for the calculation of the concentration of oleic acid per gram of the soap solution. This method was checked by analysis of a prepared standard soap solution 0.01522 N , (weight normality), with the following results. Theoretical: 4.298 mg. oleic acid per gram of soap solution. Found. 4.295 mg. per gram. In the first two experiments standard acid was added to the soap in the extraction tubes and the sodium was determined by titrating the excess acid with standard alkali. Difficulties in obtaining check results led to the use of the following method. A sample of the aqueous phase remaining in the extraction tube after the removal of the benzene was weighed into a tared platinum crucible. The water was evaporated in the oven by the same method as descrited for the benzene. The sodium sulfate was ignited in a muffle to constant weight and weighed to five decimal places in grams on the Kuhlmann balance. A p p a r a t u s and procedure .for carrying out a n experiment. The apparatus
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K. M. SEYMOUR, H. V . TARTAR AND
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was a modified form of the one used by Laing, McBain, and Harrison (10) and is shown in figure 1. A and B are reservoirs for the equilibrium benzene solution. This arrangement was used in order to maintain a constant head of solution in B. Tube M connected bulb B to a gas washing bottle used to saturate the incoming air with benzene vapor. Stopcock C permitted regulation of the flow of benzene solution through the tip E. This tip was made by a careful thickening of a length of small Pyrex tubing and drawing it out to the proper diameter. The orifice was circular and the end was ground smooth. Preliminary work showed that the tip should be of a size to produce a benzene droplet approximately 2 mm. in diameter.
FIQ.1. ADSORPTION APPARATUS
With larger spheres the volume of benzene needed t o form a sufficient number of droplets would be too great. Smaller droplets were very difficult to count accurately when the benzene flow was rapid enough to complete the run in a reasonable time. Benzene droplets of this size did not appear to be deformed in any way during the passage through the adsorption tube and over the bend I, and in our calculations we have considered them to be perfect spheres. L is the reservoir for the equilibrium soap solution. The tube H, through which the soap solution entered the adsorption tube G, projects about two-thirds of the way across the tube G, since preliminary work showed this was necessary in order to prevent the solution simply running
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down the bottom wall of G and out the tube F without appreciably mixing with the solution in the tube. Tube G was approximately 100 cm. long, with an inside bore of 1 cm., and a volume of about 75 CC. This tube made an angle of about 10" with the horizontal except for the last three centimeters before the bend at I. Here the slope was slightly steeper, and the tube was narrowed down until a t the top of the bend the diameter was between 2 and 2.5 mm. The change in slope was small, as i t was found that when the tube was steeper than 15"to 20"the benzene droplets jammed together and lost their spherical shape. The slight change in slope at the end of the tube served to separate any groups that formed as the droplets travelled up the tube. The actual time each droplet was in the tube varied in the different runs from seventy seconds to nearly two minutes. This was much longer than the time the nitrogen bubbles used by Laing, NIcBain, and Harrison (10) and by Harkins and Gans (6) remained in the adsorption tube. The collection flasks used were made by sealing a 100-cc. round bottom flask to the bottom of a similar 300-cc. flask (N, figure 1). Then the neck of the large flask was drawn out and a piece of tubing a few centimeters long and 1 cm. in diameter was sealed to it. Before each experiment the entire apparatus was carefully cleaned with potassium dichromate-sulfuric acid cleaning solution, thoroughly rinsed with distilled water, and dried. I n making a run the apparatus was filled and the benzene flow adjusted so that about three droplets were formed per second. Then the stopcock D was regulated to allow between 100 and 200 cc. of soap solution to flow out during the run. Since the size of the drops from tube D varied with the concentration of the soap, and since it was found very unsatisfactory to make other than the small adjustments necessary to keep the run going smoothly after the start of the experiment, the amount of soap used as back current fluctuated considerably over the range 100 to 200 cc. per run. This was found sufficient, however, to prevent changes in concentration of the soap in the adsorption tube. Finally, the flow of soap solution into the apparatus was so regulated that the droplets of benzene did not pack as they passed over the bend a t I, and yet the excess of soap solution accompanying them was kept as low as possible. Since the benzene droplets could not be carried up a steep tubeandthereby drained of the excess of soap solution, as was done by Laing, McBain, and Harrison (10) with nitrogen bubbles, the solution in the receiving flask was only slightly more concentrated than that in the adsorption tube. This factor combined with the low specific interfacial area makes the determination of the actual adsorption one requiring very carefully executed technique.
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When all the adjustments were properly made the weighed collection flask was saturated with benzene vapor, pushed up on the tube J, and lightly stoppered to prevent evaporation of benzene vapor. The time of placing the flask in position was recorded and the length of the run was accurately timed. Every five minutes during the course of the run the speed of formation of droplets was measured. This was done by noting, with the aid of a stopwatch, the time required for fifty droplets to pass a given point on the tube. From these readings, and the time elapsed during the run, the total number of droplets was computed. From the weight and density of the benzene collected the volume and the surface of the droplets were calculated. The temperature of the room was held very constant during the course of a run. At rare intervals and then only for a few minutes at a time did the tsmperature vary more than f 0.5"C. from 25OC. Except for the first two experiments, duplicate runs were made consecutively with no break in time. In the first two experiments only one run was made in a day and the check run was made the following day. After completion of a run the analytical procedure described above was immediately carried out. The data of the experiments are given in table 1. Determination of weights of materials used and calculations of results. Previous t o placing flask N in position its weight (WE) was carefully determined to f 2 mg. At the completion of the run the weight (WT) of the flask plus the solutions was found. Then a quantity of sulfuric acid was added in excess of that necessary to react with the sodium present. The flask was again weighed (WA) and sealed. By shaking the flask frDm time to time the oleic acid was extracted from the soap sohtion by the benzene. When extraction was complete the flask was opened and the greater portim of the benzene phase transferred by a siphon to a glass-stoppered bottle. The residue of the benzene was removed with a pipet and the upper bulb rinsed with petroleum ether. The last traces of the ether were driven out with a gentle stream of air, which was saturated with water vapor in order to prevent evaporation of the aqueous phase. After the removal of the benzene, the weight (Wu) of the flask and aqueous solution was determined. The benzene layer was analyzed for oleic acid and the aqueous phase for sodium in the manner described above. From these data the weights of the solutions used were found as follows: WT - WE = WR, total weight of solutions used in the run. WA - Ww = WBO, weight of benzene solution plus oleic acid from soap solution. WR - WBO = W s , weight of soap solution used minus its oleic acid. The weight of the oleic acid ( W o )removed from the soap solution by extraction can be found from the analysis of equilibrium soap and Ws. Then W s + W o = Wgu, weight of soap solution used. WBO - WO = WB, weight of benzene solution used in forming droplets.
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Letting C represent the concentration (milligrams of oleic acid per gram of solution) of the solutions whose subscripts are used, we find the total gain in oleic acid as follows: WBOCBO - WBCB - WsCs = mg. of oleic acid total gain. The computation of the sodium gain was somewhat simpler. The difference between the normality of the sodium concentration in the equilibrium soap bolution and the normality found for the soap solution collected gave the change in normality. This value multiplied by the number of cubic centimeters of soap solution carried over during the run and divided by 1000 gave the gain in equivalents of sodium. The formulas for comparing the results obtained in these experiments with the gains which would have been required for monomolecular layers were derived as follows: Let M equal the weight in grams of benzene solution used, N the total number of benzene droplets formed, and G the milligrams of oleic acid gained during the run. The density of the benzene solutions used was equal to 0.873. The surface of a sphere is equal to 12.57 (’)’ where 2, is the volume. Other constants used are: 6.06 X
(4.189)’
Avogadro’s number; 282.3, the equivalent weight of oleic acid; and 20.6 8..2 the mean cross-section (1) of a fatty acid molecule. From these values the following formula was derived: “Layer” =
(20.6)[(4.189)(0.873)1213(6.06) 107G (12.57) (1000) (282.3) (M2N)1/3
This reduces to: (835.4)G Layers” = ___
((
(M9N)1/3
This formula was modified to use with sodium gains, since by the method of calculation the gains were in equivalents. Using G in equivalents and multiplying the above formula by 1000 and 282.3, it becomes: “Layers” =
2.358 X 1OSG
( M 2N )l / 3
It should be understood that the calculation of gain in “layers” is purely for the purpose of comparison with the amount necessary for the production of a monomolecular layer, and does not mean that the authors conceive of any such phenomenon as twenty or thirty layers of films a t the interface. RESCLTS AND DISCUSSION
The results obtained were rather striking. In no case did the adsorption, as calculated from the gain in oleic acid, approach a monomolecular film.
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In fact, with the one exception of experiment No. 2, the gain was more than twenty times that sufficient to form a film one molecule deep. The adsorption calculated from the gain in sodium was much less. Though we found no adsorption so small as a monomolecular layer, three runs gave values less than sufficient for a dimolecular film, and in only four runs were the gains greater than five times that for a single layer of molecules. A consideration of the experimental errors in analytical results and in counting the droplets showed that should all possible errors be cumulative they would not exceed 5 per cent of the amount of adsorption. It should be pointed out that there are several factors which, while probably negligible, would tend to make our results low rather than high. Though there was very slight drainage of the droplets it is possible that some of the adsorbed material could be brushed away from the surface in the adsorption tube. Another possibility is that oleic acid might be adsorbed from the benzene to the interface. Such an adsorption would not be shown by our procedure. It is somewhat difficult to find a completely satisfactory explanation of the cause for this great difference in the amount of adsorption as measured by the gains found in the two constituents of the soap solution. A careful consideration of the results of other workers on the problem of adsorption of soap a t the oil-solution interface, as well as of the studies made on the structure of the particles of sodium oleate in solution, brings out considerable suggestive material. Laing (9) and Laing, McBain, and Harrison (10) have demonstrated the adsorption of acid soap a t a nitrogen-sodium oleate solution interface. They reported a mean ratio of oleic acid to sodium of 1.70, which would account for an acid soap of the composition NaOl .0.77 H01. Nickerson and Serex (19) studied the conductance of sodium oleate solutions in contact with benzene with results from which they conclude: “More acid sodium oleate is adsorbed a t the oil-solution interface than at the vapor-solution interface,” and “the r81e of acid sodium oleate has been much underrated.” Adsorption of the simple acid soap molecules, while undoubtedly a factor in the experiments we have made, would hardly account for the large ratio found between the gain jn oleic acid and sodium in some of the runs. From the results of experiments on the adsorption of sodium oleate and sodium palmitate a t benzene and toluene interfaces, Nonaka (20) reached the conclusion that the adsorption of both soaps was of monomicellar rather than monomolecular order. He conceives of the micelle as consisting of the combination of the fatty acid micelle and neutral soap micelle in a tangled state. Later cataphoretic studies, in which Nonaka (21) used the same materials, led him to believe that the surface active material of the soap solutions must be the ionic micelle in all cases,
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Thiessen and Spychalski (22) and Thiessen and Trubel (23) have made careful examination of the structure of sodium oleate in solutions by means of x-ray and double refraction studies, and conclude that the same micelles exist in solution as in gels (see also McBain and McBain (16)). These micelles Thiessen finds are rod-like, with the long chain of the fatty acid lying a t right angles to the long axis of the rod. Two of the long faces of the micelles are made up of the carboxyl end of the soap. Thus in a neutral soap these faces would present a solid array of sodium atoms. If the micelle is not neutral some of the sodium atoms would be replaced by hydrogen. If micelles of this form should be adsorbed, the oleic acid gain would be much greater than that of the sodium. Most of the work on the presence of micelles in soap solutions has been done at higher temperatures or a t higher concentrations than we have used. However, in their determination of the micelle content of solutions of sodium oleate by means of ultrafiltration, McBain and Jenkins (15) worked with more dilute solutions and a t 18°C. They could find no micelles in a 0.01 N solution, but using a 0.144 N solution the filtrate was only 0,001 N , indicating that nearly all of the material was of a colloidal nature. In discussing this work McBain says, “The 0.01 N solutions were filtered immediately after being prepared. It is possible that the colloidal acid soap formed by hydrolysis had not yet agglomerated sufficiently to be held back.” Later work by Laing (8,9) in McBain’s laboratory has afforded convincing evidence of the presence of ionic micelles in sodium oleate solution as dilute as 0.01 N . The work alluded to indicates the probable presence of large molecular aggregates in sodium oleate solutions. It is not inconsistent to expect that such aggregates should be adsorbed at the benzene-solution interface. Such adsorption is in accord with the findings reported herein. SUMMARY
1, An analytical method has been devisedwhich is more accurate than the customary method of determining oleic acid. 2. A study has been made of the adsorption of sodium oleate at the benzene-sodium oleate solution interface. The adsorption found, as calculated from the gain in oleic acid, was, with the exception of experiment No, 2, equivalent to that necessary to form from twenty to thirty monomolecular films. The adsorption, as calculated from the sodium gain, varied from that nearly small enough for a monomolecular layer to that sufficient for ten such films. 3. Experiments have been made using solutions at equilibrium as well as non-equilibrium solutions. Similar results were found in both cases.
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