968
JOSEPH STEIGMAN A N D NORMAN SHAKE
Micelle Formation in Concentrated Sulfuric Acid as Solvent
by Joseph Steigman and Norman Shane Department of Chemistry, Polytechnic Institute o f Brooklyn, B r o o k l y , N e w Y o r k
(Received October 10, 1964)
Micelle formation in concentrated sulfuric acid solutions by long-chain fatty acids (which form cationic bisulfates) was studied by means of surface tension and light scattering measurements. It was found that micelle formation in 95% sulfuric acid took place a t a higher concentration than in water, that 5 IM potassium bisulfate had a small or negligible effect, and that water (presumably present as hydronium bisulfate) had a very marked effect, lowering the c m c . It was concluded that electrical repulsion as a factor in micelle formation was less important in sulfuric acid than the organization of the structure of the solvent, and that water (as hydronium bisulfate) was an order-destroying substance in that medium. It was suggested that the organization of the solvent structure might also be involved in micelle formation by soaps in water, and that the effect of added electrolytes in lowering the c.m.c. of soaps might be partly electrical and partly order-destroying. If so, orderforming salts like quaternary ammonium compounds should raise c.m.c. values of cationic soaps in water.
At present, there is no theory of micelle formation which is sufficiently comprehensive to include all the varieties of niolecular structure for which micelle formation has been reported in water.' For the particular case of straight-chain paraffin compounds carrying a single charged head, Debye formulated a theoretical basis for micelle formation in water.2 The onset of micelles was ascribed to the interplay between the long-range repulsive electrostatic forces of the charged heads and the short-range attractive van der Waals forces exerted by the hydrocarbon tails for each other. The electrical work performed in assembling the micelle was calculated on the assumption that the effective dielectric constant would lie between that of the solvent and that of a hydrocarbon. The work gained by bringing together the hydrocarbon portions was assumed to vary linearly with the number of methylene groups in the hydrocarbon tail. The role of the solvent was considered to be solely electrical, in that it modified the work done against electrostatic repulsion between like charges when the micelle was formed. I n a solvent with a higher dielectric constant than water, the critical micelle concentration of a simple soap should be lower than in water, since the electrical work performed against repulsion should be decreased, and (presumably) the van der Waals exchange forces should not be affected. The new solvent must be inThe J o u w a l of Physical Chemistry
organic and should resemble water in its general solvent properties. Concentrated sulfuric acid is such a solvent. The present work was undertaken in order to study micelle formation in concentrated sulfuric acid solution for a number of straight-chain carboxylic acids. The very high acidity of concentrated sulfuric acid causes weak carboxylic acids to behave like bases3
RCOOH
+ HZSO,
RCOOH2+
+ HSO4-
McCulloch* found that higher fatty acids in cold concentrated sulfuric acid gave soap-like solutions with persistent foams. I n this respect sulfuric acid appears to resemble water, that is, there is apparently an acidophobic tail and an acidophilic head in the soap molecule, corresponding to the hydrophobic and hydrophylic portions encountered in water. Micelle formation was detected by measuring the surface tension of sulfuric acid solutions of fatty acids a t different solute concentrations. These data were supplemented by observations of light scattering by several of the systems studied.
(1) W.Philippoff, Discussions Faraday Soc., 11, 96 (1951). (2) P. Debye, J . P h y s . Colloid Chem., 53, 1 (1949). (3) A Hantzsch, 2. physik. Chem., 65,41 (1909). (4) L. McCulloch, J . Am. Chem. Soc., 68, 2735 (1946).
MICELLEFORMATION IN CONCENTRATED SULFURIC ACIDSOLUTIONS
Experimental JIaterz’als. Sulfuric acid was pure llerck reagent grade, ACS specification for microanalysis. Acidimetric analysis showed that it was 95.01 (10,05j% by weight in HsSOI. I t was recrystallized twice and reanalyzed. I t was found that surface tension ineasuremerits of lauric acid arid stearic acid solutions in the unpurified acid nere virtually identical with those of the corresponding solutions prepared from the recrystallized acid. A4ccordingly, the Slerck acid was used without further purification for most of the experiments. Dilutions with distilled water produced 85.00 (10.05) % acid, and 82.00 (10.05) % acid. Fuming sulfuric acid was obtained from the J. T. Baker Chemical Co. It was added to the llerck acid to prepare sulfuric acid solutions of acid content greater than 95%. Analysis showed that the more acid solutions were 95.51 (10.03) yo by weight, 96.20 ( = t O . O 3 ) yo by weight, and 97.31 (hO.05) yoby weight, These acids n’ere used without further purification, sirice two recrystallizations of the 97.3% acid did not affect the surface tension of solutions of lauric and stearic acids. Caprylic arid capric acids were chemically pure, purest grade, obtained from Eastnian Organic Chemicals, Distillation Products Industries. Lauric, myristic, palmitic, and stearic acids were cheniically pure products of the Fisher Scientific Co. The boiling points of the caprylic and capric acids and the melting points of the other acids agreed closely with those reported in the ‘‘1nternational Critical Tables.” Other chemicals were of reagent grade and were used without further purification. Stability and Pi-eparation of Sulfuric ilcid Solutions of Fatty Acids Fatty acid solutions in sulfuric acid of eoncentration higher than 97.3Y0 by weight turned brown quickly, indicating decomposition. Accordingly, 97.3Vo sulfuric acid was the most concentrated sulfuric acid which was used. The surface tension nieasurenients were made less than 1 day after the preparation of the solution, in order to minimize possible decomposition. Fatty acid solutions in more dilute acid remained colorless for at least 2 days. The solid fatty acids were recoverable without change in the melting point on dilution of their sulfuric acid solutions with water after 2 days. Each solution whose surface tension was measured was prepared individually in glass-stoppered bottles. The various solutions were made up on a molar basis. Appai.atus. Surface tension ineasureinents were made at room temperature (27”) with a Cenco Du Kouy tensiometer equipped with a 4-cni. ring. After each nicasurenent the ring was cleaned with cleaning
969
solution and distilled water, and was then heated until it was cherry red. The experimental precautions suggested for this instrument by Harkins5 mere consistently observed in this work. Frequent measurements of the surface tension of distilled water were made, and always showed readings of 72.2 dynes’cm. A petri dish was used to hold the different solutions during the measurements. After each deterinination, it was cleaned with cleaning solution, then with distilled water, and was dried a t 150”. A larger inverted dish with a small hole bored in the bottom served to cover the test solution, limiting the access of moisture to the sample. Control tests showed that if measurements were made within 15 sec. of solution exposure to air, the readings could be reproduced on successive samples. If the samples were exposed to air for sonie minutes, the surface tension decreased noticeably. If samples were quickly measured, placed in a desiccator, and measured again, the readings were constant. A so-called dry beaker was constructed according to Kunxler’s recommendation6 and a series of ineasuremerits was carried out in an atmosphere of dried nitrogen gas. However, these were discontinued, since nieasureinents made in the dry beaker were riot significantly different from those made within 1.5 sec. of sample exposure t o air. Light Scattering ineasurenients were niade with a Brice-Phoenix light scattering photometer, using a special turbidity cell which consisted of a closed container with Teflon grease surrounding its penny-head stopper. 3Ieasurements were made at 4360 -1. on solutions of lauric and stearic acids in 95% sulfuric acid, as well as on the solvent itself. The solutions and solvent prior to measurement were suction-filtered in a dry nitrogen atmosphere thr’ough glass wool laid over a sintered glass filter. Estimation of the Critical JIicelEe Concentration Surface Tension. For each fatty acid a series of solutions of different concentrations was prepared, and the surface tension was measured. Additional solutions were prepared if they mere needed. For stearic acid, a total of six series, each series with six solutions, was run. The critical micelle concentration was determined by an ad hoc procedure. Preliniinary graphical plots of the surface tension (in dynes cm.j against the log of the fatty acid concentration (expressed initially as grains of fatty acid per 100 nil. of 95% sulfuric- acid) showed that there was an approxiniately linear decrease followed by readings which were independent of ( 5 ) W. D Harkins and H F Jordan, .7. A m (’hem Soc (1930). (6) J E Kunzler, Anal Chem , 2 5 , 93 (1953)
5 2 , 1751
Volume 69. Xzimber 9 .Ifarch 2.965
JOSEPH STEIGMAN A N D NORMAN SHANE
970
the soap concentration. The curves resembled those found in water' except that there was no minimum. It was assumed that the critical micelle concentration was to be found a t the intercept of two straight linesthe first of negative slope, before the c.m.c., and the second of zero (or almost zero) slope, after the c.m.c. This intercept was determined by formulating the equation for the first straight line by least squares, averaging the approximately constant surface tension values after the c.m.c., and from the equation for the first straight line determining the soap concentration corresponding lo the averaged limiting value. This procedure was followed for all solutions except those in sulfuric acid more dilute than 95% and those in 95% H2S04containing 5 111 KHSO4, for which the intercepts were determined graphically. Table I shows some typical values for stearic acid in 95.0% HzS04.
Table 11: Light Scattering a t 4360 A. by Lauric Acid in 95% Sulfuric Acid
Lauric acid, g./lOO ml. of H&0,
1.0012 1.0755 1,1802 1.2125 I ,2725 1,2775 1.2825 I . 2925 1.3002 1.3487
Table 111: C.m.c. for Lauric Acid and for Stearic Acid in 95y0 Sulfuric Acid Lauria acid 0.m.c..
Stearic acid c.m.c.,
Method
g./100 ml.
g./100 ml.
Surface tension Light scattering
1.11 1.18-1.21
0.013 0.0089-0.0093
~
Table I : Surface Tension of Stearic Acid Solutions in 95% Sulfuric Acid a t 27", Series C g . / l O O ml. of HzSOI
Surface tension, dynes/cm.
0.0006 0.0030 0.0060 0,0090 0.0120 0.0150
57.02 50.89 44.76 44.15 41.25 41.25
Concn. of stearic acid;
Light Scattering. The experimental terms of the absolute turbidity equation of Debye2 were used for the determination of the c.m.c. These are found in the difference
Here G900 refers to the galvanometer reading of the beam a t 90" for the sample, Goo gives the galvanometer reading for the undeviated beam (plus neutral filters), F is the product of transmittances of the neutral filters used in the determination of the ratio, and the same terms with the zero superscripts refer to the same measurements for the pure solvent. I t was assumed that the c.ni.c. was to be found in the region of abrupt change of turbidity with concentration. Table I1 gives the experimental values for lauric acid in 95.0% H2SO4,and Table I11 shows the agreement between results obtained from surface tension measurements be Seen that there light scattering' It "*' and is a fair nieasure of agreement between the two methods. The Journal of Physical Chemistry
0.026 0.033 0.049 0.112 0.292 0.301 0.309 0.327 0.354 0.382
Results and Discussion Table IV shows the results of the surface tension determinations of the c.m.c. for the various fatty acids in a number of sulfuric acid-water solutions. The values in water for the potassium salts of the same carboxylic acids are listed for comparison. This comparison is assumed to be a valid one since (in water) the number of carbon atoms in the hydrocarbon portion of a group of related surface-active agents has been shown to be the most important factor in determining the c.m.c., and the nature of the charged head is much less important. It is evident that micelle formation in 95% sulfuric acid takes place a t a higher molar concentration than in water (except for stearic acid), and that the c.m.c. increases as the sulfuric acid content increases. If the dielectric constant of the solvent is an important factor in determining the formation of micelles,2 then micelle formation in sulfuric acid should occur at a lower concentration than in water. Undoubtedly, there is a markedly decreased electrostatic repulsion between like ions in sulfuric acid. However, this factor, which would lower the c.m.c., must be less important than some other factors which raise the c.m.c. values. One such factor would be a change in the van der Waals or exchange forces between paraffin chains be(7) J. Powney and C. C. Addison, Trans. Faraday &., (1937).
33, 1243
MICELLEFORMATION IN CONCENTRATED SULFURIC ACIDSOLUTIONS
97 1
Table 1V: C.m.c. of Soaps in Water and of Fatty Acids in Various Sulfuric Acid Solutions (the latter a t 27') (moles/l.)
No. of C atoms
C.m.c. in H?O at
C.m.c. in 82 %
2500
HzSOi
8 10 12 14 16 18
0.39 0.0!18 0.0255 0.0066 0 0018 (35') 0 00045 (45")b
a
C.m.c. in 85% H2SO4
-0.14 -0.024
