Oct., 1958
MICELLAR BEHAVIOR OF HALFESTER SOAPSIN BENZENE
1257
MICELLAR BEHAVIOR OF HALF ESTER SOAPS IN BENZENE BY BAMUEL KAUFMAN AND C. R. SINGLETERRP U . 8. Naval Research Laboratory, Washington 66,D. C. Received March 8. I068
The degrees of aggregation of the sodium, barium and lead soaps of 2-ethylhexyl sebacate in moist benzene have been studied by the fluorescence depolarization method, All of these compounds are crystalline, and all are micelle formers in benzene, although the sodium soap is soluble only above 47". The gram micelle volume of the lead soap is 3000 CC. and its CMC is in the region of lo-* equivalents per liter. These values represent the smallest micelle size and the highest CMC yet found for oil-soluble metal soaps of high molecular weight. The micellar properties of this soa are attributed to the weak ionic properties of lead and its strong tendency to coordinate. Viscosities of the barium soap so%tions indicate the preseqce of either markedly asymmetric aggregates, or those of lesser asymmetry whose extremely irregular surfaces strong1 fluence the viscosity. Due to experimental uncertainties, the sizes of the barium and sodium soap micelles could on y Inbe estimated. The spherical sizes hydrodynamically equivalent to their respective gram micelle volumes are 30,000 to 40,000 cc. and 10,000 to 20,000 cc. As oil-soluble micelle formers, these half ester soaps are believed to be intermediate in properties between the arylstearates and the dinonylnaphthalene sulfonates, on the one hand, and the non-ionic surfactants on the other.
ry
Introduction The authors' previous studies of the micellar phenomena exhibited by oil-soluble soaps have been concerned with systems such as the soaps of the arylstearic acids or dinonylnaphthalene sulfonic acid, which were believed to provide useful approximations to the structure and properties of the petroleum sulfonates and naphthenates in practical use. Investigations elsewhere have utilized the petroleum acid salts, Aerosol OT, amine salts of fatty acids, metal salts of the fatty acids at higher temperatures, and non-ionic amphipaths derived from normal fatty acids and either a polyhydroxy alcohol or polyethylene oxide.' The aryl stearates, dinonylnaphthalenesulfonates and their petroleum analogs differ from the amine salts of the fatty acids, the heavy metal soaps of normal fatty acids, and the non-ionic amphipaths so far examined in an interesting respect. Micelle formation predominates for the former even at concentrations as low as 10-6 or 10-6 mo1ar12-6and the micelles of a given soap characteristically show little variation in size over a hundred- or thousandfold concentration range unless some other variable is also introduced. The amine-fatty acid s ~ a p s l . ~and - ~ the oil-soluble non-ionic surface active agents, on the other hand, usually form association colloids at concentrations in the range of lom2molar or above, and the nature of the aggregates varies substantially with concentration. A recent investigation of aliphatic diesters'o made available the pure half ester, hydrogen 2ethylhexyl sebacate, and indicated that it was capable of forming oil-soluble salts. The structure of this acid consists essentially of a 17-atom chain carrying an ethyl radical and a carbonyl oxygen as side groups. Its geometry and the presence of a neutral ester linkage near the center of the molecule suggest that the micellar behavior of its salts should be between the patterns of the two groups (1) C. R. Singleterry, J . Am. Oil Chemista' SOC.,32, 446 (1955). (2) C. R. Singleterry and L. Arkin, J . Am. Cham. Boc., 70, 3965 (1948). (3) C. R. Singleterry and L. A. Weinberger, ibid., 73, 4574 (1951). (4) 8. Kaufman and C. R . Singleterry, J . Colloid Sci., 7,453 (1952). (5) S. Kaufmen and C. R. Singleterry, ibid., 10, 139 (1955). ( 6 ) S. Kaufman and C. R. Singleterry, ibid., 12, 465 (1957). (7) A. Kitahara, Bull. Cham. SOC.Japan, 28, 234 (1955). ( 8 ) A. Kitahara, ibid., 29, 15 (1956). (9) A. Kitahara, J . Colloid Sci., 12. 342 (1957). (10) F. L. James and J. G. O'Rear, publication in preparation,
contrasted above and perhaps should explain their differences. This paper presents the results of an examination of the colloidal and micellar phenomena shown by the sodium, barium and lead salts of this acid. These were chosen as representative of classes of metal ions which might be expected t o exhibit distinctly different types of behavior.
Experimental . Materials.-Hydrogen
2-ethylhexyl sebacate (mol. wt. 314.452) was furnished by F. L. James and J. G. O'Rear,lO of the U. S. Naval Research Laboratory. Its density ( d z o 4 ) was 0.9592 and its neutral equivalent was 317.5 (theoretical: 314.452). It is insoluble in water, but soluble in benzene. Sodium 2-ethylhexyl sebacate (mol. wt. 336.441) was prepared by otentiometric titration of the half ester with sodium hytroxide in the presence of a large excess of boiled water, in which the soap is freely soluble. Complete neutralization occurred at pH 10 and the amount of alkali required for neutralization corresponded to that theoretically expected for the unhydrolyzed half ester. The neutral solution was filtered through a Millipore (0.45 p ) filter and most of the water was removed a t reduced pressure in a rotating flash evaporator. The residue was repeatedly dissolved in hot anhydrous benzene and evaporated to sweep out residual moisture and finally freed of solvent a t 0.1 mm. and 80". Lead 2-ethylhexyl sebacate (mol. wt. 834.098) was precipitated by addition of a solution of the sodium soap to a solution of lead nitrate in 100% excess. The lead nitrate solution was prepared with boiled water adjusted to pH 4 with a trace of nitric acid to prevent hydrolysis. Precautions were taken to prevent a local excess of the sodium soap. The precipitate flocculated completely after two hours of stirring. The solid was dissolved in benzene and was repeatedly extracted, f i s t with a dilute aqueous solution of lead nitrate adjusted to pH 4 and finally with water a t this pH, until the sulfate test indicated absence of lead ions in the washings. The faintly cloudy benzene solution was centrifuged, filtered, lyophilized and dried a t 83" and 4 ,u Hg. The dry soap was sealed in ampoules under nitrogen and refrigerated until used. It melted at 80-81" and cooled to a transparent, faintly ellow birefringent glass. After several days of refrigeratedJrstorage, the mass became translucent and appeared microcrystalline between crossed polaroids. Fragments of the soap precipitated from the lead nitrate solution showed extinction and crystalline form under the polarizing microscope. The compound was freely soluble in benzene, chloroform, ethyl ether, petroleum ether, dioxane, acetone, methanol, ethanol and isopropyl alcohol. Analysis of this soap indicated that it was the normal lead salt of the half ester. Barium 2-ethylhexyl sebacate (mol. wt. 764.248) was prepared by precipitation analogous to that for the lead soap. No adjustments of pH were made, since no hydrolysis was anticipated. Ultimate tem erature and ressure following lyophilization were 90" a n f 3 p Hg. TRe product was a snow white porous solid. It is soluble in benzene, leum ether, ethyl ether, dioxane, chloroform, m % :!et; ethanol and isopropyl alcohol, but not in acetone.
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Vol. 62
SAMUEL KAUFMAN AND C. R. SINGLETERRY
The benzene and Rhodamine B used here have been described elsewhere.6 Measurements.-Methods for measuring fluorescence and spectral absorption have been described by the authors in earlier publications.'-6 Viscosities of the barium soap solution were determined in a sealed three-level viscometer.11 To prevent accumulation of moisture and particles of lint during preparation of the test solution and in filling the viscometer, the soap was introduced by pressure filtration of a solution through a Millipore (0.45 p ) filter directly into the instrument. The soap was lyo hilized and dehydrated in the viscometer and dissolved in Kltered anhydrous benzene just before sealing. Dissolution of the barium soap and of water in the solvent was accelerated ultrasonically by immersing the viscometer in a water-bath excited a t 40 kc. This treatment reduces to minutes the duration of a process which otherwise requires hours or days. Except as otherwise noted, all measurements were made at 26".
4
o
t l I I I
0
I
o
I I I I1 II I I I
I
o
I I I I I l l I I 2
I 1 I 1 1 31
looxconcentrotion. g-equivalents/liter,
Results and Discussion
Fig. 1. Micelle size of lead Zethylhexyl sebacate.
Figure 1 is a plot of the gram micelle volume of lead 2-ethylhexyl sebacate as a function of the soap concentration in benzene solutions containing one mole of water per equivalent of soap plus 15% of the water required to saturate the solvent. This curve is derived from fluorescence depolarization measurements with the assumption that the aggregates were smooth unsolvated spheres. At concentrations above 3 X equivalent per liter, the apparent micelle volume approaches a limiting value of 3000, which is taken as the true micelle size, but below this concentration the curve slopes typically, as in the case of the sulfonates investigated earlier.6p6 At 8.53 X equivalent per liter, the lowest concentration of lead soap studied, the apparent gram micelle volume found was 1410 cc., a value only slightly higher than that equivalent to a 1:l complex of soap and dye molecules. If the densities of soap and dye are estimated as 1.26 and 1.2 g./ml., respectively, the corresponding gram molecular volume of the complex is about 1060 cc. At 1 X 10-3 equivalent per liter, the data indicate that micellar organization no longer dominates the system, and that monomers predominate. In the light of these results, a reasonable estimate of the critical micelle range4 for this system is 1 X 10dat o 2 X 10-3 equivalent per liter. It is noteworthy that this concentration differs by orders of magnitude from the critical concentration for the dinonylnaphthalene sulfonates or the carboxylates studied earlier.8-6 With these, clear evidence of micelle formation was obtained at concentrations of 10-6 equivalent per liter and lower. The assumption of sphericity for these micelles is justified on the basis of their determined gram micelle volume of 3000 ce., which is consistent with the aggregation of 3 to 4 soap molecules to a micelle. Such a small unit is not likely to deviate substantially from sphericity, and there were no obvious viscosity effects indicating asymmetry at the concentrations represented in the data. The lead soap, then, is characterized by a higher CMC, smaller micelle size, and a lower melting point than the metal salts previously studied by the authors. Lead ions are known to coordinate strongly with the carboxylate ion; the resulting
neutral coordination complexes are less strongly ionic than the alkali and alkaline earth salts. This decreased polarity should result in a higher molecular solubility in hydrocarbons (ie., higher CMC), in weaker interactions between molecules to form micelles, and in melting points characteristic of a molecular rather than an ionic array in the solid state. Other heavy metals forming covalent type coordinate linkages may be expected to show a similar trend. A similar series of solutions of barium %-ethylhexyl sebacate was studied, containing 1 mole of water per equivalent of soap plus 15% of the water sufficient to saturate the benzene present. Determinations of micelle size by the fluorescence depolarization method with Rhodamine B were less reliable in this case than in the case of the lead compound. The dye was solubilized less, and the spectral distribution was atypical. This latter effect necessitated long extrapolations for the estimates of 7 , the excited lifetime of the fluorescent species.6 Nevertheless, the polarizations of the emitted fluorescence from these barium soap solutions are significant and are reported in Table I. The values range between 0.40 and 0.42 and a reasonable estimate for the spherical micelle volume consistent with these polarizations is 30,000 to 40,000 cc. It is of interest that in this case there is no evidence of the CMC within the concentration range studied, whereas the monomer concentration begins to manifest itself markedly in the lead soap solutions at concentrations as high as 3 X lod3 equivalent per liter.
(11) J, Q. Honig and C, R, Singleterry, Anal. Chsm., 26,677 (19641.
,
TABLE I FLUORESCENCE POLARIZATION OF RHODAMINE B IN BENZENE SOLUTIONS OF BARIUM 2-ETHYLHEXYL SEBACATE Concn. (equiv./l.)
x 1.11 x
5.55
10-4 10-3
3.15 X 6.30 x lo-* 1.24 X lo-* 2.69 X
Polarization
0.404 .421 .417 .418 .422 ,409
The above estimate for the micelle size of the barium soap assumes sphericity for the aggregates, but here there is distinct viscometric evidence for departure from the smooth, spherical form. The
Oct., 1958
MICELLAR BEHAVIOR OF HALFESTERSOAPS IN BENZENE
observed viscosity number, (q - q0)/qoc,l2 for an anhydrous benzene solution containing 0.01107 g./ml. of this soap was 56.1, and the value for the same solution containing 1 mole of water per equivalent of soap was 62.3. (A second addition of water to a total of 3 moles of water per equivalent of soap did not dissolve completely, indicating that the limit of water solubilizing power for this solution lies between these two water/soap ratios.) The magnitudes of the viscosity numbers found for these barium soap solutions indicate marked asymmetry and/or solvation of the solute particles. However, the possibility of particle-particle interaction in this case precludes quantitative estimates of asymmetry or solvation from these data. I n earlier workaJa with solutions containing moderately asymmetric micelles at 1 to 3% concentrations it appeared reasonable to assume that the viscosity numbers found approximated those at infinite dilution, and Simha's treatment14p16 was applied to the data. Here the assumption is less valid because the ester group in the tail of the monomer provides an interaction site near the surface of the micelle, which was not present with pure hydrocarbon tails. The high viscosity numbers observed may thus be a combined effect of asymmetry, solvent entrapment and particle-particle interaction; the data are not adequate to clarify this uncertainty, although they leave no doubt that micelles are present. Sodium 2-ethylhexyl sebacate, which is insoluble in benzene at room temperature, dissolves readily at elevated temperatures. On cooling, the solution becomes viscous, then forms an unstable gel which reverts in time to insoluble, colorless, transparent crystals, distinctly discernible between crossed polaroids. These crystals dissolve in benzene at ca. 47". A sample of benzene equilibrated at room temperature with the crystalline sodium soap was passed through a Millipore (0.45fi) filter and 15% of the water necessary to saturate the benzene was added. This solution developed no color or ffuorescence with Rhodamine B, and therefore exhibited no evidence of micelles. Evaporation of 50 ml. of this fluid at 105" left 0.8 mg. of an oily residue which may have been di-2-ethylhexyl sebacate carried over from the original preparation. lo A solution of 1.2 X equivalent per liter of the sodium soap was prepared and examined at ca. 55" by the fluorescence technique. The observed polarizations of the emitted fluorescence were 0.32 and 0.28. Again in this case, the spectral character of the emission was such that definite quanti(12) The viscometric nomenclature and notation used here are those recommended by the International Union of Pure and Applied Chemistry: . I Polymer . Sei., 8, 257 (1952). (13) J. G. Honig and C. R. Singleterry, TRIBJOURNAL, 60, 1108 (1956). (14) R. Simha, ibid., 44, 25 (1940). (15) R. Simha, Proc. Intern. Coner. RheoEoey, Part 11, 70 (1948).
1259
tative conclusions cannot be drawn concerning the particle size present, although there is a clear indication of micelle formation. The gram micelle volume consistent with the polarization data would lie between 10,000 and 20,000 cc. The behavior of the sodium soap is interesting because it separates from benzene at room temperature in well formed crystals. The liquid in equilibrium with these crystals contains no micelles detectable by the sensitive fluorescence polarization technique, and in fact contains too small an amount of soap to permit any reliable estimate of the amount. The typical oil-soluble soaps are non-crystallizing and are usually miscible with the hydrocarbon in all proportions. A question naturally arises as to whether a distinctly crystalline and strongly ionic soap (such, for example, as sodium stearate) can be micelle-forming in a hydrocarbon solvent. The behavior of sodium 2-ethylhexyl sebacate suggests that the crystal and micellar organizations either cannot coexist in a given solvent, or at most can exist in equilibrium only at a transition temperature or in a narrow transition range. Like soaps in aqueous solvents, crystalline soaps in hydrocarbon solvents show a very large increase in solubility over a narrow temperature range, in analogy to the Krafft point. The explanation given by Murray and HartleylBin the case of aqueous systems must apply also in this case, Le., the low solubility of the crystalline soap increases with rising temperature until the saturation concentration of the soap monomer reaches the CMC for the micellar system. At this temperature micellization sets in and only a slight further rise in temperature is needed for the complete solution of the crystals. The transition occurs over a small range in temperature because the concentration of monomer in the micellar system increases slightly with increasing micelle concentration, in conformity with the equilibrium constant for the reaction n8
IT8,
where S is a soap monomer and n is the number of monomers in a typical micelle. The transition temperature may be thought of as a modified melting point, lowered from that of the soap alone in proportion to the effectiveness of the liquid as a solvent for the tails of the soap molecules. When regular crystalline packing becomes improbable because the hydrocarbon tails are branched, or are a mixture of branched or ring-containing isomers, the soap becomes micelle-forming in oils at room temperature or lower. Micellar soaps are usually miscible in all proportions with compatible hydrocarbons because the hydrocarbon exteriors of the micelles resemble the solvent so closely that the possible decrease in free energy by segregation as a separate phase is negligible. (16) R. C. Murray and G. 9. Hartley, Trans. Faraday SOC.,31, 183 (1935).
