J. Phys. Chem. 1985,89, 1537-1541 allows quantitative treatment of the frequency shifts upon mixing of two liquids and, as a result, enables us to discuss the solute effect spectroscopically.
Acknowledgment. The authors express their gratitude to Prof.
1537
Willis B. Person of University of Florida for discussions and a critical reading of this paper. _ _ Registry NO. CH,OH, 67-56-1; CCI4, 56-23-5; C6H6, 71-43-2; CF3COOH, 76-05-1.
Evidence for Aggregates of Cationic Surfactants in Dilute Methanollc Solution of Benzene Arun Kumar Chattopadhyay, Maurice Drifford, Departement de Physico- Chimie, CEN-Saclay, 91 -Gif-sur- Yvette, France
and Claude Treiner* Laboratoire d'Electrochimie, Universite Pierre et Marie Curie, Bat.- F 4, Place Jussieu, 75005 Paris, France (Received: May 17, 1984; In Final Form: November 21, 1984) Static light-scattering and total vapor pressure techniques have been applied to the study of the behavior of trimethyl-nalkylammonium bromide (decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl) surfactants in methanol + benzene mixtures. Both experimental techniques provide evidence that, in presence of at least 2 wt % benzene, surfactant aggregates are formed most probably of the direct type with 3 to 5 monomers per aggregate. At a given benzene concentration, the critical aggregation concentration is a linear function of the number of carbon atoms in the linear hydrocarbon chain. It is also shown from precise vapor pressure measurements that the cationic surfactants salt-in benzene at concentrations below the critical aggregation concentration, as in the classical case of aqueous solutions;however, the salting-in constant of benzene in methanolic surfactant solutions becomes less negative with the increase in hydrocarbon chain length in opposition to the trend observed in aqueous solutions.
Introduction In the course of a study of the free energy of interaction between inorganic and organic electrolytes with benzene in methanol by a vapor pressure method,' it was observed that the activity of the mixed solvent (benzene + methanol) after an initial decrease upon addition of a cationic surfactant, remained constant above a critical concentration. Sudden change in the slope of the variation of total vapor pressure vs. surfactant concentration, which was specific to surfactants, could not be observed in the absence of benzene. This observation could be attributed as evidence for the formation of aggregates comparable to micelle formation in aqueous or nonaqueous solvents. The aggregation of ionic surfactants in nonaqueous solvents is a well-established field of research with applications particularly for the catalysis of chemical reactiom2 It has eventually been studied in nonaqueous solvents of very low dielectric constant such as aliphatic and aromatic hydrocarbons and chlorinated solvents. Evidence for ionic aggregation in nonaqueous solvents of high dielectric constant is scarce and their nature not known. Micelle formation of amphiphatic compounds in water is assumed to be the consequence of the specific physicochemical properties of the solvent; in solvents of low dielectric constant so-called inverse micelles seem to be favored by dipole-dipole interactions between the ionic surfactants associated in ion pairs and stabilized by small quantities of water molecule^.^ Neither phenomena can be advocated in nonaqueous solvents of high dielectric constant; ionic surfactants are essentially dissociated into free ions and the hydrophobic phenomenon is ruled out. Nevertheless, aggregation of ionic surfactants has been observed in ethylene glycol4 (e = 40.7) dimethyl sulfoxide5 (e = 46.7), and ( I ) C. Treiner and A. K. Chattopadhyay, J. Chem. SOC.,Faraday Trans. I , 79,2915 (1983). ( 2 ) J. H.Fendler and E. J. Fendler in "Micellar and Macromolecular Catalysis", Academic Press, New York, 1975. (3) H.F. Eicke in "Topics in Current Chemistry", Springer-Verlag,Berlin,
1979.
~.
(4) L. G. Ionescu and D. S. Fung, J. Chem. SOC.,Faraday Trans. 1, 77, 2907 (19811. --, ( 5 ) E.J. Fendler, V. G. Constien, and J. H.Fendler, J. Phys. Chern., 79, 917 (1978). -. .
\ - -
hydrazine6 (e = 51.7). There is no evidence in the literature for such aggregates in methanol (e = 32.7), in fact it has been shown that aggregates of barium dinonylnaphthalenesulfonate in toluene are destabilized by the addition of small quantities of methanol.' There is an indication that alcohols such as 1-pentanol*might favor this type of association with dialkylsulfosuccinate electrolytes such as AOT. In view of this scattered information on ionic aggregation of surfactants in these solvents, it was deemed worthwhile to investigate the origin of such thermodynamic observations on methanol + benzene surfactant solutions with a more specific technique such as light scattering; furthermore, as the role of benzene in the stabilization of the aggregates was not known, attempts were made to restrict the investigationsto the low benzene concentration range needed for the existence of such aggregates and investigate the effect of the series of ionic surfactants on the aggregation phenomenon. We present in this report static vapor pressure and light scattering measurements on the behavior of the alkyltrimethylammonium series ranging from decyl- to octadecyltrimethylammonium bromide in methanol in presence of a low concentration of benzene. Vapor pressure measurements are essential to study the behavior of low surfactant concentrations where only monomer ions are present. The interaction of these surfactant monomer with benzene may be measured by a salting (Setchenov) constant just as in aqueous solutions and, consequently, it may help in understanding the behavior of the more complicated aggregates formed at higher surfactant concentration; light scattering measurements provide information on the aggregates themselves. The essential conclusion which may be drawn from this preliminary study is that small aggregates of about 3 to 5 surfactant monomers are definitely stabilized in methanol by benzene molecules. A critical micelle concentration reflected by a neat break in the vapor presence of intensity of light vs.
+
(6) M. S.Ramadan, D. F. Evans, and R. Lumry, J. Phys. Chem., 87,4538 (1983). (7)A.J. Fryer and S . Kaufman, J. Colloid Interface Sci., 29,444(1969). ( 8 ) (a) J. B. Peri, J. Colloid Interface Sci., 29,6(1969). (b) A. S. Kertes
and H.Gutman in "Surface and Colloid Science", Vol. VIII, E. Matijevic, Ed., Wiley-Interscience, New York, 1976.
0022-3654/85/20S9-1537$01.50/00 1985 American Chemical Society
1538 The Journal of Physical Chemistry, Vol. 89, No. 8, 1985
Chattopadhyay et al.
f
surfactant concentration may be defined; the cmc is a linear function of the number of carbon atoms in the linear chain of the surfactant. Plausible interpretations are proposed for the mechanism of aggregate stabilization.
Experimental Section n-Decyltrimethylammonium bromide (DTAB, Eastman-Kodak), dodecyltrimethylammonium bromide (DDTAB, Sigma), tetradecyltrimethylammonium bromide (TTAB), Hexadecyltrimethylammonium bromide (CTAB), and octadecyltrimethylammonium bromide (OTAB) (all from Fluka) were purified by recrystallization from acetone. Methanol (Merck) and benzene (Merck) were dried Qver 3-A molecular sieves before use. Apparatus and Procedure. i. Light Scattering. The surfactant solutions were prepared in methanol benzene (5 wt %) and the net water content in the resulting solutions was determined by Karl-Fisher titration, which showed that the maximum water mol/L; though the content in the solution was
