Langmuir 1992,8, 2107-2109
2107
Articles Ultrasonic Relaxation and Micelle Formation in Solutions of Cetylpyridinium Chloride in Formamide M. A. Thomason, D.M. Bloor, and E.Wyn-Jones* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT,U.K. Received January 27, 1992. In Final Form: May 15,1992 An ultrasonic relaxation has been observed in solutions of cetylpyridinium chloride in formamide at surfactant concentrationsranging from0.2 to 0.99mol dm-3. The behavior of the sound absorption coefficient at different concentrations and frequencies is consistent with the origin of the relaxation being associated with the perturbation of monomerlmicelleequilibria, and the mechanism is the same as the corresponding process in water. By comparison of the present results with previous resulta of micellar kinetics, it appears that the micellar aggregates are much smaller in formamide than in water.
Introduction The formation of micelles and liquid crystals have been A reported in a number of nonaqueous polar survey of reportdofocusing on the aggregation of cationic surfactants in formamide has shown that in several cases ambiguous conclusions can arise and the following issues have been raised (i) Do the surfactants form micelles or not? (ii) If micelles are formed are they large or small aggregates? (iii) What is the mechanism associated with the formation of micelles? The first issue concern the Krafft point of the various surfactants in formamide, and in general it has been found that a t high enough temperatures (i.e., 40-60 "C) most surfactants are above their Krafft point and form aggregates in formamide. The last two issues have been highlighted in recent publications7J0*21-2s which illustrate the ambiguous conclusions (1) Reinsborough, V. C. A u t . J. Chem. 1970, 23, 1473. (2) Ray, A. J. Am. Chem. SOC.1969, 91 (23), 6511. (3) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. J. Colloid Interface Sci. 1982, 88,89. (4) Evans, D. F.; Yamauchi, A.; Wei, G.; Bloomfield, A. J. Phys. Chem. 1983,87, 3537. (5) Rico, I.; Lattes, A. J. Phys. Chem. 1986, 90,5870. (6) Belmajdoub, A.; Emayaed, K.; Brondeau, J.; Cannet, D.; Rico, I.; Lattes, A. J. Phys. Chem. 1988,92, 3569. (7) Binana-Limbele, W.; Zana, R. Colloid Polym. Sci. 1989,267,440. (8) Fletcher, P. D. I.; Gilbert, P. J. J. Chem. SOC.,Faraday Trans. 1, 1989,85, 147. (9) Backlund, S.; Bergenstahl, B.; Molander, 0.;Warnheim, T. J . Colloid Interface Sci. 1989, 131, 393. (10) Sjoberg, M.; Henrihon, U.; Warnheim, T. Langmuir 1990,1205. (11) Gharibi, H.; Palepu, R.; Tiddy, G. J. T.;Hall, D. J.; Wyn-Jones, E. J . Chem. SOC.,Chem. Commun. 1990, 115. (12) Warnheim, T.;Jonsson, A. J. Colloid Interface Sci. 1988, 125, 627. .~ (13) Warnheim, T.; Bokstrom, J.; Williams, Y. Colloid Polym. Sci. 1988,266,562. (14) Jonstromer, M.; Sjobera, - M.; Warnheim, T. J. Phys. Chem. 1990, 94, 7549. (15) Warnheim, T.;Sjoberg, M. J. Colloid Interface Sci. 1988, 131, 402. (16) Rong, G.; Friberg, S. E. J. Dispersion Sci. Technol. 1988, 9 ( 4 , 401. (17) Friberg, S. E.; Rong, G. Langmuir 1988, 4, 796. (18) Friberg, S. E.; Blute, I.; Stenius, P. J. Colloid Interface Sci. 1989, 127, 573. (19) Ward, A. J. I.; Rong, G.; Friberg, S. E. Colloid Polym. Sci. 1989, -267. - - , 730.
(20) Evans,D. F. Langmuir 1988,4,3. (21) Belmajdoub, A.; Boubel, J. C.; Canet, D. J. J. Phys. Chem. 1989, 93,4844.
that can arise from different experimental measurements. Since these two questions are concerned withmechanisms, we have attempted to address the problem by carrying out kinetic studies, and in this paper we report our ultrasonic relaxation data on solutions of cetylpyridinium chloride ( C & C l ) in formamide. We have recently shown that the approach was successful in a study of nonionic surfactants in formamide.26
Experimental Section The ultrasonic absorption and sound velocities in the frequency range 0.4-18 MHz were measured using a modifiedversionof the Eggers resonance techniq~e.~'.~ This modified version utilizes a Hewlett-Packard 4195A network analyzer which is used in a spectrum analysis mode, and the whole setup is controlled by a microcomputer. In order to accommodateformamide as asolvent, the procedure used in assembling the mechanical cell has also been The X-cut quartz crystals were mounted onto the stainleansteel body in the usual fashionby using silicon rubber as a sealant. The narrow circular gap between the X-cut quartz crystal and the cell body is then sealed by the silicon rubber and is covered with a coating of silver dag, following which the cell was placed in an oven at 110 O C for 2 h in order to bake the silver dag. This treatment ensured that good electrical contact was achieved between the gold-platedcrystal and the cell body and also that the heat-treated silver dag was resistant enough to formamide as e solvent. To avoid any uptake of water by the solution, the measurements were carried out under an inert nitrogen atmosphere. In addition measurements were alsotaken with a pulse apparatus in the range 25-95 MHz. The surfactant used in this work (C,SyCl) was a purified commercial sample, recrystallizedat least three times from ethanoVacetonemixtures. The formamidewas a spectrophotometricgrade product supplied by Aldrich (99+% and carefullydried before use. Surfacetension ~~
(22) Auvray,X.I.;Petipas,C.;Anthore,R.;Rico,I.;Lattee,A.;AhmahZadeh Samii, A.; de Savignac, A. Colloid Polym. Sci. 1987,265, 925. (23) Auvray,X. I.;Petipas, C.; Anthore, R.;Rico,I.; Lattee, A. J.Phye. Chem. 1989,93, 7458. (24) Auvray, X . I.; Perche,T.; Anthore,R.;Petipas, C.;Rico, I.;Lattes, A. Langmuir 1991, 7, 2385. (25) Auvray, X. I.; Petipas, C.; Perche, T.; Anthore, R.; Marti, M. J.; Rico, I.; Lattes, A. J. Phys. Chem. 1990,94,8604. (26) Thomason, M. A.; Bloor, D. M.; Wyn-Jones, E. J. Chem. Phys. 1991, 95, 6107. (27) Eggers, F. Acowtica 1965, 19, 3223. (28) Eggers, F.; Funk, T. L.; Richmann, K. H. Reu. Sci. Instnrm. 1976, 42, 361.
0743-7463/9212408-2107$03.0010 0 1992 American Chemical Society
Thomason et al.
2108 Langmuir, Vol. 8, No. 9, 1992 450
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