3189
J. Phys. Chem. 1994,98, 3189-3193
Study of Aerosol OT Reverse Micelle Formation by Infrared Spectroscopy M. D’Angelo, G. Onori,’ and A. Santucci Dipartimento di Fisica, Universita’ di Perugia. V. A . Pascoli, 1-06100 Perugia, Italy Received: August 6, 1993; In Final Form: December 14, 1993”
IR spectroscopy was applied to the study of micelle formation in aerosol OT [bis(2-ethylhexyl)sodium sulfosuccinate] systems. Dependence of the infrared spectra on surfactant concentration at fixed [H20] / [surfactant] ratios in the range 0.2-10 has been determined at very low surfactant concentrations (1 X 10-5 to 1.4 X mol.L-l} where microaggregate formation is observed. Effects on the molar extinction coefficients of the bands due to C-H stretching vibrations can be used as indicators of micelle formation, and they appear to be independent of the amount of water in the system. The results are discussed in terms of pseudophase and multiple equilibrium models of micelle formation.
Introduction Despite vigorous recent discussion, the controversy concerning the nature of association of surfactant molecules in nonaqueous solventsand the applicabilityof the critical micelle concentration (CMC) concept remain unresolved.1-3 The situation is further complicated because it is believed that aggregation in apolar solvents is not possible in absence of a minimum amount of cosolubilized water. Some authors4v5suggest that the CMC must decrease when water is added; but according to Borkovec,6 the addition of water should increase the CMC. Many of the common methods for CMC determination in aqueous media are inapplicable to organic solvents because of the relatively small aggregationnumbers of surfactant molecules in the latter systems as compared to aqueous solutions. So a particular problem encountered with nonpolar surfactant solutions is the lack of suitable methods for determining critical micelle concentrations. In a previous paper’ we reported on the applicability of IR spectroscopy as a sensitive tool for the study of the state of water within reverse AOT micelles. The IR technique is noninvasive, functional-groupselective,sensitiveto the chemical environment, and it is extensively used to monitor the interactions between solutes or between solvents and solutes. So, IR spectroscopy seems to be a reliable method of solving the problem concerning the influence of water content on the formation of reversed micelles, and the existence of a well-defined critical concentration in these systems. In order to gain more insight into these problems, we report in this paper IR investigations of water-AOTarbon tetrachloride reversed micelles in the 4000-2500-~m-~range, where absorptions due to 0-H and C-H stretching modes of H2O and AOT, respectively,are present. Contributions due to vibrational modes of water and AOT are well characterized because they appear in distinct spectral ranges. This allows examination of both H20 and surfactant molecules at the same time. In this paper the surfactant concentration dependence of the IR spectra at a fixed [HzO]/[surfactant] ratio (W)in the range 0.2 < W -
D'Angelo et al.
0.4 0.3
0.2 0.1
0.0 0
2
4
6
a
10
[HzOI,/[AO T I Figure 6. Dependence of A,/A on the mole ratio of water in micellar cores to AOT (A,),area of the ith Gaussiancomponent of the entrapped waterspactrum;(A),totalpcakarea. PointsrefertoGaussiancomponents centered at (0)-3603 cm-l, (0) -3465 cm-l, and (A) -3330 cm-l. The lines serve as a guide to the eye.
the supposed competitive solubilization of water into the micelle and bulkphases and with theindicationofanappreciablevariation of the state of water within the micelle at [AOT] I 4 X 10-3 mo1.L-l (see Figure 6). The values found for [HzO]s are appreciably higher than the measured water solubility in CC4 (8 X 10-3 mo1.L-1); this fact points to an increase of water solubility in the presence of AOT and suggests (see eq 4) that the observed process should be only slightly affected by variations in [HzO],. In order to verify this prediction, we performed a new set of measurements by diluting a concentrated AOT/HzO/CCh sample at W = 10 by addition of CC4 saturated with HzO ([HzO]% = 8 X mol-L-l). The values of [HzO],/[AOT] relative to this set of measurements are reported in Figure 7 along with those previously obtained. As expected,only small differenceswere observed. Also in this case, eq 11 fits the experimental data for [AOT] > 4 X 10-3 mo1.L-1 with a constant value [HzO]s = (20 f 4) X 10-3 mo1.L-1. This result confirms that previously obtained and further supports the validity of the proposed model. Conclusions
be explained by assuming a competitive solubilization of water
into the micellar and bulk phases. Before dilution, the water concentration in the sample is
W2OI = [H201so+ WAOTI
(10)
Le., [ H z O ]=~ 2 X 10-3 mo1.L-1 present in the solvent plus the quantity added. Dilution does not alter the previous relation. Therefore by writing [HzO] in eq 10 as a sum of water dispersed in bulk phase, [HzOIs, and of entrapped water, [HzO],, one obtains
As long as the chemical potential of water inside the micellar core is constant, one expects a constant value for [HzO]~.For [HzOIs - [HzO]% = constant > 0, eq 11 predicts a progressive lowering of the [HzO],/[AOT] ratio as [AOT] is lowered. Equation 11 reproduces adequately the experimental data only for [AOT] L 4 X 10-3 mo1.L-1 and gives for [ H Z O ]a~value of (18 f 4) X 10-3 mol-L-1 and (15 f 4) X 10-3 mo1.L-1 for W = 10 and W = 7, respectively; deviations are present in the same concentration range wherevariationsin the spectrum of entrapped water are observed (Figure 6). Such a result is consistent with
Our data show that formation of aggregates from AOT molecules in the AOT/H2O/CC4 system is accompanied by a significant decrease in the molar extinction coefficient of the bands due to C-H stretchingvibrations. The experimental results indicate that, in agreement with findings on similar systems? aggregation starts already at surfactant concentrations as low as 10-5 mol-L-1. The process has been observed in the 10-5-10-3 mo1.L-l [AOT] range, and it appears to be independent of the amount of water in the system. The present IR data are equally compatible with single- or multiple-equilibriummodels. In this condition, any definition of CMC becomes arbitrary. Similar results areobtained from analysisof the concentration dependence of NMR chemical shifts for solutions of alkyl ammonium carboxylates in organic solvents.10 Moreover, our data show that when a concentrated AOT/ Hz0/CC14 sample is diluted with CC4, water is progressively released from the micelles. The effect appears at [AOT] 5 20 X 10-3mol.L-1. Thisprocessismodifiedonlya littlewhendilution is with CCl4 saturated with H20. This fact can be explained by assuming a competitive solubilization of water in the micellar and bulk phases and an increased water solubility in C C t in the presence of surfactant. The observed decrease of the entrapped water in the micellar core suggests a concurrent lowering of the micellar radius as [AOTI'is lowered. This result agrees with the
Aerosol OT Reverse Micelle Formation observation reported in ref 11 relative to a lowering of the Stokes radii for water and AOT in isooctane on dilution with isooctane.
References and Notes (1) Eicke, H.F. Top. Curr. Chem. 1980,87,86and references therein. (2) O'Connor, C. J.; Lomax,T. D.;Ramage, R. E. Adv. Colloidlnterface Sci. 1984,20, 21 and references therein.
The Journal of Physical Chemistry, Vol. 98, No. 12, 1994 3193 (3) Chcvalier,Y.;Zemb,T.Rep. Prog. Phys. l990,53,279andreferences therein* (4) Eicke, H.F.;Sheperd, J. C. W. Helv. Chim. Acta 1974,57, 1951. ( 5 ) Kotlarchyk, M.;Huang, J. S.;Chen, S. H. J. Phys. Chem. 1985,89, 4382. -. (6) Borkovec, M. J. Chem. Phys. 1989,91,6268. (7) Onori, G.; Santucci, A. J. Phys. Chcm. 1993,97,5430. ( 8 ) Chcn, S.H. Annu. Rev. Phys. Chem. 1986.37, 351. (9) Tamura, K.; Schelly, 2. A. J. Am. Chem. Soc. 1981,103,1013. (10) Muller, N.J. Phys. Chem. 1975,79, 287. (11) Zulauf, M.;Eicke, H. F. J. Phys. Chem. 1979,83,480.
