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Langmuir 1996, 12, 2900-2905
Microviscosity and Aggregation Number of Potassium N-Acylalaninate Micelles in Potassium Chloride Solution Shigeyoshi Miyagishi,* Hiroshi Suzuki, and Tsuyoshi Asakawa Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920, Japan Received November 1, 1995. In Final Form: March 18, 1996X Microviscosity, micellar aggregation number, and the critical micelle concentration (cmc) of potassium N-acylalaninates were determined in KCl solutions by fluorescence probe methods together with their solution viscosity. Two inflection points were observed on the plot of microviscosity vs KCl concentration. The first and second inflection points were attributed to the beginning of micellar growth and intermicellar interaction, respectively. At a KCl concentration above the first inflection point, the micellar aggregation number steeply increased and the log-log plot of cmc against KCl concentration deviated downward from a linear Corrin-Harkins relation. Above the second inflection point, the micellar solutions of N-acylalaninates became very viscous and exhibited a constant microviscosity. Comparison with the previous data for N-acylvalinates (Langmuir 1995, 11, 2951) revealed that microviscosity on the micellar surface was larger in N-acylalaninates than in N-acylvalinates, while microviscosity in the micellar core was the same in the two surfactant series. Each N-acylalaninate solution was divided into four regions (solution of monomer surfactant, region of globular micelles, region comprising large micelles followed by micellar growth, and region where there is strong intermicellar interaction). Each boundary between the regions shifted to lower concentrations of KCl and N-acylalaninate as the acyl chain became long.
Fluorescence probe methods have often been used for studying the structure of micelles, microemulsions, and vesicles.1,2 They are particularly effective for determination of microviscosity, micropolarity, and aggregation number of a micelle. Recent studies of surfactant solutions using fluorescence probes revealed that microviscosity probes are very effective for determination of the transition point of micelle shape and the beginning point of intermicellar interaction.3,4 It was found in the studies that microviscosity in a micelle increases with increasing micellar size and exhibits a steep increment at a sphereto-rod micellar transition point, while micropolarity is not sensitive to such a transition. The microviscosity of alkyltrimethylammonium bromide reached a constant value above a micelle overlapping point (at which the micelles begin to contact each other).3 On the other hand, the microviscosity of sodium N-acylvalinate still continued to increase above its micelle overlapping point and reached a constant value at a point where the micelles interacted strongly with each other (entanglement point).4 If the micellar interaction might be stronger in the former system than in the latter, these results suggest that strong micellar interaction induces intermicellar exchange of a probe molecule, following virtually constant fluorescence intensity. However, when the micellar interaction is not strong, micellar growth is a more dominant factor in the fluorescence intensity compared with the intermicellar exchange of the probe. In addition, there is the fact that the aggregation numbers of rodlike micelles of alkyltrimethylammonium bromides depend little on the surfactant concentration at a constant * To whom correspondence may be addressed: Fax, 81-762-344800; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19, 675. (2) Zana, R. Surfactant SolutionssNew Methods of Investigation; Zana, R., Ed.; Marcel Dekker, Inc.: New York, 1987; p 241. (3) Miyagishi, S.; Kurimoto, H.; Asakawa, T. Bull. Chem. Soc. Jpn. 1995, 68, 135. (4) Miyagishi, S.; Kurimoto, H.; Asakawa, T. Langmuir 1995, 11, 2951.
S0743-7463(95)00554-3 CCC: $12.00
salt concentration,5 while the aggregation numbers of N-acylvalinate micelles obviously increase with increasing surfactant concentration.4 In the present stage, no welldefined relation has been found among intermicellar growth, and microviscosity. More experimental data are necessary to clarify the relation and then to utilize the relation as a reliable means of identifying micellar states. Microviscosity probes were also found in our previous report6 to be useful in determining critical micelle concentrations. These reports stimulate further utilization of microviscosity probes in the research field of micellar solutions. It is very significant to confirm and develop the probability of microviscosity probes in order to examine micelle formation, micellar states, vesicle states, and colloidal domains. From this point of view, the fluorescent probe method was used in the present work. Amino acid surfactants are soluble even at relatively high salt concentrations and then are expected to dissolve in different micellar states depending on both the surfactant concentration and salt concentration. In this and subsequent papers, we present the application of the fluorescence probe method to some amino acid surfactant solutions of which the micellar structure, aggregation number, solution viscosity, and the salt effect on them have not been known. Experimental Section Materials. N-Acylalaninates were synthesized by the reaction of alanine with acyl chlorides as described previously.7,8 The solutions of the corresponding potassium salts were prepared by dissolving the N-acylalaninates in 10 mM excess potassium hydroxide solution. Fluorescence probes, auramine (guaranteed reagent, Kanto Chemical Co.), 1,3-dipyrenylpropane (P3P, Dojin, Wako Pure Chemical Ind, Ltd.), and pyrene (guaranteed reagent, (5) Imae, T.; Abe, A.; Ikeda, S. J. Phys. Chem. 1988, 92, 1548. (6) Miyagishi, S.; Kurimoto, H.; Ishihara, Y.; Asakawa, T. Bull. Chem. Soc. Jpn. 1994, 67, 2398. (7) Miyagishi, S.; Nishida, M. J. Colloid Interface Sci. 1978, 65, 380. (8) Miyagishi, S.; Higashide, M.; Asakawa, T.; Nishida, M. Langmuir 1991, 7, 51.
© 1996 American Chemical Society
Microviscosity in N-Acylalaninate Micelles
Langmuir, Vol. 12, No. 12, 1996 2901 Table 1. Micellar Aggregation Number of Potassium N-Acyl-DL-alaninates dodecanoyl 25 50 100 hexadecanoyl KCl/mol mmol mmol mmol tetradecanoyl dm-3 dm-3 dm-3 100 mmol dm-3 100 mmol dm-3 dm-3
Figure 1. Dependence of micellar aggregation number on KCl concentration in 100 mM potassium N-acylalaninates.
0 0.01 0.05 0.1 0.2 0.3 0.45 0.5 0.6 0.7 0.9 1.0 1.2 1.5 1.7 2.0 2.5
57
57
69
70
58 60 70
78
78
93
72
91 94 108 109 110 118 125 135 173
84 90 103 107 110
89
91
105 113 123
106 118 129
118 127 134 171
116 138 162
Table 2. Cmc’s of Potassium N-Acyl-DL-alaninates
Katayama Chemical Co.) were used as received. The typical concentrations of these probes were 1 × 10-5, 1 × 10-6, and 1 × 10-7 mol dm-3, respectively. Procedures. Fluorescence intensities of the probes in the surfactant solutions were measured on a Hitachi fluorescence spectrophotometer F-3010 equipped with a temperature control unit. The fluorescence intensity of auramine was measured at 464.4 nm (excited at 390 nm). The ratio of the fluorescence intensity in a surfactant solution (I) and the intensity in an aqueous solution containing no surfactant (I0) was used as a measure of microviscosity. The monomer fluorescence intensity (IM) of P3P and its excimer intensity (IE) were measured at 377 and 484 nm, respectively. Microviscosity was estimated from the ratio of IM against IE. Aggregation numbers of the micelles were estimated by a static quenching method of pyrene fluorescence using cetylpyridinium chloride as a quencher.9 Solution viscosities and the critical micelle concentrations (cmc) were determined in the same manner as the procedures given in our previous paper.3,4 All experiments were performed at 25 °C.
Results and Discussion Aggregation numbers. Micellar aggregation numbers (N) can be determined from the slope of the natural logarithmic plot of In/Iq of pyrene vs the quencher concentration where In and Iq are the fluorescence intensity of pyrene in the absence and presence of a quencher.9 Such plots had a linear relation for each N-acylalaninate. The values of N determined for potassium N-dodecanoyl-, N-tetradecanoyl-, N-hexadecanoyl-dl-alaninates are given in Figure 1 and Table 1. The N-acylalaninate micelles of a longer acyl group always had a larger aggregation number at a constant KCl concentration. The higher the KCl concentration, the greater was the dependence of the aggregation number on surfactant concentration. Namely, addition of more KCl have a threshold KCl concentration strikingly facilitated micellar growth. Some ionic surfactant micelles exhibit the so-called sphere-rodlike transition when surfactant and/or salt concentrations reach a threshold value.4,10-12 This transition follows steep growth of micellar aggregation number with increasing salt addition. A similar increasing trend of the micellar aggregation number was observed in the (9) Perez-Beeito, E.; Rodenas, E. J. Colloid Interface Sci. 1990, 139, 93. (10) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90, 5435. (11) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1982, 87, 424. (12) Ikeda, S. Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 825.
cmc/mmol dm-3 KCl/mol dm-3 0 0.01 0.05 0.1 0.2 0.3 0.45 0.5 0.6 0.7 0.9 1.0 1.2 1.5 1.7 2.0 2.5 3.3 a
dodecanoyl 8.5 5.5 3.5 2.4 2.0 1.6
8.8a 4.0a 1.6a
tetradecanoyl 1.1 0.68 0.44 0.33 0.16 0.13
1.2
0.083
0.89 0.69 0.66
0.077 0.048 0.042 0.042 0.030 0.025 0.012
0.39 0.33 0.24 0.18 0.080
0.36a
1.2a 0.40a 0.14a
0.049a
hexadecanoyl 0.13 0.076 0.044 0.026 0.018 0.013 0.012 0.011 0.0088 0.0072 0.0071 0.0053 0.0030
0.12a 0.044a 0.013a
0.0054a
0.030a
0.19a
Auramine method.
systems of N-acylalaninates (Figure 1). The KCl concentration at the corresponding transition point was 1.36, 0.66, and 0.44 mol dm-3, respectively, in 100 mmol dm-3 solution of N-dodecanoyl-, N-tetradecanoyl-, and Nhexadecanoylalaninates. The aggregation numbers were not as large at low KCl concentration compared with those above the transition points and were comparable with that of globular micelles. Cmc’s. The critical micelle concentration (cmc) can be easily determined from a plot of fluorescence intensity of auramine vs surfactant concentration as the intensity increases steeply at its cmc.6 The results for cmc obtained by the auramine method are in good agreement with those by a surface tension method, as compared in Table 2. This agreement provides further support of our emphasis that auramine is one of the most useful probes for the micellar solutions. Generally, the relation of cmc to salt concentration can be represented as a Corrin-Harkins plot for ionic surfactants. In the present systems, the following equations were obtained:
for the potassium N-dodecanoylalaninate (
