Study on the aggregation number of mixed micelles in aqueous binary mixtures of the bile salts and nonionic surfactant

Abstract

The mixed micellar behavior of sodium cholate, NaC, and octa-oxyethylene glycol n-decyl ether, C 10E 8, has been studied from the viewpoints of the aggregation number and the polarity of the interior of the micelle. The polarity of the interior of the mixed micelle obtained from I 1 I 3 and the lifetime of pyrene fluorescence shows that the solubilization site of pyrene varies with the molar fraction. This fact is considered to be caused by the differences in the chemical structure and the hydrophobic nature between NaC and C 10E 8. Concentration dependencies are seen for the aggregation number and I 1 I 3 . These dependencies indicate that the size and composition in the mixed micelles vary with the total concentration of surfactants, and that the initial micelles formed near the CMC contain higher molar fractions of C 10E 8 than do analytical molar fractions. Furthermore, theoretical calculations using regular solution theory and excess thermodynamic quantities have been performed with the values of CMCs. The calculated values of the molar fraction in the micelle were lower than those of the analytical molar fraction; however, there was a large difference between the values of the two methods. The experimental results agreed best with ones obtained from the excess thermodynamic quantities.