Micelle Formation of Anionic Surfactant with Divalent Counterion of Separate Electric Charge

The critical micelle concentration (CMC) of the anionic surfactant, 1,1'-(1,omega-decanediyl) bispyridinium hexadecane-1-sulfonate (C10BP(C16)2) was determined by electrical conductivity measurements at various temperatures. The degree of counterion binding to micelles was evaluated from the change in CMC with total counterion concentration. The molecular weight of the micelles was determined by static light scattering. The mass action model was applied to micelle formation in order to calculate the three micellization parameters, the micellization constant, the micelle aggregation number, and the number of counterions per micelle. Thermodynamic parameters (DeltaG0, DeltaH0, -TDeltaS0) for the micellization were evaluated by their temperature dependence. The findings were: (1) Micelle formation was entropy-driven over the whole temperature range examined. (2) C10BP(C16)2 had a higher degree of counterion binding to micelles compared with those of monovalent counterion. (3) The plots of log CMC against the carbon number of the homologous surfactant ions gave a straight line, indicating that free energy change per methylene group for micelle formation was -1.18RT for surfactant ions. Copyright 1999 Academic Press.