EVALUATION OF VISCOSITY MEASUREMENTS OF DILUTE-SOLUTIONS OF IONIC SURFACTANTS FORMING ROD-SHAPED MICELLES

A theoretical model that describes the association of ionic surfactants into rodlike micelles under equilibrium conditions is constructed. The model is based on the concept of counterion binding. Moreover, a simplified formula for the concentration-dependent viscosity increase of a solution of rodlike colloids is developed, that provides a good approximation of the exact equation of Kuhn and Eisenschitz. In combination with the association model it is possible to evaluate all thermodynamic model parameters from viscosity and surface tension measurements. This approach is applied to micellar solutions of hexadecylpyridinium nitrate and bromide at various electrolyte concentrations. It is shown that the pronounced effect of counterion concentration on the growth of rod-shaped micelles reflects a small excess of counterion association in the cylindrical part of the micelle. The degree of counterion association is only by about 0.025 larger in this part of the micelle than in the hemispherical caps. © 1990 American Chemical Society.