Viskoelastische Tensidlösungen

Abstract

Abstract Classical and dynamic light scattering, electric birefringence, kinetic, surface tension, conductivity and rheological measurements were carried out on so called viscoelastic detergent solutions of Cetylpyridiniumsalicylate (CPySal). The results indicate that spherical micelles are formed in these solutions above the critical micelle concentration (cmc) = 1.5 · 10 −4 M. From the cmc on the concentration of the micelles increases linearly with the total detergent concentration up to a second critical concentration (cmc 11 = 4.5 · 10 −4 M). Upon further increase of the total concentration, the concentration of the micelles remains constant and the globular micelles grow to rodlike micelles. The length of the rods increases linearly with the total concentration. The residence time of a detergent molecule in the rod is longer than in the globular micelle. The surface charge density of the rods is smaller than for the spherical micelles. In unstirred solutions the rods are randomly oriented but their center of masses are ordered. As a consequence the rods can rotate freely as long as their main axis is shorter than the mean distance between the aggregates but their translational diffusion is hindered by the existence of lattice like ordering. Shearing of the solution leads to large areas of a new metastable nematic type phase in which the rods are oriented with respect to each other. These oriented domains are the source for the apparent elasticity of the solutions. When the shearing of the solution is stopped, the elasticity decays with rather long time constants that can amount to several minutes. — The complete equilibration of the solution after the system was perturbed by dilution or a change in temperature takes up to several hours.