Viscoelastic detergent solutions

Abstract

Aqueous solutions of some cationic detergents show viscoelastic behaviour at very low concentrations. Rheological measurements indicate that the strong viscoelasticity of these solutions can be traced back to the existence of a three-dimensional structure built up from rod-like micelles. As long as the lengths L of the rods are smaller than their mean distance of separation D, the unsheared solutions are not elastic. The L values increase linearly in this concentration range with the detergent concentration and the axes of the rods seem to be randomly oriented.

In the concentration range of overlapping rods some of the experimental data can be explained on the basis of the Doi and Edwards theory for stiff rods. In order to explain the elasticity of the solution it must be assumed that a three-dimensional network is formed by the rods as soon as L > D. The network is of a temporary nature. The dynamic aspect comes about because of the continuous formation and breaking of contacts between individual free rods. The viscoelastic properties of the network solutions have been evaluated by dynamic and static rheological measurements. The effects of temperature and the chain length on the viscoelastic properties were determined and are discussed.