INTERACTION BETWEEN GELATIN AND ANIONIC SURFACTANTS

The effects of alkyl chain length of homologous alkyl sulfate surfacta nts ranging from C-8 to C-14 on the diffusion behavior of both the sur factant and gelatin have been investigated by pulsed-gradient spin-ech o NMR spectroscopy. Changes in the diffusivity of the surfactant can b e rationalized in terms of a two-state model consisting of gelatin-bou nd micelles in equilibrium with freely diffusing unimeric surfactant. A minimum in the diffusivity of gelatin is observed when the binding o f surfactant amounts to about 1 micelle/strand. The depth of of this m inimum increases with the chain length of the surfactant. These effect s are explained in terms of micelle-mediated transient cross-links as proposed by Greener et al. (Macromolecules 1987, 20, 2490). The effect ive strength of the cross-links is a decreasing function of the number of micelles/strand because of the electrostatic repulsion between the micelles; the strength increases with an increase in the size of the micelles.