DIFFUSION AND INTERACTION IN GELS AND SOLUTIONS .5. NONIONIC MICELLAR SYSTEMS

Studies are presented on the nonionic micellar systems hexaethylene (C12E6) and octaethylene (C12E8) glycol dodecyl ether in gels and solutions of kappa-carrageenan. Aggregation numbers and size distributions of the micelles were studied with a fluorescence quenching method, while self-diffusion coefficients of the surfactants were determined with a tracer technique. It was found that the interaction between the surfactants and the polymers was repulsive, as revealed by a slight increase in micelle size in the presence of polymers. The behavior of C12E8 could be described by hard-sphere diffusion of the micelles, both in water and in the polymer systems. Thus, the diffusion in a K+-kappa-carrageenan gel system, where the chains adopt a helical conformation, was found to be faster than in a Na+-kappa-carrageenan solution of polymers in the coil conformation. Concerning C12E6, it was found that these micelles were larger than the C12E8 micelles and adopted a prolate shape, of which the axial ratio increased with concentration. However, in contrast to C12E8, the observed C12E6 diffusion in polymer systems did not depend on the structure in the polymer system. Instead, the intermicellar diffusion of the monomer was substantially contributing to the observed results, due to first, a strongly reduced aggregate diffusion and, second, the micelles being pushed closer together by the polymer chains. The latter effect was also shown by calculations of the radial pair distribution function for hard spheres in polymer networks. By considering a water phase in equilibrium with a polymer phase, both containing hard spheres, a measure of the local micelle concentration in the polymer networks was obtained from a simple theory. These results compared well with simulations and then formed the basis for quantitative predictions with the prolate cell model.