SURFACTANT LIMITED AGGREGATION OF HYDROPHOBIC MOLECULES IN WATER

The precipitation in water of hydrophobic molecules has been studied i n presence of added surfactants. Amorphous particles grow through aggr egation of clusters of hydrophobic molecules; the growth is terminated by adsorption of surfactant. The particle sizes vary according to the concentrations of hydrophobic molecules and of surfactant molecules. Two regimes have been found for the use of surfactant molecules: at lo w surfactant concentrations, an efficient regime where all surfactant molecules are adsorbed on the surfaces of the growing particles; at hi gh surfactant concentrations, a wasteful regime, where excess surfacta nt molecules are left in water. Attempts to reduce the particle sizes by adding increasing amounts of surfactant become inefficient at some point where most of the added surfactant remains in water. These resul ts are explained by a kinetic aggregation model which simulates the co mpetition between aggregation of hydrophobic molecules and adsorption of surfactant. The results of experiments are well reproduced by simul ations where aggregation is allowed to proceed unimpeded for a time ta u, and then adsorption of the surfactant starts. In these conditions, particle sizes are determined by the rate of aggregation and by the va lue of this time delay.