Surfactant Limited Aggregation of Hydrophobic Molecules in Water

(Received 28 July 1996, received in final form 11 October 1996, accepted 25 October 1996)

Abstract
The precipitation in water of hydrophobic molecules has been studied in presence of addded surfactants. Amorphous particles grow trough aggregation 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 low surfactant concentrations, an efficient regime where all surfactant molecules are adsorbed on the surfaces of the growing particles; at high surfactant concentrations, a wasteful regime, where excess surfactant 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 results are explained by a kinetic aggregation of hydrophobic molecules and adsorption of surfactant. The results of experiments are well reproduced by simulations where aggregation is allowed to proceed unimpeded for a time , and then adsorption of the surfactant starts. In these conditions, particle sizes are determined by the rate of aggregation and by the value of this time delay.