Dynamical Behavior Of Micellar Surfactant Solutions

Abstract

Surfactants in solution often self-assemble into spherical micelles. Micellar surfactant solutions display a wide range of dynamic processes. The mechanisms of some such processes remain poorly understood. Three distinct dynamic pro- cesses are studied in this work: single molecule exchange, creation and destruc- tion of micelles, and adsorption of surfactant from a micellar solution onto an initially bare interface. Exchange of individual molecules between a micelle and the surrounding solutions is the most important elementary dynamical process in a micellar so- lution. A combination of molecular dynamics simulation and self consistent field calculations is used to study this process for micelles of diblock copolymer surfactants in both small molecule and polymeric solvents. For micelles in a polymeric solvent considered here, it is found that the corona does not pose a significant barrier to insertion. Studies of micelles of copolymer with a long corona block in a small molecule solvent instead show a measurable insertion barrier due to coronal stretching. Overall rates of insertion are found to depend very weakly on corona block length, however, because of compensating effects of the changes in the concentration of free molecules that coexist with micelles and changes in the barrier to insertion. Studies of relaxation of homogeneous micellar solutions after a weak pertur- bation show the existence of fast and slow processes with disparate relaxation times. The fast process is a relaxation in aggregation number via insertion and expulsion of individual molecules. The slow process is associated with creation and destruction of entire micelles. Novel simulation techniques are used here to determine the mechanism of the slow process. In systems of more soluble surfactants, micelles are created and destroyed by a sequence of single-molecule insertion or expulsion events. In systems of more sparingly soluble surfactants, the number of micelles changes by fission and fusion events. When a micellar solution is exposed to an initially bare interface the re- sulting relaxation is highly nonlinear and may exhibit several distinct behavior regimes. This process is studied by developing approximate models for particu- lar time and parameter regimes and comparing the resulting predictions to the results of full numerical solutions for the relevant diffusion reaction equations for a polydisperse solution. Micelle dissociation is found to be rapid near the inter- face during the early phases of adsorption, even in systems in which micelles are very long-lived in equilibrium, because a depletion of local free molecule concen- tration near the interface removes the barrier to stepwise micelle dissociation. Dissociation of micelles near the interface can lead to formation of a micelle free region near the interface.