The interactions between surfactants and vesicles: dissipative particle dynamics.

The interactions between surfactants and vesicles formed by double-tail amphiphiles are investigated by the dissipative particle dynamics. As the surfactant concentration is increased, vesicle solubilization can be generally described by the three-stage hypothesis including vesicular region, vesicle-micelle coexistence, and mixed micellar region. We study the partition of surfactants between the bilayer phase and the aqueous phase where a higher value of K indicates that more surfactant molecules are incorporated in the bilayer. It is found that ln(K(-1)) is proportional to the hydrophile-lipophile balance (HLB), which depicts the degree of hydrophilicity associated with a surfactant. As the overall hydrophilicity of surfactants increases, i.e., higher HLB, K declines and vice versa. When the amounts of surfactants reach a critical point, the solubilization begins and the coexistence of vesicles and mixed micelles is observed. Further increase in the surfactant concentration results in total collapse of the vesicle. Consistent with experimental observations, the three stages are identified through the vesicle size-surfactant concentration relation. Our simulations clearly demonstrate the process of the vesicle solubilization and confirm the validity of the three-stage hypothesis.