Interplay between micelle formation and waterlike phase transitions

摘要

A lattice model for amphiphilic aggregation in the presence of a structured waterlike solvent isstudied through Monte Carlo simulations. We investigate the interplay between the micelleformation and the solvent phase transition in two different regions of temperature-density phase diagram of pure water. A second order phase transition between the gaseous (G) and high densityliquid (HDL) phases that occurs at very high temperatures, and a first order phase transition betweenthe low density liquid (LDL) and (HDL) phases that takes place at lower temperatures. In both cases, we find the aggregate size distribution curve and the critical micellar concentration as a function of the solvent density across the transitions. We show that micelle formation drives theLDL-HDL first order phase transition to lower solvent densities, while the transition G-HDL is driven to higher densities, which can be explained by the markedly different degrees of micellizationin both cases. The diffusion coefficient of surfactants was also calculated in the LDL and HDL phases, changing abruptly its behavior due to the restructuring of waterlike solvent when we crossthe first order LDL-HDL phase transition. To understand such behavior, we calculate the solvent density and the number of hydrogen bonds per water molecule close to micelles. The curves of the interfacial solvent density and the number of hydrogen bonds per water molecule in the firsthydration signal a local phase change of the interfacial water, clarifying the diffusion mechanism of free surfactants in the solvent.