Collective degrees of freedom involved in absorption and desorption of surfactant molecules in spherical non-ionic micelles

Abstract

Dynamics of absorption and desorption of a surfactant monomer into and out of a spherical non-ionic micelle is investigated by coarse-grained molecular dynamics (MD) simulations. It is shown that these processes involve a complex interplay between the micellar structure and the monomer configuration. A quantitative model for collective dynamics of these degrees of freedom is developed. This is accomplished by reconstructing a multi-dimensional free energy landscape of the surfactant-micelle system using constrained MD simulations in which the distance between the micellar and monomer centers of mass is held constant. Results of this analysis are verified by direct (unconstrained) MD simulations of surfactant absorption in the micelle. It is demonstrated that the system dynamics is likely to deviate from the minimum energy path on the energy landscape. These deviations create an energy barrier for the monomer absorption and increase an existing barrier for the monomer desorption. A reduced Fokker-Planck equation is proposed to model these effects. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4762816]