Molecular Dynamics Simulations of Dioctadecyldimethylammonium Chloride Monolayers

Abstract

Molecular dynamics are performed on systems of two-dimensional periodicity composed of 64 ionic dioctadecyldimethylammonium chloride amphiphiles arranged in a monolayer at 298 K with surface coverages ranging from 57 A 2 per amphiphile to 150 A 2 per amphiphile. Bond lengths are constrained whereas valence and torsional angles interactions are described by conventional expressions. Nonbonded interactions are introduced through an anisotropic united atom method due to Toxvaerd. The amphiphiles adhere to the surface through electrostatic and nonbonded interactions between the amphiphiles and an underlying substrate. Results are presented for the density normal to the monolayer plane, order parameters, and conformational transition rates. The structure of the two-dimensional layer at the lowest headgroup area studied is quite disordered. Small islands of empty surface surrounded by amphiphiles are formed. As the headgroup area increases, we observed further translational disordering and an increase in the number of amphiphiles aligned with the surface. The g + g + tg + g + defect observed in one of the chains is stable on a timescale of 250 ps. The equilibrium structures of the two chains are quite different but the conformational dynamics as observed by the transition rates are almost indistinguishable