MOLECULAR-DYNAMICS SIMULATIONS OF DIOCTADECYLDIMETHYLAMMONIUM CHLORIDE MONOLAYERS

Molecular dynamics are performed on systems of two-dimensional periodi city composed of 64 ionic dioctadecyldimethylammonium chloride amphiph iles arranged in a monolayer at 298 K with surface coverages ranging f rom 57 Angstrom(2) per amphiphile to 150 Angstrom(2) per amphiphile. B ond lengths are constrained whereas valence and torsional angles inter actions are described by conventional expressions. Nonbonded interacti ons are introduced through an anisotropic united atom method due to To xvaerd. The amphiphiles adhere to the surface through electrostatic an d nonbonded interactions between the amphiphiles and an underlying sub strate. Results are presented for the density normal to the monolayer plane, order parameters, and conformational transition rates. The stru cture of the two-dimensional layer at the lowest headgroup area studie d is quite disordered. Small islands of empty surface surrounded by am phiphiles are formed. As the headgroup area increases, we observed fur ther translational disordering and an increase in the number of amphip hiles 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 equilibri um structures of the two chains are quite different but the conformati onal dynamics as observed by the transition rates are almost indisting uishable.