A MOLECULAR-DYNAMICS STUDY OF THE STRUCTURE OF A LONG-CHAIN AMPHIPHILE MONOLAYER ADSORBED ON ICE IH

We report the results of molecular dynamics simulations of a monolayers of long chain carboxylic acids and alcohols supported on the 0001 face of ice Ih. The simulations are designed to provide information concerning the influence of the atomic structure of the head group and of the nature of the head group-surface interactions on the collective tilt of the molecules in the monolayer. The results of the simulations show that the structure of the head group is a more important influence on the collective tilt of the molecules than is the specific character of the head group-surface interactions. It is also shown that the constraint imposed by the potential field of the rigid ice Ih 0001 surface renders the ice supported monolayer different from the liquid water supported monolayer. Specifically, the collective tilt of the molecules in the ice supported monolayer is not accompanied by a distortion of the projection of the unit cell of the monolayer in the plane of the surface; that projection remains hexagonal for all tilt angles. In contrast, the introduction of a collective tilt of the molecules in a liquid water supported monolayer always leads to a distortion of the projection of the unit cell in the plane of the surface, typically from hexagonal at zero tilt to centered rectangular for all nonzero tilt. The tilting of the monolayer supported on ice Ih is favored by the molecular spacing induced by the field of the 0001 surface, which is slightly greater than closest packing separation of the molecules. The equilibrium tilt angle appears to be determined primarily by the chain-chain interactions since the surface pressure attained at equilibrium is sensibly independent of the precise value of the tilt angle and of the character of the head group-head group and the head group-surface interactions.