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 monolayer s of long chain carboxylic acids and alcohols supported on the 0001 fa ce of ice Ih. The simulations are designed to provide information conc erning the influence of the atomic structure of the head group and of the nature of the head group-surface interactions on the collective ti lt of the molecules in the monolayer. The results of the simulations s how that the structure of the head group is a more important influence on the collective tilt of the molecules than is the specific characte r of the head group-surface interactions. It is also shown that the co nstraint imposed by the potential field of the rigid ice Ih 0001 surfa ce renders the ice supported monolayer different from the liquid water supported monolayer. Specifically, the collective tilt of the molecul es in the ice supported monolayer is not accompanied by a distortion o f the projection of the unit cell of the monolayer in the plane of the surface; that projection remains hexagonal for all tilt angles. In co ntrast, the introduction of a collective tilt of the molecules in a li quid water supported monolayer always leads to a distortion of the pro jection of the unit cell in the plane of the surface, typically from h exagonal at zero tilt to centered rectangular for all nonzero tilt. Th e tilting of the monolayer supported on ice Ih is favored by the molec ular spacing induced by the field of the 0001 surface, which is slight ly greater than closest packing separation of the molecules. The equil ibrium tilt angle appears to be determined primarily by the chain-chai n interactions since the surface pressure attained at equilibrium is s ensibly independent of the precise value of the tilt angle and of the character of the head group-head group and the head group-surface inte ractions.