SIMULATIONS OF SELF-ASSEMBLED MONOLAYERS WITH THE SAME SURFACE-DENSITY BUT DIFFERENT GRAFTING PATTERNS

Molecular dynamics simulations have been carried out for self-assembled monolayers of alkylsiloxane, molecules on planar silica substrates. The simulation system consisted Of 36 alkylsiloxane molecules terminated by hydroxyl groups arranged in a simulation cell with periodic boundary conditions in the plane of the surface. Two models have been studied as alternative representations of the attachment of the alkylsiloxanes to the surface: a square lattice (model I) and a triangular lattice (model II), formed by the head groups of the chains with the same:surface density. After equilibration at 300 or 350 K, the two models yield monolayers in which the chains exhibit a different orientation, which suggests that the tilt angle is not determined by surface density alone. The average thickness of the monolayers for model I is smaller than that for model II because the chains in model II have a smaller tilt;angle. When the tilt angle of a chain is large, the chain is found to favor an orientation in which the plane containing the carbon backbone is perpendicular to the plane formed by the chain axis and the surface normal. The fraction of gauche conformation is 2.6%, and most of the gauche defect occurs at the last C-C bond near the tail of the chains. The surface pattern formed by the tail oxygen atoms is disordered, and the approach between these oxygen atoms may be;the result of the attraction between oxygen and hydrogen atoms.