The structures of hexadecylamine films adsorbed on iron-oxide surfaces in dodecane and hexadecane

Molecular-dynamics simulations are used to gain insights on recent sum-frequency spectroscopy and polarised neutron reflectometry measurements of the structure of hexadecylamine films adsorbed on iron-oxide surfaces in dodecane and hexadecane. Simulations were carried out under quiescent and high-shear conditions. Mass-density profiles, molecular-height and molecular-orientation probability distribution functions, and in-layer radial distribution functions were calculated. The simulation results show that at high surface coverage, the film thickness is about 15 Å, and that the molecules are mainly pointing upwards from the surface at an angle of 40-50°. The results are compared critically against published experimental results, and the agreement is found to be good. The in-layer ordering of the hexadecylamine head-group atoms is found to be dictated by the crystalline structure of the iron-oxide surface, but this influence rapidly diminishes along the molecular backbone. The tail-group atoms show almost no positional ordering. Finally, an example calculation of the kinetic friction coefficient under high-shear conditions is presented. The lateral (friction) force is measured as a function of the normal (applied) force, and the kinetic friction coefficient is determined to be about 0.09, which is typical for this kind of system.