Chemical Effects on the Adhesion and Friction between Alkanethiol Monolayers: Molecular Dynamics Simulations

Abstract

The chemical effects on the adhesion and friction between alkanethiol self-assembled monolayers (SAMs) on gold are studied using molecular dynamics simulations. Alkanethiol SAMs (S(CH2)nX) are investigated as a function of the terminal group (X = CH3, OH, COOH) and chain length n. The adhesive force is calculated as a function of the separation between two SAMs with the same terminal group. The maximum attraction for methyl-terminated SAMs is much weaker than for the OH- and COOH-terminated SAMs which form interlayer hydrogen bonds. The COOH-terminated SAMs have the strongest attraction. For the COOH- and OH-terminated SAMs, the adhesion−separation curves depend strongly on n. In both cases the change in the position of the attractive minimum is large as a function of n. The magnitude of the attractive minimum varies little for OH-terminated SAMs, and the variation is also small for COOH-terminated SAMs once the odd:even effect is considered. The source of the n dependence is structural changes within the...