Thermal stability of octadecyltrichlorosilane self-assembled on a polycrystalline aluminium surface

In search for a good boundary lubricant additive for aluminium substrates we investigate the molecular disorder retained in a monolayer self-assembled on an aluminium surface subjected to thermal cycling. Our target molecule is octadecyltrichlorosilane (C18H37SiCl3, OTS) but we also study octadecanoic acid (C17H35COOH) to explore the effect of changing the molecular head group. The two molecules have the same alkyl back bone, same terminal group but different head groups; –SiCl3 and –COOH respectively, both however adhere to the aluminium substrate through oxygen atoms by chemisorption bonding. We conduct the study using grazing angle Fourier transform infrared spectroscopy. Our studies show that OTS which is able to polymerize into a two-dimensional network when self-assembled on the aluminium surface is able to retain partial molecular order when subjected to thermal cycles. In contrast a self-assembled monolayer (SAM) of the octadecanoic acid, which does not polymerize on the aluminium surface, become totally disordered when heat treated.