Ageing of alkanethiol self-assembled monolayers

Self-assembled monolayers of three straight-chain alkanethiols [CH3(CH2)(n)SH, n = 11, 15, 17] have been formed on a gold substrate from cyclohexane solution and characterised using IR spectroscopy. Over a period of ca. six months, the relative intensities of the C-H stretching bands in the IR spectra were observed to change. These changes are explained by a tilting of the molecules from their initial position away from the surface normal upon oxidation of the thiolate root. This temporal dependence of the tilt angle has implications for the stability of self-assembled monolayers.