In Situ, Vibrationally Resonant Sum Frequency Spectroscopy Study of the Self-Assembly of Dioctadecyl Disulfide on Gold

We report the results of an in situ, vibrationally resonant sum frequency generation (SFG) spectroscopy study of the assembly of perdeuterated dioctadecyl disulfide on gold substrates from ethanol solutions under laminar flow conditions. The coverage evolution of the SFG spectra can be well described by the coexistence of two distinct phases: a low-coverage, disordered phase and the full-coverage crystalline phase. The structure of the adsorbed thiolate fragments in the low-coverage phase is disordered but upright (as opposed to lying completely in the surface plane), characterized by significant gauche defects in the backbone but a near-normal orientation for the terminal methyl group. The crystalline phase is marked by an erect all-trans configuration of the alkane chain. The kinetics of the evolution of these two phases can be quantitatively described by a simple model, consistent with phase coexistence above a critical density of the disordered phase.