PHASE-TRANSITIONS IN THIN ALKANE FILMS AND ALKANETHIOLATE MONOLAYERS ON GOLD DETECTED WITH A THICKNESS-SHEAR MODE DEVICE

A thickness shear mode (TSM) device has been used to characterize phas e transitions in thin films of n-alkanes and in self-assembled monolay ers (SAMs) of n-alkanethiolates on gold. For alkane films, both the fr equency and conductance of the TSM device are sensitive to the viscosi ty changes associated with rotator (solid-solid) and melt (solid-liqui d) transitions. The transitions are distinguishable through the change s they effect in the resonance frequency profiles. The thiolate monola yers exhibit two disordering transitions, manifested as changes in the TSM conductance. The structural changes associated with them have bee n determined by surface-enhanced Raman (SER) spectroscopy. The lower-t emperature transition involves the appearance of methylene group disor der near the air/monolayer interface, while the higher temperature tra nsition is more melt-like, as it involves the appearance of gauche rot amers deeper into the chains. This transition occurs ca. 60 degrees C above the melting point of the bulk thiol, showing the effects of adhe sive interactions on phase behaviour. Irreversible changes in the SER spectra and TSM response of alkanethiolate SAMs at higher temperatures may be due to adsorbate-induced restructuring of the gold surface and the movement of gold thiolates (Au-S-R). Both alkane thin films and a lkanethiolate monolayers exhibit small structural changes that affect the adlayer viscoelasticity; we have demonstrated that the structural changes, even in monolayers, can be detected through changes in the TS M response. Our results also suggest the feasibility of using TSM devi ces to characterize adhesive interactions and the acoustomechanical co nsequences of substrate restructuring.