Phase transitions in thin alkane films and alkanethiolate monolayers on gold detected with a thickness shear mode device

Abstract

A thickness shear mode (TSM) device has been used to characterize phase transitions in thin films of n-alkanes and in self-assembled monolayers (SAMs) of n-alkanethiolates on gold. For alkane films, both the frequency and conductance of the TSM device are sensitive to the viscosity changes associated with rotator (solid–solid) and melt (solid–liquid) transitions. The transitions are distinguishable through the changes they effect in the resonance frequency profiles. The thiolate monolayers exhibit two disordering transitions, manifested as changes in the TSM conductance. The structural changes associated with them have been determined by surface-enhanced Raman (SER) spectroscopy. The lower-temperature transition involves the appearance of methylene group disorder near the air/monolayer interface, while the higher temperature transition is more melt-like, as it involves the appearance of gauche rotamers deeper into the chains. This transition occurs ca. 60°C above the melting point of the bulk thiol, showing the effects of adhesive 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 alkanethiolate 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 TSM response. Our results also suggest the feasibility of using TSM devices to characterize adhesive interactions and the acoustomechanical consequences of substrate restructuring.