Phase transitions of alkanethiol self-assembled monolayers at an electrified gold surface

We report an in situ vibrational study of the reductive desorption and the oxidative adsorption of an alkanethiol monolayer on Au(111) in an alkaline solution between 5 and 37 degreesC. Cyclic voltammograms show that the reduction of a hexadecanethiol monolayer gives rise to two current peaks at low temperature. These two peaks slowly merge into a single peak as the temperature increases. The oxidative chemisorption displays a similar behavior. Three oxidative current peaks are observed at low temperature and only one at higher temperature. In situ vibrational spectroscopy shows that these electrochemical changes are related to a sharp phase transition. A sudden increase of the wavenumber of the methylene asymmetric CH stretching d(-) band from 2918 cm(-1) below 12 degreesC to 2927 cm(-1) above 12 degreesC indicates an abrupt disordering of the monolayer. A sharp transition at the same temperature of 12 degreesC is observed for the intensity of the differential reflectance CH stretching bands. This shows that changes in orientation of the alkane chains occur simultaneously with the disordering of the monolayer. We suggest that the phase transition from an ordered phase to a disordered phase is related to a sudden increase of ion permeation into the hexadecanethiol monolayer above 12 degreesC.