Quantitating the balance between enthalpic and entropic forces in alkanethiol/gold monolayer self assembly

We have studied the temperature and adsorbate concentration dependence of alkanethiol/gold monolayer formation in situ and in real time using a quartz crystal microbalance to monitor the rate of reaction. The temperature dependence of the experimental rate constants, for a given concentration, demonstrates that the formation of these monolayers is not diffusion rate limited for the conditions we report here. Our data are modeled accurately by the Langmuir adsorption isotherm and, using this model, we have determined the adsorption and desorption rate constants for monolayer self assembly. We have extracted from these data the thermodynamic quantities Delta G(ads), Delta H-ads, and Delta S-ads for monolayer formation. For 1-octadecanethiol adsorption from n-hexane onto gold, Delta G(ads) is temperature dependent and is ca. -5.5 kcal/mol, Delta H-ads = -20 +/- 1 kcal/mol, and Delta S-ads = -48 +/- 1 cal/(mol . K). These data have direct bearing on the broader field of interface chemistry because they underscore the dominant role that entropy plays in determining the course of chemical reactions at interfaces and the robustness of the resulting molecular assembly. We discuss the implications of these data on the stability of the monolayers and the limits that these values place on the extent of experimental control available over their formation.