Alkane adsorption from vapour onto hydrophobic solid/vapour and hydrophobic solid/water interfaces

Abstract

In connection with our interest in the effect of adsorption of alkanes on the wettability by water of hydrophobic solid surfaces, we have determined adsorption isotherms for a range of alkane vapours onto methyl-terminated self-assembled alkanethiol monolayers. The adsorption has been measured using a quartz crystal microbalance. The isotherms are well-fitted using the isotherm equation proposed by Aranovich and Donohue. It is found that alkane adsorption is only slightly affected by the alkanethiol chain length (between 10 and 18). Mixed alkanethiol layers (10+18) however lead to enhanced alkane adsorption. We have compared adsorption of alkanes onto the crystalline thiol layers with that observed for liquid-like surfactant monolayers on water, where mixing of alkane with surfactant alkyl chains is known to occur. Using surface pressures of "‘saturated’' alkane layers on the solid surfaces, together with various measured contact angles, we have obtained surface pressures of alkanes at the hydrophobic solid/water interface. The adsorption of alkanes onto solid surfaces, the solid/water interface and into surfactant monolayers has been treated successfully using simple theory of van der Waals forces together with reasonable values of Hamaker constants.