Alcohol cluster formation on silica surfaces in cyclohexane

Abstract

Adsorption of ethanol onto silica surfaces from ethanolcyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy and Fourier transform IR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long range attraction was found between the silica surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol%. At 0.1 mol% ethanol, the attraction appeared at a distance of 35 ±3 nm and turned into a repulsive force below 3.5 ± 1.5 nm upon compression. The ATR spectra demonstrated that ethanol adsorbed on the surfaces formed hydrogen-bonded clusters even in a low ethanol concentration range of 0.1–0.5 mol%, where the attractions were especially long-ranged and practically no ethanol cluster formed in the bulk solutions. The spectra also indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol molecules. We account for the observed longrange attraction by the bridging of opposed adsorption layers of ethanol on the silica surfaces.