Surface induced hydrogen-bonded macrocluster formation of methanol on silica surfaces

Recently, we have succeeded in identifying the structure of the adsorption layer of ethanol on a silica surface in cyclohexane to be a hydrogen-bonded linear aggregate (polymer), which we call a surface molecular macrocluster. In this work, we studied the effect of the miscibility of liquids on the formation of the surface molecular macroclusters for confirming that this is a surface induced phenomenon. We investigated the interaction and the structure of methanol adsorbed on a silica surface in methanol-cyclohexane binary liquids by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode, and compared the results with those of the ethanol-cyclohexane and 1-propanol-cyclohexane binary liquids. The former system is immiscible at methanol concentrations of ca. 8-90 mol %, and the latter two are miscible at any composition. At 0.03 mol % methanol, which is far from the critical concentration for the phase separation, the contact of the methanol macrocluster layers formed on the silica surface brought about the attraction from a distance of 42 +/- 5 nm which was similar to that observed in ethanol-cyclohexane. At a methanol concentration of 9.0 mol %, above bulk phase separation, completely different force profiles were observed. These results demonstrated that the molecular macrocluster formation was different from the wetting induced by the bulk.