Macrocluster formation of alcohol on silica surface in cyclohexane: Analysis of interfacial energy between adsorption layer and bulk solution

Recently, we have found that alcohols adsorbed on the silica, surface in non-polar liquids (cyclohexane, etc.) form ordered structures extending to several tens of nanometer through hydrogen bonding between the surface silanol groups and the adsorbed alcohols as well as between the adsorbed alcohols. We call this structure the surface molecular macrocluster. Surface forces measurement has revealed that the long range attraction appears from distances about twice of the macrocluster layer thickness. The bridging of the adsorption layers brings about the long range attraction due to the interfacial energy between the adsorption layer of molecular macroclusters and the bulk solution. In this study, the interfacial energies (γ) between the methanol adsorption layer and the bulk solution were evaluated by analyzing the long range attraction as well as pull-off force in methanol-cyclohexane binary liquid mixtures. The interfacial energy (γ) was evaluated to be 7.2 ± 0.3 mN/m, which was more than 10 times larger than that for methanol/cyclohexane interface at the phase separation (0.6 mN/m). This larger interfacial energy should be caused by the rather fixed orientation of methanol molecules at the interface due to the ordered structure of methanol molecules in a macrocluster.