Competitive Molecular Adsorption at Liquid/Solid Interfaces: A Study by Sum-Frequency Vibrational Spectroscopy

摘要

We used sum-frequency vibrational spectroscopy to study competitive adsorption of water-alcohol binary liquid mixtures at hydrophilic fused silica and hydrophobic alkyl silane-covered substrates. Monitoring the strength of the methyl stretching modes of alcohols in the mixtures allowed deduction of the surface coverage of alcohols and their adsorption isotherms. It was found that for both types of substrates, alcohol adsorbs preferentially than water at liquid/solid interfaces. The driving force behind preferential alcohol adsorption appears to be the strong hydrogen-bonding interaction among water molecules as they would like to maintain the three-dimensional hydrogen-bonding network and minimize loss of hydrogen bonds in the interfacial layer. The mechanism is believed to be generally true if interaction among molecules of one species is significantly stronger than other intermolecular interactions in a mixture and the interactions of different molecular species with the substrate are about the same.