Energy Landscape of Water and Ethanol on Silica Surfaces

Abstract

Fundamental understanding of small molecule–silica surface interactions at their interfaces is essential for the scientific, technological, and medical communities. We report direct enthalpy of adsorption (Δhads) measurements for ethanol and water vapor on porous silica glass (CPG-10), in both hydroxylated and dehydroxylated (hydrophobic) forms. The results suggest a spectrum of energetics as a function of coverage, stepwise for ethanol but continuous for water. The zero-coverage enthalpy of adsorption for hydroxylated silica shows the most exothermic enthalpies for both water (−72.7 ± 3.1 kJ/mol water) and ethanol (−78.0 ± 1.9 kJ/mol ethanol). The water adsorption enthalpy becomes less exothermic gradually until reaching its only plateau (−20.7 ± 2.2 kJ/mol water) reflecting water clustering on a largely hydrophobic surface, while the enthalpy of ethanol adsorption profile presents two well separated plateaus, corresponding to strong chemisorption of ethanol on adsorbate-free silica surface (−66.4 ± 4.8 ...