Evolution of the Adsorbed Water Layer Structure on Silicon Oxide at Room Temperature

TLDR

The molecular configuration of water adsorbed on a hydrophilic silicon oxide surface at room temperature was determined as a function of relative humidity using attenuated total reflection infrared spectroscopy. The study shows that as relative humidity rises, a hydrogen‑bonded icelike water network builds up to three layers at 0–30 % RH, then liquid water begins to appear between 30–60 % RH while the icelike layer continues to grow, with only a single additional molecular layer thickening until 60 % RH, after which liquid water overlays the icelike layer, and these structural transitions in the outermost adsorbed layer drive changes in the adsorption isotherm and interfacial energy, explaining many water‑adsorption phenomena.

Abstract

The molecular configuration of water adsorbed on a hydrophilic silicon oxide surface at room temperature has been determined as a function of relative humidity using attenuated total reflection (ATR)−infrared spectroscopy. A completely hydrogen-bonded icelike network of water grows up to three layers as the relative humidity increases from 0 to 30%. In the relative humidity range of 30−60%, the liquid water structure starts appearing while the icelike structure continues growing to saturation. The total thickness of the adsorbed layer increases only one molecular layer in this humidity range. Above 60% relative humidity, the liquid water configuration grows on top of the icelike layer. This structural evolution indicates that the outermost layer of the adsorbed water molecules undergoes transitions in equilibrium behavior as humidity varies. These transitions determine the shape of the adsorption isotherm curve. The structural transitions of the outermost adsorbed layer are accompanied by interfacial energy changes and explain many phenomena observed only for water adsorption.