The local structure of water on chemically and structurally different surfaces is subject of ongoing research. In particular, confined spaces as found in mesoporous silica have a pronounced effect on the interplay between adsorbate-adsorbate and adsorbate-surface interactions. Mid-infrared spectroscopy is ideally suited to quantitatively and qualitatively study such systems as the probed molecular vibrations are highly sensitive to intermolecular interactions. Here, the quantity and structure of water adsorbed from the gas phase into silica mesopores at different water vapor pressures was monitored using mid-infrared attenuated total reflection (ATR) spectroscopy. Germanium ATR crystals were coated with different mesoporous silica films prepared by evaporation induced self-assembly. Quantitative analysis of the water bending vibration at 1640 cm-1 at varying vapor pressure allowed for retrieving porosity and pore size distribution of the mesoporous films. The results were in excellent agreement with those obtained from ellipsometric porosimetry. In addition, different degrees of hydrogen bonding of water as reflected in the band position and shape of the stretching vibrations (3000-3750 cm-1) were analyzed and attributed to high-density, unordered bulk and low-density, surface-induced ordered water. Thereby, the progression of surface-induced ordered water and bulk water as a function of water vapor pressure was studied for different pore sizes. Small pores with 5 nm diameter showed a number of two ordered monolayers, while for pores > 12 nm the number of ordered monolayers is significantly larger and agrees with the number observed on planar SiO2 surfaces.

Ellipsometry porosimetry provides full adsorption/desorption isotherms and is among the most accurate techniques to probe porosity in thin films. Here we address a novel technique based on desorption scanning for a precise assessment of the porosity features revealing the size distribution of interconnections between the various populations of pores in the coatings. This investigation was performed on model (multimodal) mesoporous silica films prepared by evaporation-induced self-assembly in the presence of micellar templating agents, before and after pore network modification through chemical attack in NH4F. The systems were also analyzed by grazing incidence small-angle X-ray scattering to confirm the multimodal periodic organization and homogeneity of the porosity. Conventional pore-blocking effect could not explain by itself the unconventional behaviors observed in the desorption scans. We propose a new desorption mechanism, addressed as pore transit desorption, to justify the experimental features and provide a full description of the porous network complexity. Based on these, we discuss several model cases of porosities combining up to two populations of pores and three populations of interconnections. Overall, interpretations made in this work can be generalized and transposed to any mesoporous material.