Water uptake at 298 K by two mesoporous silicas with different pore sizes was studied using volumetric vapor sorption. Through variation of sample pretreatment temperature and time, the number of surface hydroxyl groups was varied, leading to marked changes in the water sorption behavior. The BET model was used to measure surface hydroxyl density from low pressure parts of water adsorption isotherms. An adsorbed phase model is utilized to calculate, for the first time, the distribution of pore water molecules between adsorbed and pore condensation phases and characterize the density and thickness of the water sorption phase as functions of surface hydroxylation and pore size. With increasing surface hydroxyl density, the adsorption of water to the pore surfaces increases, leading to formation of thicker and denser water sorption layers. Monolayer coverage is reached at reduced pressure of ca. 0.3. The onset of adsorption pore condensation of water shifts to lower reduced pressure with increasing surface hydroxyl density, indicating growing thickness of adsorption layers. However, the water pore condensation step of the desorption branch shifts to smaller reduced pressure with increasing surface hydroxylation, reducing the adsorption−desorption hysteresis width. In the smaller pores, the adsorbed phase forms a sparse monolayer, while approximately a bilayer is formed in the wider pores. The previously reported silica nanopore underfilling by water is confirmed and rationalized by a significantly reduced water density in the adsorbed phase with respect to bulk, while the pore core is filled with water of approximately bulk density.

Quasi-elastic neutron scattering (QENS) and molecular dynamics simulations (MDS) reveal the effects of water on the structure and dynamics of propane confined in 1.5 nm wide cylindrical pores of MCM-41-S.

A combination of QENS experiments and MD simulations quantifies the structure and dynamics of water on the olivine surface.

The dynamics of water on or in a mineral substrate plays an important role in interfacial processes. This is because the structure and dynamics of interfacial water deviate from those of bulk water due to the change of interactions between surface water molecules and the interactions between the surface water and the substrate. This work presents a study of the vibrational behavior of water on a forsterite (Mg- end member of olivine) surface using inelastic neutron scattering (INS) and molecular dynamics (MD) simulations as complementary tools. The synthetic nano-forsterite used (dominated by the (010) crystal face), i.e., “dry” sample, inherently has a partial hydration/hydroxylation layer on the surface, as shown by previous studies and TGA. In the INS experiments, three water loadings (0.5, 1, and 2 monolayers) were added to the nano-forsterite surfaces. For samples with lower water loadings, i.e., dry and 0.5 monolayers, the INS spectra exhibited a red shift (lower frequency) of the water libration band and strengthening (blue shift, higher frequency) of the O–H stretching modes, implying weakening of the hydrogen bonding acting on the water molecules. In the simulations, we modeled the forsterite (010) surface and titrated it with two water loadings representing the lower and higher experimental water loadings. The lower loading in the simulation is equivalent to the dry and 0.5 monolayer samples in the experiment, thus suggesting weak hydrogen bonding between water molecules. The higher-loading simulation emulates the multilayer adsorption experiment. This produced a more significant shift of the vibrational bands, implying increased hydrogen-bonding strength and disorder between water molecules. The MD simulations complement the INS study by providing a detailed interfacial structure, and the combination of the two approaches provides a fundamental understanding of how the presence of the olivine surface impacts the vibrational behavior of water under different degrees of hydrationa phenomenon widely associated with terrestrial and extraterrestrial surfaces and near-surface processes.