The structure and dynamic properties of interfacial water at the graphite and silica solid surfaces were investigated using molecular dynamics simulations. The effect of surface properties on the characteristics of interfacial water was quantified by computing density profiles, radial distribution functions, surface density distributions, orientation order parameters, and residence and reorientation correlation functions. In brief, our results show that the surface roughness, chemical heterogeneity, and surface heterogeneous charge distribution affect the structural and dynamic properties of the interfacial water molecules, as well as their rate of exchange with bulk water. Most importantly, our results indicate the formation of two distinct water layers at the SiO2 surface covered by a large density of hydroxyl groups. Further analysis of the data suggests a highly confined first layer where the water molecules assume preferential hydrogen-down orientation and a second layer whose behavior and characteristics are highly dependent on those of the first layer through a well-organized hydrogen bond network. The results suggest that water−water interactions, in particular hydrogen bonds, may be largely responsible for macroscopic interfacial properties such as adsorption and contact angle.

Inter facial water properties at the alumina surface were investigated via all-atom equilibrium molecular dynamics simulations at ambient temperature. Al-terminated and OHterminated alumina surfaces were considered to assess the structural and dynamic behavior of the first few hydration layers in contact with the substrates. Density profiles suggest water layering up to ∼10 Å from the solid substrate. Planar density distribution data indicate that water molecules in the first interfacial layer are organized in well-defined patterns dictated by the atomic terminations of the alumina surface. Interfacial water exhibits preferential orientation and delayed dynamics compared to bulk water. Water exhibits bulk-like behavior at distances greater than ∼10 Å from the substrate. The formation of an extended hydrogen bond network within the first few hydration layers illustrates the significance of water-water interactions on the structural properties at the interface.

Molecular dynamics simulations were employed to study the dynamic properties of water at the silica−liquid interface at ambient temperature. Three different degrees of hydroxylation of a crystalline silica surface were used. To assess the water dynamic properties we calculated the residence probability and in-plane mean-square displacement as a function of distance from the surface. The data indicate that water molecules at the fully hydroxylated surface remain longer, on average, in the interfacial region than in the other cases. By assessing the dynamics of molecular dipole moment and hydrogen−hydrogen vector an anisotropic reorientation was discovered for interfacial water in contact with any of the surfaces considered. However, the features of the anisotropic reorientation observed for water molecules depend strongly on the relative orientation of interfacial water molecules and their interactions with surface hydroxyl groups. On the partially hydroxylated surface, where water molecules with hydrogen-down and hydrogen-up orientation are both found, those water molecules associated with surface hydroxyl groups remain at the adsorbed locations longer and reorient more slowly than the other water molecules. A number of equilibrium properties, including density profiles, hydrogen bond networks, charge densities, and dipole moment densities, are also reported to explain the dynamics results.

All-atom molecular dynamics simulations were employed for the study of the structure and dynamics of aqueous electrolyte solutions within slit-shaped silica nanopores with a width of 10.67 A at ambient temperature. All simulations were conducted for 250 ns to capture the dynamics of ion adsorption and to obtain the equilibrium distribution of multiple ionic species (Na+, Cs+, and Cl(-)) within the pores. The results clearly support the existence of ion-specific effects under confinement, which can be explained by the properties of interfacial water. Cl(-) strongly adsorbs onto the silica surface. Although neither Na+ nor Cs+ is in contact with the solid surface, they show ion-specific behavior. The differences between the density distributions of cations within the pore are primarily due to size effects through their interaction with confined water molecules. The majority of Na+ ions appear within one water layer in close proximity to the silica surface, whereas Cs+ is excluded from well-defined water layers. As a consequence of this preferential distribution, we observe enhanced in-plane mobility for Cs+ ions, found near the center of the pore, compared to that for Na+ ions, closer to the solid substrate. These observations illustrate the key role of interfacial water in determining ion-specific effects under confinement and have practical importance in several fields, from geology to biology.