Dynamical and Structural Properties of Water in Silica Nanoconfinement: Impact of Pore Size, Ion Nature, and Electrolyte Concentration

In this study, we characterized the structure and the dynamics at a picosecond scale of water molecules in aqueous solutions with cations having various kosmotropic properties (XCl2 where X = Ba2+, Ca2+, and Mg2+) confined in highly ordered mesoporous silica (MCM-41 and grafted MCM-41) by Fourier transform infrared spectroscopy and quasi-elastic neutron scattering. We pinpointed the critical pore size and the electrolyte concentration at which the influence of the ion nature becomes the main factor affecting the water properties. These results suggest that whatever the ions kosmotropic properties, for pore sizes phi(p) < 2.6 nm and [XCl2] <= 1 M, the water dynamics is mainly slowed down by the size of the confinement. For pore sizes of 6.6 nm, the water dynamics depends on the concentration and kosmotropic properties of the ion more than on the confinement. The water properties within the interfacial layer were also assessed and related to the surface ion excesses obtained by sorption isotherms. We showed that, for pore sizes phi(p) >= 2.6 nm, the surface ion excess at the pore surface is the main driver affecting the structural properties of water molecules and their dynamics within the interfacial layer.