Structures of Water Molecules at the Interfaces of Aqueous Salt Solutions and Silica: Cation Effects

Structures of water molecules at water/silica interfaces, in the presence of alkali chloride. were investigated using infrared-visible sum frequency vibrational spectroscopy. Significant perturbations of the interfacial water structure were observed on silica surfaces with the NaCl concentration as low as 1 x 10(-4) M. The cations, which interact with the Silica Surface via electrostatic interaction, play key roles in Perturbing the hydrogen-bond network of water molecules at the water/silica interface. This cation effect becomes saturated at concentrations around 10(-2) to 10(-1) M, where the sum frequency generation peaks at 3200 and 3400 cm(-1) decrease by 75%. Different alkali cation species (Li+, Na+, and K+) produce different magnitudes of perturbation, with K+ > Li+ > Na+. This order can be explained by considering the effective ionic radii of the hydrated cations and the electrostatic interactions between the hydrated cations and silica Surfaces. The interfacial water structure associated with the 3200 cm(-1) band is more vulnerable to the cation perturbation, Suggesting that the more ordered water structure on silica is likely associated with the vincinal silanol groups, which create a higher local surface electrical field on silica.