(485h) Opposing Trends in Surface Water Mobility for Ordered Mesoporous Silicas As a Function of Surface Polarity
Silica materials are widely used in diverse applications such as drug delivery, catalysis, and membranes [1-3]. Many of the phenomena critical to their performance occur at solid-water interfaces. Even though it is well-documented that geometrical confinement affects interfacial water dynamics , the effect of silica surface chemistry and polarity on the mobility of interfacial water is still largely unknown. In order to study the role of silica surface chemistry and polarity on water dynamics, we prepared two types of periodic mesoporous silicas: (1) pure silicas, and (2) organosilicas. For the pure silicas, surface polarity was controlled using thermal treatment temperatures up to 1,000 ºC. The polarity of the organosilicas was modulated using the co-condensation method. Because surface water dynamics can be affected by confinement effects, the pore size of the silicas was controlled by the choice of surfactant. This systematic strategy allows us to correlate surface chemistry with water dynamics by including the confinement effects. The surface polarities of all materials were estimated using adsorption of an organic dye. Translational dynamics of surface water were investigated using Overhauser dynamic nuclear polarization. In pure silicas, condensation of polar silanols to give moderately nonpolar siloxane groups leads to an increase in translational water diffusivity due to their weak interactions with water. In contrast, increasingly nonpolar surfaces achieved by incorporating organic groups, such as phenylene, biphenylene and ethylene bridges, lead to a gradual decrease in surface water diffusivity. The opposite trend for water diffusivity observed for the nonpolar, organic surfaces is likely due to the formation of a strong hydrogen bonding network at the organic-water interface. In this presentation, we will discuss experimental evidence supporting formation of water structure around hydrophobic moiety. The findings suggest that surface chemistry, in addition to polarity, is a significant design parameter for modulating solid-water interactions.
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