Evidence for Entropically Controlled Interfacial Hydration in Mesoporous Organosilicas

At aqueous interfaces, the distribution and dynamics of adsorbates are modulated by the behavior of interfacial water. Hydration of a hydrophobic surface can store entropy via the ordering of interfacial water, which contributes to the Gibbs energy of solute binding. However, there is little experimental evidence for the existence of such entropic reservoirs, and virtually no precedent for their rational design in systems involving extended interfaces. In this study, two series of mesoporous silicas were modified in distinct ways: (1) progressively deeper thermal dehydroxylation, via condensation of surface silanols, and (2) increasing incorporation of nonpolar organic linkers into the silica framework. Both approaches result in decreasing average surface polarity, manifested in a blue-shift in the fluorescence of an adsorbed dye. For the inorganic silicas, hydrogen-bonding of water becomes less extensive as the number of surface silanols decreases. Overhauser dynamic nuclear polarization (ODNP) relaxometry indicates enhanced surface water diffusivity, reflecting a loss of enthalpic hydration. In contrast, organosilicas show a monotonic decrease in surface water diffusivity with decreasing polarity, reflecting enhanced hydrophobic hydration. Molecular dynamics simulations predict increased tetrahedrality of interfacial water for the organosilicas, implying increased ordering near the nm-size organic domains (relative to inorganic silicas, which necessarily lack such domains). These findings validate the prediction that hydrophobic hydration at interfaces is controlled by the microscopic length scale of the hydrophobic regions. They further suggest that the hydration thermodynamics of structurally heterogeneous silica surfaces can be tuned to promote adsorption, which in turn tunes the selectivity in catalytic reactions.

Automated summary

In this study, the behavior of water at the interfaces of mesoporous silicas was controlled by modifying their thermal and chemical properties. The results showed that the decrease in surface polarity of inorganic silicas reduced the hydrogen-bonding of water and increased the surface water diffusivity. In contrast, the decrease in polarity of organosilicas decreased the surface water diffusivity, indicating enhanced hydrophobic hydration. These findings validate the control of hydrophobic hydration at interfaces by the microscopic length scale of the hydrophobic regions, and suggest that the hydration thermodynamics of silica surfaces can be tuned to promote adsorption and selectivity in catalytic reactions.