Surface chemical heterogeneity modulates silica surface hydration.

Area of Science:

• Surface science
• Physical chemistry
• Materials science

Background:

• Water interaction with amorphous silica is crucial for interfacial hydration and industrial processes like catalysis and chromatography.
• Silica surfaces are tunable, with adjustable hydrophilic silanol and hydrophobic siloxane groups.
• The impact of surface hydrophilicity and topology on interfacial water properties remains poorly understood.

Purpose of the Study:

• To investigate the influence of silanol density and distribution on interfacial water properties.
• To measure surface water diffusivity and silica-silica interaction forces as a function of silanol density.
• To correlate experimental findings with molecular dynamics simulations.

Main Methods:

• Controlled alteration of surface silanol density on amorphous silica.
• Measurement of surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP).
• Measurement of silica-silica interaction forces using a surface forces apparatus (SFA).
• Molecular dynamics simulations of model silica-water interfaces.

Main Results:

• Increased silanol density generally correlates with slower water diffusivity and stronger silica-silica repulsion at short separations (<4 nm).
• Sharp transitions in hydration properties were observed at intermediate silanol densities (2.0-2.9 nm⁻²).
• Simulations confirmed increased water diffusivity with silanol density and revealed ~10% variation due to silanol spatial distribution.

Conclusions:

• Silanol density and spatial distribution significantly impact interfacial water dynamics and silica surface interactions.
• A critical silanol cluster size or connectivity may explain observed sharp transitions.
• Findings are relevant for understanding wettability, colloidal interactions, and surface reactions on heterogeneous materials.