Surface solvation and hindered isomerization at the water/silica interface explored with second harmonic generation

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Water structure and dynamics at solid-liquid interfaces are crucial for understanding chemical reactions.
• Previous studies suggested nonpolar solvation for solutes at hydrophilic interfaces due to restricted water dynamics.
• Conflicting experimental and simulation data highlight the need for further investigation.

Purpose of the Study:

• To investigate the local dielectric environment of water at the water-silica interface using Coumarin 152 (C152) as a probe.
• To compare SHG results with time-resolved fluorescence and simulations regarding interfacial water properties.
• To elucidate the influence of solvent-substrate interactions on solute solvation and reactivity.

Main Methods:

• Resonantly enhanced second harmonic generation (SHG) spectroscopy of C152 adsorbed at the water-silica interface.
• Comparison with time-resolved fluorescence experiments and molecular dynamics simulations.
• Investigation of both hydrophilic and hydrophobic silica surfaces.

Main Results:

• SHG spectra indicate C152 experiences a local dielectric environment slightly more polar than bulk water at the water-silica interface.
• This contradicts previous reports suggesting alkane-like permittivity for interfacial water.
• Eliminating hydrogen bonding to hydrophobic silica surfaces led to solvation properties similar to bulk acetone or methanol.

Conclusions:

• Restricted water dynamics at hydrophilic interfaces can lead to apparent nonpolar solvation despite a large static electric field.
• Solvent-substrate interactions significantly influence solute solvation and reactivity at buried interfaces.
• SHG provides a valuable tool for probing the local dielectric environment at solid-liquid interfaces.