Discrete partitioning of solvent permittivity at liquid-solid interfaces

Electronic excitation energies of chromophores adsorbed to strongly associating, alcohol-hydrophilic quartz, liquid-solid interfaces are measured using resonant second harmonic generation (SHG). Shifts in the SHG spectra relative to excitation spectra in various bulk solvents are used to infer the dielectric character of the interfacial environment. At interfaces formed between n-alcohol solvents and hydrophilic quartz surfaces, the local dielectric environment is less polar than bulk solution. This trend becomes more pronounced with longer chain n-alcohol solvents. The interfacial region formed between a branched alcohol (2-propanol) and hydrophilic quartz is more polar than bulk solution. These results are interpreted in terms of a solvent's ability to order at the interface and cancel dipolar contributions from surface hydroxyl groups to the local solvation environment experienced by the chromophore solute.