| Abstract
| - A new family of neutral, solvatochromic surfactants has been used to probe solvent polarity across stronglyassociating solid/liquid interfaces. The surfactants consist of p-nitroanisole-based chromophores and polar−OH groups separated by alkyl chains of different lengths. The solid substrate is silanol-terminated, hydrophilicsilica, and the strongly associating solvents are 1-butanol and 1-octanol. To understand how the chromophoreitself interacts with the hydrophilic substrate, we acquire resonance enhanced second harmonic (SH) spectraof the bare chromophore adsorbed to the silica/cyclohexane, solid/liquid interface. Spectra show two featuresdespite the chromophore having only a single electronic resonance in the wavelength region (∼300 nm)examined. These features are assigned to the chromophore adsorbed to the surface in two different orientations.Second harmonic spectra of the bare chromophore adsorbed to the hydrophilic/butanol and hydrophilic/octanolinterfaces again show two features with the less polar, shorter wavelength feature being more pronounced inthe octanol spectrum. Despite exhibiting pNAs-like behavior in bulk solution, molecular rulers present a verydifferent picture of interfacial polarity. SH spectra from the hydrophilic/butanol interface indicate that interfacialpolarity is enhanced over bulk solution limits. Furthermore, spectra lose their bimodal appearance, indicatingthat local polarity is rather uniform across the interfacial region. In contrast, the hydrophilic/octanol interfacecontinues to show bimodal behavior for all of the molecular rulers studied, suggesting that surface-inducedchanges in solvent structure partitions solvent polarity into regions that are more polar and less polar thanbulk solution. Data from the hydrophilic/octanol system suggest that variations in solvent polarity extend nomore than ∼1.2 nm into solution or approximately the length of a fully extended, 1-octanol molecule.
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