Molecular rulers: new families of molecules for measuring interfacial widths

Homologous series of solvatochromic neutral alcohols and ionic sulfates are synthesized and characterized. Each surfactant series consists of hydrophobic, p-nitroanisole-based chromophores attached to polar or ionic headgroups by n-alkyl spacers. UV absorption measurements show that the optical properties of surfactant chromophores closely track those of the parent chromophore. Interfacial tension measurements are used to calculate surface excess concentrations of ionic surfactants adsorbed to an aqueous-cyclohexane interface. With a hydrophobic chromophore, a hydrophilic headgroup, and a variable-length, alkyl spacer, these surfactants have the potential to function as molecular rulers: probes of molecular-scale variation in solvation forces across condensed-phase interfaces. Changing the separation between the hydrophobic, solvatochromic probe and the hydrophilic headgroup should enable different members of a homologous series to span different interfacial widths, thus exposing the chromophore to different chemical environments. This idea is explored by using surface-specific, nonlinear optical spectroscopy. Resonant second harmonic spectra of p-nitroanisole and the surfactant product 4a adsorbed to an aqueous-cyclohexane interface show the surfactant spectrum blue-shifted 9 nm relative to the spectrum of adsorbed p-nitroanisole. On the basis of chromophore solvatochromism, these results are consistent with a less polar environment surrounding the surfactant chromophore. Significant differences in interfacial solvation resulting from a approximately 5 A separation between the surfactant headgroup and chromophore support recently proposed models of molecularly sharp, microscopically flat aqueous-alkane interfaces.