Investigations of Solvent Polarity at Liquid/Liquid Interfaces by SHG Spectroscopy Using Molecular Rulers

Homologous series of solvatochromic surfactants have been synthesized to study polarity at liquid/liquid interfaces. Each surfactant series consists of hydrophobic, para-nitroanisole-based chromophores attached to ionic headgroups by n-alkyl spacers. By incorporating these components these molecules have the capability of functioning as molecular rulers: probes of molecular-scale variation in solvation forces across liquid/liquid interfaces. Changing the chromophore-headgroup separation 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, non-linear optical spectroscopy. Resonance-enhanced second harmonic generation spectra of molecular rulers have been collected at weakly and strongly associating liquid/liquid interfaces. At weakly associating interfaces between water and four alkanes (cyclohexane, methylcyclohexane, octane, and hexadecane), data suggest that all four water/alkane interfaces are molecularly sharp (< 9 Å), but that differences in the solvent molecular structure alter the transition from aqueous to organic solvation across the interface. Polarity across two interfaces (cyclohexane and hexadecane) changes gradually over the distance spanned by ruler surfactants. In contrast, the transitions at the interfaces between water and both methylcyclohexane and octane appear much more abrupt. These findings appear to correlate with each organic solvent's ability to pack and associated free volume. More free volume in the organic phase leads to a more abrupt water/alkane interface. At strongly associating interfaces between water and four alcohols (1-octanol, 1decanol, 3-octanol, and 2,6-dimethyl-4-heptanol), data suggest that all four water/alcohol interfaces contain a region of reduced polarity between the polar water phase and the bulk alcohol. We attribute this region to the alignment of the alkyl chains of the interfacial alcohol molecules. Polarity across the two interfaces with linear alcohols changes gradually over the distances spanned by ruler surfactants. In contrast, transitions at the interfaces between water and the two branched alcohols appear much more abrupt. These differences appear to correlate well with the solute accessible free volume within the alcohols. The width of the interfaces between water and the linear alcohols appears to be directly related to the length of the alkyl chain on the alcohol