Solvent Polarity across Strongly Associating Interfaces

Solvatochromic surfactantsalso referred to as “molecular rulers”have been used to examine changes in polarity across strongly associating liquid/liquid interfaces. The water/alcohol interfaces were formed between an aqueous subphase and either linear (1-octanol and 1-decanol) or branched (3-octanol and 2,6-dimethyl-4-heptanol) alcohols. Resonance-enhanced second harmonic generation was used to collect effective excitation spectra of species adsorbed to these interfaces. 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 interfaces formed with linear alcohols changes gradually over the distances spanned by ruler surfactants, and interfacial width scales with the length of the alcohol solvent. In contrast, transitions at the interfaces between water and the two branched alcohols are more abrupt. These...