Weak acid chemistry and photoisomerization of azobenzene surfactants at the liquid/liquid electrochemical interface by optical second harmonic generation spectroscopy

The surface-sensitive technique of optical second harmonic generation (SHG) was applied to surfactants at the liquid/liquid electrochemical interface. Utilizing molecular probes containing useful chemical moieties and high nonlinear optical activity, resonant SHG measurements were used to elucidate information about monolayer structures, electrochemical profiles, weak acid chemistry, and photochemical processes at the liquid/liquid interface.;SHG surface coverage measurements of a weak acid surfactant were used to resolve the adsorption mechanism of the carboxylate form of a weak acid surfactant at the liquid/liquid interface. Coupled adsorption and surface deprotonation equilibria result in the formation of a monolayer of anions at the water/1,2-dichloroethane interface without the involvement of charge transport. The surface coverage of this monolayer depends on the interfacial pH, which is affected by a potential applied across the interface. Analysis of the potential dependence of the SHG from the monolayer allowed an examination of the local potential at the interface.;The cis/trans photoisomerization of an azobenzene surfactant at the liquid-liquid interface was studied with SHG. The molecular nonlinear polarizabilities for the trans and cis isomers were calculated to demonstrate the feasibility of following the photoisomerization process with SHG. The photoisomerization process was found to be reversible and repeatable, with no fatigue of the sample observed. The thermal isomerization behavior of the azobenzene in solution was compared with that at the interface, allowing the following conclusions to be drawn. First, the monolayer does not exchange molecules with the adjoining solution. Second, the cis isomer of the azobenzene is apparently stable at the interface for a much longer period of time than has been observed for similar molecules in other media. In addition, an analysis of the timescale of the conversion of the entire interfacial monolayer resulted in the conclusion that surface-tension-induced convection effects are present during the photoisomerization process.