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Surface second harmonic generation (SSHG) studies of the azobenzene derivative p-dimethylaminoazobenzene sulfonate, often referred as Methyl Orange (MO), at the neat water/1,2-dichloroethane (DCE) interface is reported. The two forms of the anionic MO dye, which are usually observed in bulk solution, with one form being hydrogen bonded to a water molecule through the azo nitrogens (MO/H2O) and the other form not being hydrogen bonded (MO) have also been observed at the water/DCE interface. Their equilibrium constant has been compared with the corresponding bulk solution and found to be identical. The adsorption equilibrium of the two forms has been determined and the Gibbs energy of adsorption measured to be -30 kJmol(-1) for both forms. From a light polarisation analysis of the SH signal, the angle of orientation of the MO transition dipole moment was found to be 34 +/- 2 degrees for MO and 43 +/- 2 degrees for MO/H2O under the assumption of a Dirac delta function for the angle distribution, a difference explained by the different solvation properties of the two forms. Furthermore, the wavelength dependence analysis of these data revealed an interference pattern resulting from the electronic density redistribution within the hydrated anionic form occurring upon the formation of the hydrogen bond with a water molecule. This interference pattern was clearly evidenced with the use of another dye at the interface in order to define a phase reference to both forms of Methyl Orange.

Time-resolved laser-induced fluorescence in total internal reflection mode has been used to monitor the lifetimes of excited species located within the evanescent wave on the aqueous side of the water/1,2-dichloroethane interface, in the presence of varying concentrations of quencher molecules in the adjacent organic phase. The aqueous chromophore used was europium(III), with anthracene being employed as the organic quencher. The kinetic data are obtained from analysis of the relevant mass-transfer regime. The heterogeneous electron transfer rate constant between the two species was found to be 1.9 × 10-4 m4 mol-1 s-1.

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