Surface second harmonic generation monitoring of the anion methyl orange during ion transfer reactions across a polarised water ∣ 1,2-dichloroethane interface

The surface second harmonic generation (SHG) response at the water ∣ 1,2-dichloroethane interface of the azobenzene derivative anionic dye p-dimethylaminoazobenzenesulfonate, often referred to as methyl orange (MO), as a function of the applied potential across the interface is reported. It is shown that the nonlinear optical signal follows the surface coverage of the dye at the interface as a function of the applied potential. At low initial bulk aqueous concentrations of the dye, and for Galvani potential differences more positive than the formal ion transfer potential, the surface coverage follows an adsorption isotherm determined by the potential drop in the aqueous phase. At Galvani potential differences around the formal ion transfer potential, the dye surface coverage vanishes following the onset of the corresponding transfer current in this potential region. At higher methyl orange bulk aqueous concentrations, the surface coverage of the dye reaches the saturation regime as dictated by a Langmuir adsorption isotherm from the aqueous phase at Galvani potential differences more positive than the formal ion transfer potential.