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

The surface second harmonic generation (SHG) response at the water \ 1,2-di chloroethane interface of the azobenzene derivative anionic dye p-dimethyla minoazobenzenesulfonate, often referred to as methyl orange (MO), as a func tion 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 bul k 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 aque ous phase. At Galvani potential differences around the formal ion transfer potential, the dye surface coverage vanishes following the onset of the cor responding transfer current in this potential region. At higher methyl oran ge bulk aqueous concentrations, the surface coverage of the dye reaches the saturation regime as dictated by a Langmuir adsorption isotherm from the a queous phase at Galvani potential differences more positive than the formal ion transfer potential. (C) 2000 Elsevier Science S.A. All rights reserved .