Second harmonic generation studies of adsorption at a liquid-liquid electrochemical interface

Abstract

oo22-36~~/~~/209 ~-04a~~o~.oo/o optical response time, and theability toeasilydiscriminatebetween surface species and species in the adjacent buIk media, SHG has been employed extensively at a wide variety of surfaces.10J)-M In this paper we report the first application of surface SHG measurements to the study of an electrochemical liquid-liquid interface. Resonant SHG measurements are used to monitor the adsorption of the surfactant molecule 2-(n-octadecylamino)naphthalene-6-sulfonate (ONS) at a water/ 1 ,2-dichloroethane interface. The relative surface coverage of the anionic surfactant molecule as a function of applied potential is determined from the surface SHG signal. The results of these SHG experiments are used to provide new information complementary to that obtained from measurements of the interfacial tension of the ITIES. At a pH of 9, anionic ONS adsorption is found to mr at all potentials positive of the potential of zero charge (pzc). The adsorption of ONS can be described by a Frumkin isotherm with a free energy of adsorption that increases with the applied potential and with the surface coverage of ONS. At a pH of 3, the combination of SHG and interfacial tension measurements is used todetermine that both the anionic and the zwitterionic forms of ONS are present at the water-dichloroethane interface. At potentialsverypositiveofthepzc, theanionicONSat theinterface undergoes a field-dependent protonation of its amino group to form the zwitterion.