2ND HARMONIC-GENERATION STUDIES OF ADSORPTION AT A LIQUID LIQUID ELECTROCHEMICAL INTERFACE

The technique of optical second harmonic generation (SHG) is applied to the measurement of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant second harmonic response from 2-(n-octadecylamino)naphthalene-6-sulfonate (ONS) is used in conjunction with interfacial tension measurements to optically determine the relative surface coverage of the anionic surfactant molecule at a charged water-dichloroethane interface. At a pH of 9, ONS adsorption occurs at all potentials positive of the potential of zero charge. The potential-dependent adsorption of ONS can be described by a Frumkin isotherm with a free energy of adsorption that varies linearly with applied potential. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed ONS molecules with respect to the ITIES. At a pH of 3, both the anionic form of ONS and the protonated zwitterionic form of ONS are present at the liquid-liquid interface. At very positive potentials, the anionic ONS at the interface undergoes a field-dependent conversion to the zwitterionic form.