OPTICAL 2ND-HARMONIC GENERATION MEASUREMENTS OF MOLECULAR ADSORPTION AND ORIENTATION AT THE LIQUID-LIQUID ELECTROCHEMICAL INTERFACE

The surface-sensitive spectroscopic technique of optical second-harmon ic generation (SHG) is applied to the in situ study of molecular adsor ption at the interface between two immiscible electrolyte solutions (I TIES). The resonant SHG from molecules which exhibit a large non-linea r optical response at a specific wavelength can be used to measure the relative surface coverage of surfactants at the ITIES as a function o f the external electrochemical parameters. In addition, the polarizati on dependence of the resonant surface SHG can be used to estimate the average molecular orientation of adsorbates at the liquid/liquid elect rochemical interface. As an example, the adsorption of the surfactant 4-(4'-dodecyloxyazobenzene)benzoic acid at the water/1,2-dichloroethan e interface is characterized as a function of applied potential, surfa ctant concentration and aqueous pH with in situ resonant molecular SHG measurements. An analysis of the SHG data results in the determinatio n of the local potential and surface pH experienced by the surfactant.