OPTICAL 2ND-HARMONIC GENERATION STUDIES OF AZOBENZENE SURFACTANT ADSORPTION AND PHOTOCHEMISTRY AT THE WATER 1,2-DICHLOROETHANE INTERFACE/

The surface-sensitive spectroscopic technique of optical second harmon ic generation (SHG) is employed to study the adsorption and photochemi stry of surfactant molecules at the water/1,2-dichloroethane (DCE) int erface. Resonant SHG measurements at 730 nm are used to monitor the ad sorption of the azobenzene surfactant 4-[[(dodecyloxy)benz-4-yl]azo]be nzoic acid (DBA) from DCE solution to the interface at an aqueous pH o f 8 or greater. The concentration and pH dependence of the resonant SH G from the adsorbed monolayer indicates that the DBA exists in its ani onic (carboxylate) form at the interface. In a series of combined phot ochemical/SHG experiments the trans/cis photoisomerization of the adso rbed DBA anions is examined. SHG measurements demonstrate that illumin ation of the surface with light at 365 nm converts the adsorbed trans- DBA molecules to the cis isomer. The cis-DBA anion is found to be unus ually stable at the liquid/liquid interface, and the entire photoactiv e monolayer (total area 4 cm(2)) can be photochemically converted in a pproximately 450 s with an illumination spot of 0.12 cm(2). This relat ively short conversion time is attributed to surface-tension-induced c onvection effects that occur during the photochemical conversion of th e monolayer. Illumination of the cis-DBA anions with light at 440 nm r econverts the entire surfactant monolayer back to the trans-DBA form i n the same time frame.