2ND-HARMONIC GENERATION MEASUREMENTS OF ELECTROSTATIC BIOPOLYMER-SURFACTANT COADSORPTION AT THE WATER 1,2-DICHLOROETHANE INTERFACE/

The electrostatic coadsorption of the synthetic polypeptide poly-L-glu tamic acid (pGlu) and the cationic hemicyanine surfactant rans-4-[4-(d ibutylamino)styryl]-1-methylpyridinium iodide (D+-CH3) is observed at the water/1,2-dichloroethane (DCE) interface with the spectroscopic te chnique of optical second-harmonic generation (SHG). The adsorption is otherm of D+-CH3 to the water/DCE interface is obtained with a combina tion of resonant surface SHG and interfacial tension measurements. In the presence of polyanionic pGlu in the aqueous phase at a pH of 8.5, the limiting surface coverage of the cationic surfactant is enhanced b y a factor of 3.7. In contrast, no such enhancement is observed in the presence of p6lu for the adsorption of the neutral zwitterionic hemic yanine surfactant -(dibutylamino)styryl]-1-(3-sulfopropyl)pyridinium h ydroxide inner salt hydrate (D+-(CH2)(3)-SO3-) to the water/DCE interf ace, Additionally, no enhancement in surfactant adsorption is observed for D+-CH3 when the pGlu in the aqueous phase is replaced with a solu tion of the monomer L-glutamic acid. These observations demonstrate th at the coadsorption of pGlu and D+-CH3 is a multipoint electrostatic a dsorption interaction at the water/DCE interface.