Second-Harmonic Generation Measurements of Electrostatic Biopolymer-Surfactant Coadsorption at the Water/1,2-Dichloroethane Interface

The electrostatic coadsorption of the synthetic polypeptide poly-L-glutamic acid (pGlu) and the cationic hemicyanine surfactant trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (D-CH3) is observed at the water/1,2-dichloroethane (DCE) interface with the spectroscopic technique of optical second-harmonic generation (SHG). The adsorption isotherm of D-CH3 to the water/DCE interface is obtained with a combination 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 by a factor of 3.7. In contrast, no such enhancement is observed in the presence of pGlu for the adsorption of the neutral zwitterionic hemicyanine surfactant trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl)pyridinium hydroxide inner salt hydrate (D-(CH2)3-SO3) to the water/DCE interface. Additionally, no enhancement in surfactant adsorption is observed for D-CH3 when the pGlu in the aqueous phase is replaced with a solution of the monomer L-glutamic acid. These observations demonstrate that the coadsorption of pGlu and D-CH3 is a multipoint electrostatic adsorption interaction at the water/DCE interface.