Total internal reflection fluorescence and electrocapillary investigations of adsorption at a H2O-dichloroethane...

Total internal reflection fluorescence and electrocapillary measurements are employed to provide complementary potential-dependent information about the mechanical and photophysical properties of the interface between two immiscible electrolyte solutions, 1,2-dichloroethane-H2O. Adsorption of the zwitterionic amphiphile, di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate (I) produces an interface with mechanical (interfacial tension) and charge transport properties qualitatively like the unmodified interface. Addition of dilauroylphosphatidylcholine (DLPC) to the organic phase produces an interface dominated by DLPC adsorption and drastically alters the potential dependence of the interfacial tension, gamma, the interfacial excess populations, GammaI, the charge transport, and the fluorescence response from I. This result is explained in terms of a potential-dependent protonation of the DLPC at the interface, which causes it to desorb, and a competition for interfacial sites between DLPC and protonated and unprotonated dye I. Protonation of DLPC results in a rise in gamma, which is correlated with an increase in transport of the organic-phase anion tetraphenylborate, TPB-, and an increase in interfacially excited fluorescence from I. Both results are explained by a model in which the mechanical properties of the interface, as determined by the interfacial DLPC population, direct the ability of other species to transfer across TPB- or adsorb to I the interface.