Total internal reflection fluorescence and electrocapillary investigationsof adsorption at a H2O-dichloroethane electrochemical interface. 1. Low-frequency behavior

Total internal reflection fluorescence and electrocapillary measurements ar e employed to provide complementary potential-dependent information about t he mechanical and photophysical properties of the interface between two imm iscible electrolyte solutions, 1,2-dichloroethane-H2O. Adsorption of the zw itterionic amphiphile, di-N-butylaminonaphthylethenylpyridiniumpropylsulfon ate (I) (I) produces an interface with mechanical (interfacial tension) and charge transport properties qualitatively like the unmodified interface. A ddition of dilauroylphosphatidyl-choline (DLPC) to the organic phase produc es an interface dominated by DLPC adsorption and drastically alters the pot ential dependence of the interfacial tension, gamma, the interfacial excess populations, Gamma(I), the charge transport, and the fluorescence response from I. This result is explained in terms of a potential-dependent protona tion of the DLPC at the interface, which causes it to desorb, and a competi tion for interfacial sites between DLPC and protonated and unprotonated dye I. Protonation of DLPC results in a rise in gamma, which is correlated wit h an increase in transport of the organic-phase anion tetraphenylborate, TP B-, and an increase in interfacially excited fluorescence from I. Both resu lts are explained by a model in which the mechanical properties of the inte rface, as determined by the interfacial DLPC population, direct the ability of other species to transfer across TPB- or adsorb to I the interface.