Total Internal Reflection Fluorescence and Electrocapillary Investigations of Adsorption at the Water−Dichloroethane Electrochemical Interface. 2. Fluorescence-Detected Linear Dichroism Investigation of Adsorption-Driven Reorientation of Di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate

The potential-dependent adsorption and orientation of the zwitterionic amphiphile, di- N -butylaminonaphthylethenylpyridiniumpropylsulfonate, I, in the presence of dilauroylphosphatidylcholine (DLPC) at the H 2 O−1,2-dichloroethane (DCE) interface was investigated using a combination of steady-state fluorescence, fluorescence-detected linear dichroism, and electrocapillary measurements. From electrocapillary measurements, DLPC was found to dominate the interfacial composition at all potentials, when DLPC and I were present in bulk DCE and H 2 O at ca. 2 and 1 μM concentrations, respectively. At potentials E w − E o > 0.32 V, the affinity of DLPC for the interface is diminished, and I becomes a more effective competitor for interfacial sites. Over the potential range 0.32 V ≤ E w − E o ≤ 0.47 V, the total interfacial excess of species is reduced, but the ratio / is enhanced. The DC fluorescence signal increased in response to the enhanced interfacial population of the unprotonated monomeric I at positive potentials. Concurrent fluorescence-detected linear dichroism (FDLD) measurements found that I reorients toward the interface normal on the positive scan. Because the total interfacial excess decreases at these potentials, this behavior cannot be ascribed to a simple compression effect. Rather, it reflects the favored geometry at the compositions obtained at positive potentials. Potential step experiments showed phenomena occurring on three distinct time scales: ion reorganization on the millisecond time scale, an initial excursion of the DC fluorescence intensity over a few tens of seconds, and then a much longer evolution of the DC fluorescence (opposing the initial change) and the FDLD signal. The initial DC response can be explained by an interfacial reorganization of the chromophore that occurs in response to the applied potential before significant mass-transport occurs, while the slow time responses of both the I DC and the FDLD signal are attributed to a mass-transport-limited partitioning of the lipid species, into and out of the interfacial region.