Potential-dependent adsorption of amphoteric rhodamine dyes at the oil/water interface as studied by potential-modulated fluorescence spectroscopy

Ion transfer and adsorption of amphoteric rhodamines, that is, Rhodamine B (RB), Rhodamine 19 (R19), and Rhodamine 110 (R110), and a cationic rhodamine, Rhodamine 123 (R123), at a polarized 1,2-dichloroethane/water (DCE/W) interface, were studied by means of cyclic voltammetry and potential-modulated fluorescence (PMF) spectroscopy. For all rhodamines, a well-defined voltammetric wave was obtained and the pH dependence of the reversible half-wave potential (i.e., midpoint potential) was investigated to prepare the ionic partition diagram. Theoretical considerations of the diagrams showed that the voltammetric waves obtained for the amphoteric rhodamines were not due to a simple transfer of the protonated form (R+) but due to the transfer of H+ facilitated by the amphoteric form (R-+/-) in DCE (and partly in W for R110): H+(W) + R-+/-(DCE or W) -> R+(DCE). In PMF spectroscopy, the PMF signal due to the adsorption of R+ at the interface could be obtained for RB, R19, and R123, only when the Galvani potential difference across the interface (Delta(W)(O)phi) was lower than -0.14 V, suggesting a significant role of Delta(W)(O)phi in the interfacial adsorption of the rhodamines. The PMF spectrum obtained for the rhodamines under these conditions suggested that the xanthene ring of the adsorbed species should be located in the DCE phase. Furthermore, the dependence of PMF on the angle of polarization of the excitation beam suggested that the longitudinal axis of the xanthene ring should tilt only by 20-25 degrees with respect to the interface.