| Abstract
| - Ion transfer and adsorption of amphoteric rhodamines, that is, Rhodamine B (RB), Rhodamine 19 (R19), andRhodamine 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 pHdependence of the reversible half-wave potential (i.e., midpoint potential) was investigated to prepare theionic partition diagram. Theoretical considerations of the diagrams showed that the voltammetric waves obtainedfor the amphoteric rhodamines were not due to a simple transfer of the protonated form (R+) but due to thetransfer 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 interfacecould be obtained for RB, R19, and R123, only when the Galvani potential difference across the interface(φ) was lower than −0.14 V, suggesting a significant role ofφ in the interfacial adsorption of therhodamines. The PMF spectrum obtained for the rhodamines under these conditions suggested that the xanthenering of the adsorbed species should be located in the DCE phase. Furthermore, the dependence of PMF onthe angle of polarization of the excitation beam suggested that the longitudinal axis of the xanthene ringshould tilt only by 20−25° with respect to the interface.
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