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Ion-transfer reactions at the interface between fluorous solvent 1,1,1,2,3,4,4,5,5,5-decafluoropentane (DFP) and water (W) were investigated voltammetrically. Within the potential window at the DFP | W interface, various non-fluorinated ions, including tetraphenylarsonium and tetraphenylborate, and fluorinated ions, including fluorocomplex and perfluoro- n -alkanoate ions, gave reversible voltammetric waves for their ion transfer across the DFP | W interface. Based on the tetraphenylarsonium-tetraphenylborate assumption, the formal potentials of their ion transfer could be determined in terms of the Galvani potential difference. Also, formal potentials for the transfer of bis(perfluoroalkanesulfonyl)imide ions at the DFP | W interface were determined potentiometrically. The formal Gibbs energies for the transfer of ions from DFP to W, which would provide the criteria necessary for evaluating the fluorophilicity/hydrophilicity of ions were calculated from the formal potentials, and were compared with the transfer energies from 1,2-dichloroethane to W and those from the organic solvent to DFP, as quantitative scales of the ions' lipophilicity/hydrophilicity and fluorophilicity/lipophilicity. [ABSTRACT FROM AUTHOR]

The transfer of streptothricin antibiotics containing a β-lysine monomer (ST-F), trimer (ST-D), and tetramer (ST-C) at a polarizable nitrobenzene (NB) | water (W) interface was studied by cyclic voltammetry. Under acidic conditions, the streptothricins existed in fully protonated forms, as ST-F 2 + , ST-D 4 + and ST-C 5 + cationic species. Within the potential range tested, ST-D 4 + and ST-C 5 + ions gave reversible voltammograms due to their transfer across the NB | W interface. The formal potentials, Δ O W ϕ 0 S ’ T , of ST-D 4 + and ST-C 5 + ions were determined. In the presence of a neutral ligand (L), dibenzo-18-crown-6, in NB, ST-F 2 + , ST-D 4 + and ST-C 5 + ions gave reversible voltammograms due to their transfer assisted by the formation of ST-FL 2 2 + , ST-DL 4 4 + and ST-CL 5 5 + complexes with the ligand in NB. Using the Δ O W ϕ 0 S ’ T -values, association constants between the NH 3 + -groups of ST-D 4 + and ST-C 5 + ions and the crown ether in NB were determined. The association constants were in agreement with each other. By assuming that the association constant of ST-F 2 + was equal to those of the ST-D 4 + and ST-C 5 + ions, the Δ O W ϕ 0 S ’ T of the ST-F 2 + ion was estimated. Interestingly, the determined Δ O W ϕ 0 S ’ T -value was increasingly negative in the order of ST-F 2 + (~ 0.24 V) > ST-D 4 + (0.199 V) > ST-C 5 + (0.194 V), indicating that the lysine oligomer affected the phase-transfer characteristics of streptothricin. [ABSTRACT FROM AUTHOR]

Abstract: The transfer of a biopolymer, ε-poly-L-lysine (εPL), at a polarized nitrobenzene (NB) | water (W) interface was studied voltammetrically. Despite the polydispersity of sample (n =25–35, n being the degree of polymerization), an S-shaped current-potential curve with a well-defined limiting current was observed in normal pulse voltammogram for the transfer of εPL at the NB | W (pH 3.0) interface. In the W-phase, εPL would exist in the fully protonated form, and the limiting current can be explained by the diffusion of the polycationic εPL species. Also a pair of anodic and cathodic peak currents was observed by cyclic voltammetry. The midpoint potential was in agreement with the half-wave potential in the normal pulse voltammogram. The influence of pH and the supporting electrolyte anions in both phases on the voltammograms has been also studied. By the addition of dibenzo-18-crown-6 (DB18C6) to NB, the half-wave potential shifted to negative potentials. This can be interpreted as being due to the εPL-transfer facilitated by the formation of εPL-DB18C6 complex in NB. The concentration dependence of the half-wave potential can be explained by the formation of 1:n εPL-DB18C6 complex. The results suggest the applicability of the voltammetric technique to compare the phase-transfer characteristics of εPL and the derivatives. [Copyright &y& Elsevier]

Abstract: Potential dependent behaviors and structures of binary-component bilayers and multiple layers of two different water-insoluble 4-pyridyl-terminated surfactants (C15–O–Py and C15–(C;Py, see Fig. 1) on a Au(111) electrode were investigated by voltammetric techniques. Film preparation was based on repetitive horizontal touching of the electrode to the spread film of the surfactants at a gas/water interface. When bilayer formation was made by sequential touching to the neat spread films of the two different surfactants, the resultant bilayer structure was always of Au(111)/C15–O–Py/C15–(C;Py, being independent of whether the first touching was made with C15–O–Py or C15–(C;Py. This reflects the stronger affinity of C15–O–Py for the electrode surface, which works dominantly over mixing. When multiple touching was made to the spread film of two-component mixture at a given mixture ratio, the resultant first layer appeared a C15–O–Py monolayer as long as its monolayer amount was supplied. Further increase in the number of the multiple touchings to the mixed spread film resulted in the formation of C15–O–Py bilayer in the bilayer thickness region of the film. Implications of these results were discussed in regards to the structures and properties of the films. When binary-component bilayers and multi-layers are prepared by horizontal touching methods such as a Langmuir–Schaefer (LS) technique, care must be taken to the fact that the resultant film structure does not always turn out as the procedural deposition order. [Copyright &y& Elsevier]

Abstract: The major factors determining the potential-driven phase changes of the films of 4-pyridyl terminated water-insoluble neutral surfactants on a Au(111) electrode were described using the results of dc and ac voltammetric measurements. In total six surfactants were used to elucidate the effects of electronic property of the 4-pyridyl head group, hydrogen bonding ability between amide groups, direction of the amide bond, and bulkiness around the head group upon the potential dependent reorientation and adsorption–desorption processes. Comparison of the behavior of pentadecyl 4-pyridyl ether (C15–O–Py) and 1-pyridin-4-yl-hexadecan-1-one (C15–(C O)–Py), possessing respectively, the highest and lowest excess electronic charge on the pyridyl nitrogen among the six surfactants, revealed that the latter exhibits more distinct reorientation of pyridyl group corresponding to phase transition between condensed and loose-packed states and narrower potential region of the compact film formation. Introduction of amide group in the alkyl chains of the surfactants reduced the packing of the adsorbed film in the potential region of the compact film formation, as evidenced by an increase of the differential capacitance. It was found that the existence of amide group facilitates the potential-driven phase transition of closely packed film from a condensed state to a loosely packed state, whereas it suppresses the transition of loosely packed film. The concomitant steric effect was also discussed. [Copyright &y& Elsevier]