No abstract

Various kinds of cations (alkali and alkaline earth metal ions, Me4N+, Et4N+, n-Bu4N+, and tetraphenylarsonium ion) and anions (halide ions, SCN-, ClO4 -, NO3 -, and tetraphenylborate ion) have been extracted from water to nitrobenzene (NB) using several extractants:  viz. tetraphenylborate and dipicrylaminate for the cations; n-Bu4N+, n-Pen4N+, n-Hep4N+, and tris(1,10-phenanthroline)iron(II) for the anions. The determination of water content in NB by means of the Karl Fischer method has confirmed that some water molecules are coextracted by hydrophilic inorganic cations and anions. Accurate numbers (n) of the coextracted water molecules per ion have been established. On the basis of these findings, a new model has been proposed for a better understanding of the Gibbs energy of ion transfer at the organic solvent/water interface. If hydrated radii of ions evaluated from n are used, conventional (Born-type) electrostatic solvation models are invalid. A new approach recognizing short-range ion−solvent interactions (e.g. hydrogen bonds) has given a better account of for the hydrated ions.

A non-Bornian theory has been proposed for the standard Gibbs free energy (∆G°t r ,OfW) of ion transfer at the organic solvent (O)/water (W) interface. In this theory the ionic charge (z)-dependent term of ∆G°t r ,OfW , which has so far been considered as describing electrostatic (i.e., long-range) ion-solvent interactions, is treated as describing the specific (short-range) interactions. A quantum chemical consideration has shown that the short-range interaction energy can be given by a quadratic function of the surface field strength (E) of an ion. In the present theory it should also be noted that a hydrophilic ion, being preferentially hydrated in the O phase, is assumed to transfer across the interface as the hydrated ion. The new, non-Bornian theory has been applied to 34 ions in the nitrobenzene (NB)/W system. In the data analyses the ions have been classified into five groups, including hydrated cations, nonhydrated cations, hydrated anions, nonhydrated anions, and polyoxometalate anions (nonhydrated), where "hydrated" or "nonhydrated" means that the ion is associated with some water molecules in NB or not. Regression analyses have shown that in accord with the theory, the z-dependent term of ∆G°t r ,OfW can be given by a quadratic function of E for each group of ions. Furthermore, the present theory has been found to be useful for the prediction of ∆G°t r ,OfW for some hydrophilic ions.

No abstract

The transfer of proteins by the anionic surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) at a polarized 1,2-dichloroethane/water (DCE/W) interface was investigated by means of ion-transfer voltammetry. When the tetrapentylammonium salt of AOT was added to the DCE phase, the facilitated transfer of certain proteins, including cytochrome c (Cyt c), ribonuclease A, and protamine, could be controlled electrochemically, and a well-defined anodic wave for the transfer was obtained. At low pH values (e.g., pH 3.4), the anodic wave was usually well-separated from the wave for the formation of protein-free (i.e., unfilled) reverse micelles. The anodic wave for the protein transfer was analyzed by applying the theory for facilitated transfer of ions by charged ligands and then supplying information regarding the number of AOT anions reacting with one protein molecule and the total charge carried by the protein transfer. However, controlled-potential electrolyses performed for the transfer of Cyt c, which is red, revealed that the protein-AOT complexes were unstable in DCE and liable to aggregate at the interface when the pH of the W phase was 3.4. At pH 7.0, when formation of unfilled reverse micelles occurred simultaneously, the protein-AOT complexes appeared to be stabilized, probably via fusion with unfilled reverse micelles.