Transfer of ions Gibbs energy

A relative scale of the standard Gibbs energies of ion transfer or the standard ion transfer potentials can be established based on partition and solubility measurements. The partition eqnilibrium of the electrolyte can be characterized by a measnrable parameter, the partition coefficient P x-... 

The standard Gibbs energy of electrolyte transfer is then obtained as the difference AG° x ° = AG° ° - AG° x. To estabfish the absolute scale of the standard Gibbs energies of ion transfer or ion transfer potentials, an extrathermodynamic hypothesis must be introduced. For example, for the salt tetraphenylarsonium tetraphenyl-borate (TPAs TPB ) it is assumed that the standard Gibbs energies of transfer of its ions are equal. 

Table 2.1. Values o f standard Gibbs energies of ion transfer from water to nitrobenzene in electron volts. From P. Vanysek, Thesis, J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague (1982).

Gibbs energy of ion and dipole transfer — The standard -> Gibbs energy of ion transfer (see also -> ion transfer at liquid-liquid interfaces) can be represented as the difference of two -> solvation energies A Gf = A acGA -... 

Cyclic voltammetry has been used mainly for the determination of the standard ion-transfer potential Aq (or the standard Gibbs energy of ion transfer A ttx °), and e ion diffusion coefficient. The Figure shows an example of the cyclic voltammogram for the Cs+ ion-transfer reaction at ITIES in the electrochemical cell... 

It should also be mentioned that the Gibbs energy of ion transfer can be affected by complexation phenomena at the liquid liquid boundary. A classical example is given by the work of Koryta [25], who studied the transfer of from water to nitrobenzene assisted by dibenzo-18-crown-6. The complexation step decreases the energy of solvation of the cation in the organic phase, decreasing the formal transfer potential as defined in Eq. (5). Various mechanisms have been proposed for assisted ion-transfer processes, namely, aqueous complexation followed by transfer, transfer followed by complexation in the organic phase, transfer by interfacial complexation, or transfer by interfacial dissociation [26,27]. 

Updated lists of the values of standard Gibbs energies of transfer for various individual ions are given in Professor Hubert H. Girault s website at the EPFL of Switzerland (36). More valnes can be found in a data survey of Gibbs energy of ion transfer for 57 different solvents pubhshed by lUPAC (37). Table 17.3.1 lists some data for W/NB and W/DCE systems. 

Standard Gibbs energies of ion transfer are reported for various ions in Tables 20.10 and 20.11. Values are given for the two most common solvents used with water to form immiscible electrolyte boundaries, namely nitrobenzene and 1,2-dichloroethane. 

Table 1. Standard Gibbs energies of ion transfer and corresponding standard Galvani potentials in the water-nitrobenzene systems...

In 2005, an interesting critical summary of the current knowledge on capacitance measurements and potential distribution, together with a new model, was published by Monroe et al. [64]. In this work, the good old Verwey-Niessen theory was extended to allow ionic penetration at the interface. With this adaptation, several features could be accounted for, such as asymmetry and shifts of the capacitance minimnm, that could not be described by the classical Gouy-Chapman or Verwey-Niessen theories. Gibbs energies of ion transfer were used as input parameters to describe ionic penetration into the mixed-solvent interfacial layer, and experimental data were successfully reproduced. 

Scholz, R and R. Gulaboski, Determining the Gibbs energy of ion transfer across water-organic liquid interfaces with three-phase electrodes, Chemphyschem, Vol. 6, (2005) p. 16. 

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