The Measurement of the Gibbs Energy of Transfer Between Oil and Water Using a Nano-Carbon Paste Electrode

The use of nano-carbon paste electrodes for the measurement of Gibbs energies of transfer between oil and aqueous phases is reported. In this method the oil of interest is used as the binder for the nano-carbon paste electrodes and the molecule of interest is dissolved in the organic or aqueous phase. Voltammetry is performed over a period of time and used to monitor the transfer of the molecule between the two phases. The method is illustrated for the transfer of ferrocenemethanol between water and oil using the ferrocenemethanol / ferroceniummethanol (FcCH2OH/FcCH2OH+) redox couple. Three pairs of voltammetric peaks were observed in a 0.1M KCl solution when the nano-carbon paste electrode was modified by dissolution of FcCH2OH in the binder oil: P1 [E=0.23V, 0.17V vs. Ag/AgCl (1M KCl)], P2 [E=0.36V, 0.32V vs. Ag/AgCl (1M KCl)] and P3 [E=0.55V, 0.46V vs. Ag/AgCl (1M KCl)]. These are assigned to the FcCH2OH species existing in the aqueous solution [FcCH2OH(aq)/FcCH2OH+(aq)], originating in the oil (o) [FcCH2OH(o)/FcCH2OH+(aq)] and to oxidation of adsorbed (ads) material on the nano-carbon [FcCH2OH(ads)] respectively. When supporting electrolyte containing the anions Cl-, NO3- or SCN- was used, an expulsion of the oxidised ferrocene occurred and the difference in midpoint potentials (Emid) between the peaks P1 and P2 observed in these experiments allowed the calculation of the Gibbs energy (ΔG°) of transfer of ferrocenemethanol from water to oil. The average ΔG° value thus obtained was (-12.7±0.2) kJmol-1. For more hydrophobic anions (X-=PF6-, AsF6-), the electron transfer is coupled to the transfer of the anion into the oil and the ΔG° for the transfer of the ion pair of FcCH2OH+ and X- ions from water to oil was found to be -1.3 and -3.9kJmol-1 for PF6- and AsF6- respectively. © 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim