Mechanism and rate of anomalous extraction of chloride salts from water to 1,2-dichloroethane

Abstract

Open circuit potential (OCP) measurements, potentiostatic pulse (PP) amperometry and cyclic voltammetry (CV), were used to investigate and to sort by rate the anomalous extraction of the chloride salts RCl of the cations R+ = tetrabutylammonium (TBA+), tetrapentylammonium (TPeA+), tetrahexylammonium (THexA+), tetraheptylammonium (THepA+) and bis(triphenyl-phosphoranylidene)ammonium (BA+) from water to 1,2-dichloroethane (DCE). A tentative mechanism is proposed comprising (a) the diffusion-controlled transport of the ion-pairs (RCl)ip from the aqueous to the organic solvent phase providing a major contribution to the chloride extraction, (b) decomposition of the multiple ion-pairs or clusters (RCl)ipx, which are formed by agglomeration of the single ion-pairs (RCl)ip in the aqueous phase (w), and (c) the exponential decay of the ion-pairs (RCl)ip accumulated on the organic solvent side of the interface at the beginning of the OCP measurements. This model enables to simulate successfully the experimental time dependence of the chloride concentration. A conclusion is made that the effect of the composition of the organic solvent electrolyte RX, where X- = tetraphenylborate (TPB-) or tetrakis(pentafluorophenyl) borate (TB-), on the rate and extent of the anomalous accumulation of the chloride anion in the phase (o) of the two-phase liquid system LiCl(w)/RX(o) can be related to the hydrophobicity of the electrolytes RCl and RX. The latter property can be characterized by the standard Gibbs energy of transfer from the aqueous (w) to the organic solvent (o) phase. The accumulation of the chloride anion in the phase (o) appears to be promoted by the decreasing value of the transfer Gibbs energy of RCl in the sequence TBACl > TPeACl > THexACl > THepACl > BACl, while the decreasing value of the transfer Gibbs energy of RX in the sequence TBATPB > TPeATPB > THexATPB > THepATPB > BATPB > BATB starts to block the cycle combining the energetically unfavorable extraction of RX from the phase (o) to the phase (w) with the energetically favorable extraction of RCl in the opposite direction. IR spectroscopy provides evidence of the formation of the water clusters in the organic solvent phase equilibrated with the aqueous phase in the presence of the tetraalkylammonium cation in both phases, which are supposed to drive the anomalous accumulation of the chloride anion in the phase (o).