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Thermodynamic Description of Synergy in Solvent Extraction: I. Enthalpy of Mixing at the Origin of Synergistic Aggregation
journal contribution
posted on 2016-11-02, 00:00 authored by J. Rey, S. Dourdain, J.-F. Dufrêche, L. Berthon, J. M. Muller, S. Pellet-Rostaing, T. ZembRevisiting
aggregation of extractant molecules into water-poor
mixed reverse micelles, we propose in this paper to identify the thermodynamic
origins of synergy in solvent extraction. Considering that synergistic
extraction properties of a mixture of extractants is related to synergistic
aggregation of this mixture, we identify here the elements at the
origin of synergy by independently investigating the effect of water,
acid, and extracted cations. Thermodynamic equations are proposed
to describe synergistic aggregation in the peculiar case of synergistic
solvent extraction by evaluating critical aggregation concentration
(CAC) as well as specific interactions between extractants due to
the presence of water, acid and cations. Distribution of two extractant
molecules in the free extractants and in reverse micelles was assessed,
leading to an estimation of the in-plane interaction parameter between
extractants in the aggregates as introduced by Bergström and
Eriksson (Bergström, M.;
Eriksson, J. C. A Theoretical Analysis of Synergistic
Effects in Mixed Surfactant Systems. Langmuir 2000, 16, 7173−7181). Based on this model, we study the N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA)
and di(2-ethylexyl) phosphoric acid (HDEHP) mixture and show that
adding nitric acid enhances synergistic aggregation at the equimolar
ratio of the two extractants and that this configuration can be related
to a favored enthalpy of mixing.