The role of solvent and neutral organophosphorus extractant structure in their organization and association

The choice of organic solvent is known to impact metal ion partitioning in solvent extraction systems, although an understanding based on molecular interactions has been largely qualitative. To provide a more quantitative and molecular scale understanding of extractant and solvent association in liquid-liquid solvent extraction systems, molecular dynamics studies of binary extractant/solvent systems are conducted for several organophosphorus solvating extractants across a range of organic solvents. Classical molecular dynamics potentials for the extractants are optimized in pure phase simulations. These potentials are then validated with binary extractant/solvent solutions by comparison to experimental data for percent volume change on mixing and mixing enthalpies. Trends in association free energy, mixing enthalpy and deviations from ideal mixing volume are reported for each extractant and solvent binary mixture. Contributions to those properties depending on extractant and solvent molecular structure are investigated, including the relative enthalpic favorability of extractant solutions in toluene, diethyl ether or n-hexane. Inclusion of the ether group in diethyl ether lowered the enthalpy of mixing to a similar degree as reducing the alkane solvent chain length from n-dodecane to n-hexane owing to dipole-dipole interactions with the “head” and “tail” regions of the extractant molecules, which depend on extractant alkyl chain length. Despite similar percent volume changes on mixing with n-hexane or diethyl ether, extractant/toluene mixtures were significantly more enthalpically favorable with an enthalpy of mixing minimum controlled by stoichiometry rather than volume fraction, as observed for alkane solvents.