(629j) Methanol Self-Association and Preferential Solvation of Chelating Agents for the Extraction of Nuclear Fission Products in Supercritical CO2

While supercritical carbon dioxide (sCO₂) is a promising solvent in heavy metal extractions due to its low toxicity, surface tension and viscosity, many chelating molecules exhibit limited solubility in sCO₂. The addition of volatile polar molecules to sCO₂, such as methanol (MeOH), improves the solubility of chelating molecules and metal-ligand complexes. MeOH self-assembly and preferential solvation of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP), a chelating ligand, in sCO₂ at 19.7 MPa and 323 K was investigated with diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) and molecular dynamics simulations. DOSY-NMR pulse sequences contain a pair of pulsed field gradients that attenuate the NMR resonance and enable calculation of diffusivity for each NMR active component in the mixture. Diffusometry results were analyzed using a model that integrates the Stokes-Einstein relationship with self-association models to approximate the size-dependent, energy of cluster formation landscape. DOSY-NMR results are consistent with molecular dynamics simulations and suggest the formation of small clusters of MeOH limited to a composition of under 10 molecules in a cluster at high concentrations of MeOH. Next, self-assembly of HEHEHP into dimers was quantified with DOSY-NMR in sCO₂ at 19.7 MPa and 323 K. The propensity of HEHEHP to self-associate into dimers in sCO₂ was evaluated with the integrated Stokes-Einstein, self-association model. The results suggest that HEHEHP is primarily composed of dimers at concentrations of over 40 mM in sCO₂. Finally, we compare the extent of dimerization of HEHEHP in sCO₂ containing 4 mole % of MeOH to the dimerization of HEHEHP in neat sCO₂ with a series of DOSY-NMR experiments. The experiments indicate that MeOH preferentially solvates HEHEHP, increasing both the solubility of HEHEHP in the supercritical fluid and the weight fraction of monomeric HEHEHP. Our results suggests that the increased amount of monomeric chelators may be an additional, synergistic factor in the enhanced heavy metal extraction in MeOH modified sCO₂.