(138c) Molecular Dynamics Simulation of Interfacial Water Extraction by TBP/n-Dodecane

Solvent extraction is a commonly used process in industrial nuclear fuel reprocessing. Metal cations, acid, and water are typically co-extracted from the aqueous phase into the organic phase mixture of an extracting agent, such as tri-n-butyl phosphate (TBP), and a diluent, such as n-dodecane. Quantification of solute extraction by molecular dynamics (MD) simulations is an open area of research. Major challenges include development of molecular models and long-time simulations to determine relevant transport rates. To this end, we have been developing practical force fields and performing corresponding two-phase simulations to predict water solubility in an organic phase consisting of TBP/dodecane. Water extraction is important for model/simulation validation and for practical operation of extraction processes since it competes for available solvent agent molecules. Extensive molecular dynamic simulations of pure water in direct contact with a TBP/n-dodecane organic phase have been performed in the NPT ensemble at 25 ^oC and 1 bar using LAMMPS with the Verlet integrator and a Nosé-Hoover thermostat and barostat with a time step of 1 fs.

 Our results indicate that the atomic charges on the TBP molecular model strongly affect water molecule extraction into the organic phase. The charges are systematically varied so to reproduce the experimental dipole moment of the TBP molecule. Higher dipole moments of TBP increase the roughness of the water/organic interface by reducing the interfacial tension. A “rougher” interface increases the interfacial area, and facilitates water hydrogen bonding breaking which is found to be the rate limiting process for water extraction. We have quantitatively characterized the aqueous/organic interface, including: the distribution profiles of water, TBP, and n-dodecane across the interface, the preferential orientation of TBP, and the interfacial coverage by TBP molecules. Moreover, we have conducted extensive MD simulations (~100 ns) on large systems (~360,000 atoms) to obtain the saturated water solubility in the organic phase, which is found to compare well with experiments. Our simulations indicate that water extraction is intricately affected by TBP amphiphilicity, interface topology, dynamics of hydrogen bond breaking, water cluster formation, and temperature.