Quantify Water Extraction by TBP/Dodecane via Molecular Dynamics Simulations

Abstract

Objective: The purpose of this project is to quantify the interfacial transport of water into the most prevalent nuclear reprocessing solvent extractant mixture, namely tri-butylphosphate (TBP) and dodecane, via massively parallel molecular dynamics simulations on the most powerful machines available for open research. Specifically, we will accomplish this objective by evolving the water/TBP/dodecane system up to 1 ms elapsed time, and validate the simulation results by direct comparison with experimentally measured water solubility in the organic phase. The significance of this effort is to demonstrate for the first time that the combination of emerging simulation tools and state-of-the-art supercomputers can provide quantitative information on par to experimental measurements for solvent extraction systems of relevance to the nuclear fuel cycle.Efforts and Results: To achieve the object described above, initially, the single component, single phase systems were studied to verify existing and develop new molecular models, followed by a single phase mixture, and then finally the complete twophase water extraction system. Specifically, the systems we studied are: (1) pure TB; (2) n-doedecane; (3) TBP/n-dodecane mixture; and (4) the complete extraction system: water-TBP/n-dodecane two phase system to gain deep insight into the water extraction process. We have completely achieved our goal of extracting water molecules into the TBP/n-dodecane mixture to saturation and obtained favorable comparison with experiment. Many insights into fundamental molecular level processes and physics were obtained from the process. Most importantly, we found that the dipole moment of the 2 Bu=C 4 extracting agent is crucially important in affecting the interface roughness and the extraction rate of water molecules into the organic phase. In addition, we have identified shortcomings in the existing OPLS-AA force field potential for long-chain alkanes. The significance of this force field is that it is supposed to be optimized for molecular liquid simulations. We found that it failed for n-dodecane and/or longer chains to correctly predict the liquid state and thus not usable for our water extraction simulation. We have proposed a simple modification to circumvent the artificial crystallization of the long alkane chains applicable at ambient condition.(b) Effort performed and the accomplishments achieved TBP Force Field Model DevelopmentWe conducted force field survey on 4 sets of force field parameters for TBP, a molecule composed of a phosphate head group and three butyl tails as shown in Figure 1. The 4 sets of parameters are based on adoption of Van der Waals parameters for the phosphoryl head group and the butyl tail group of the TBP molecules, from separate force field models already in use. In particular, we used the Amber parameters for the Van der Waals interaction of the phosphoryl group, combined with either the Amber or the OPLS-AA (Optimized Potential for Liiquid Simulation-All Atoms) force field for the butyl tail...