Quantifying Dimer and Trimer Formation of Tri-n-butyl Phosphates in Different Alkane Diluents: FTIR Study

Abstract

Tri-n-butyl phosphate (TBP), a representative of neutral organophosphorous metal-ion-extracting reagents, is an important ligand used in solvent extraction processes for the recovery of uranium and plutonium from spent nuclear fuel, as well as other non-nuclear applications. Ligand-ligand and organic solvent-ligand interactions play an important role in these processes. The self-association behavior of TBP in various alkane diluents of different chain lengths (8, 12, and 16 carbons) and a branched alkane (iso-octane) was investigated by Fourier transform infrared spectroscopic measurements. By careful deconvolution of the spectra into multiple peaks, our results indicate that TBP self-associates to form not only dimers, as previous studies showed, but also trimers in the practical concentration range. Using a mathematical fitting procedure, the dimerization and trimerization constants were determined. As expected, these equilibrium constants are dependent on the solvent used. As the alkane chain for linear hydrocarbon solvents becomes longer, dimerization decreases whereas trimerization increases. For the more branched hydrocarbon, we observe a significantly higher dimerization constant. These effects are most likely due to the intermolecular van der Waals interactions between the butyl tails of each TBP molecule and the diluent hydrocarbon chain as all solvents in this study are relatively nonpolar.

Quantifying Dimer and Trimer Formation by Tri-n-butyl Phosphates in n-Dodecane: Molecular Dynamics Simulations

“…Our simulated TBP self-association values of k 2 = 1.54 ± 0.07 M −1 and k 3 = 0.56 ± 0.09 M −1 showed excellent agreement with our experimental study reported in a separate study. 21 The results from this work validate the accuracy of our optimized force fields for TBP and n-dodecane molecules. The new insights into the conformational behaviors of the TBP molecule as a monomer and as part of an aggregate will greatly aid in the understanding of the complex formation between TBP and metal ions and the third phase formation later on when we pursue our ambitious goal of simulating a complete solvent extraction system.…”

“…We have also carried out an experimental study employing the FTIR technique in parallel with this MD work to validate our results. 21 The agreement between these studies will be discussed throughout this section.…”

“…This work provided values of 1.68 ± 0.06 and 0.78 ± 0.12 M −1 for the dimerization and trimerization constants, respectively. 21 Comparison of the mole percentage of each TBP species in the TBP/n-dodecane mixtures between MD simulations and FTIR experiments 21 shows excellent agreement (Figure 10).…”

“…To our knowledge, there is not any experimental work that has addressed this concern. Thus, we also carried out a separate experimental study employing the FTIR technique in parallel to see if we could observe the existence of trimers and validate our results from MD simulations.…”

“…To our knowledge, there is not any experimental work that has addressed this concern. Thus, we also carried out a separate experimental study 21 employing the FTIR technique in parallel to see if we could observe the existence of trimers and validate our results from MD simulations. In this work, utilizing our previously optimized force fields for TBP and n-dodecane molecules, 19 we first examine TBP isomerism by performing gas phase simulations of a single TBP molecule and constructing the potential mean force (PMF) of various TBP monomer conformations.…”