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.

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