Molecular Interactions in Tri-n-butyl Phosphate−Diluent Mixtures

Abstract

This model considers both specific chemical and nonspecific physical contributions to excess Gibbs energy. Competitive chemical equilibria are evaluated to describe specific chemical interactions. Molecular size, shape difference, and regulation solution terms, based on the "true" species, are used to describe nonspecific physical interactions. Parameter estimates are obtained by fitting vapor-liquid equilibrium and hydrogen and phosphorus NMR shift data for three tri-n-butyl phosphate (TBP) and diluent binary mixtures. The thermodynamic equilibrium constant for the TBP dimer is a shared parameter since this complex apparently exists in all binary mixtures. At 22°C, its value is 6.65 based on the behavior of TBP diluted with either benzene, chloroform, or n-heptane. Thermodynamic equilibrium constants for the 1:1 TBP/ chloroform and TBP/benzene complexes are 37.88 and 1.34, respectively. The 1:2 chloroform/ TBP dimer complex has an equilibrium constant of 37.57. Model parameters are used to predict excess molar volumes and enthalpies. Commonly reported stability constants are shown to be concentration dependent.

Microscopic Structures of Tri-n-butyl Phosphate/n-Octane Mixtures by X-ray and Neutron Scattering in a Wide q Range

“…On the basis of these direct observations and arguments, we conclude that TBP is not dispersed homogeneously in octane in the absence of water and heavy metal ions. Thus, TBP forms a supramolecular assembly, consistent with the extensive body of previous literature ,− ,− demonstrating the formation of TBP dimers.…”

“…Therefore, the calculation of N agg is justifiable in the concentration range of 0.1 ≤ ϕ TBP ≤ 0.3, where the values are shown in Table . In many previous studies, ,− ,− it has been reported that TBP forms a dimer in nonpolar solvents, such as cyclohexane, hexane, and dodecane. Our evaluations of N agg , which show about two molecules (see Table , taking errors into consideration), are consistent with the historical literature on TBP dimerization.…”

“…In the high- q region, the scattering maxima I X and I N arise from the intermolecular dipole–dipole interactions involving the PO groups of the phosphate esters ,− ,− of two TBP molecules. The scattering maxima II X , II N , and III X originate from intramolecular interactions between TBP and octane.…”

“…Osseo-Asare reviewed the experimental evidence, reported before 1991, of the existence of self-associated species in TBP/diluent systems. − The self-association of TBP has been observed by various experimental techniques including partitioning studies, infrared spectroscopy, nuclear magnetic resonance, calorimetry, − and vapor pressure osmometry . These experiments revealed that the self-association of TBP is attributable to dipole–dipole interactions involving the PO groups of the phosphate ester mediated by hydrogen bonding with water, forming TBP dimers. ,− ,− Moreover, in that review on TBP–diluent/H 2 O–acid–metal ion extraction systems, Osseo-Asare proposed that the TBP–acid–metal ion complex consists of a high-order assembly (reverse micelle or microemulsion) in organic diluents. The self-assembled architecture is composed of a hydrophilic core (PO groups of TBP), which incorporates heavy metal ions, and a hydrophobic shell (butyl groups of TBP), which is swollen by diluent.…”

Spectroscopic, thermodynamic and ultrasonic properties of pseudo‐binary mixtures of organic extractants and vegetable oil at 25–60°C under atmospheric pressure

“…On the other hand, the densities of mixtures B and D increase with mole fraction of their corresponding extractant components as the densities of TBP and D2EHPA are greater than that of the vegetable oils. Both these phosphate compounds tend to form dimers 8,63 and transform their conformation from alpha to beta multimers 64 . This leads to a compact formation with less voids in between the molecules of the solvents.…”