Aggregation Behavior of Nitrilotriacetamide (NTAmide) Ligands in Thorium(IV) Extraction from Acidic Medium: Small-Angle Neutron Scattering, Fourier Transform Infrared, and Theoretical Studies

Abstract

Two tripodal amides obtained from nitrilotriacetic acid with n-butyl and n-octyl alkyl chains (HBNTA(L I ) and HONTA(L II ), respectively) were studied for the extraction of Th(IV) ions from nitric acid medium. The effect of the diluent medium, i.e., n-dodecane alone and a mixture of n-dodecane and 1-decanol, onto aggregate formation were investigated using small angle neutron scattering (SANS) studies. In addition, the influence of the ligand structure, nitric acid, and Th(IV) loading onto ligand aggregation and third-phase formation tendency was discussed.The L I /L II exist as monomers (aggregarte radius for L I : 6.0 Å; L II :7.4 Å) in the presence of 1-decanol, whereas L II forms dimers (aggregarte radius for L II :9.3 Å; L I does not dissolve in n-dodecane) in the absence of 1-decanol. The aggregation number increases for both the ligands after HNO 3 and Th(IV) loading. The maximum organic concentration (0.050 ± 0.004 M) of Th(IV) was reached without third-phase formation for 0.1 M L I /L II dissolved in 20% isodecanol +80% ndodecane. The interaction of 1-decanol with L II and HNO 3 /Th(IV) with amidic oxygens of L I /L II results in shift of carbonyl stretching frequency, as shown by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) studies. The structural and bonding information of the Th-L I /L II complex were derived from the density functional theoretical (DFT) studies. The molecular dynamics (MD) simulations suggested that the aggregation behavior of the ligand in the present system is governed by the population of hydrogen bonds by phase modifier around the ligand molecules. Although the theoretical studies suggested higher Gibbs free energy of complexation for Th 4+ ions with L I than L II, the extraction was found to be higher with the latter, possibly due to the higher lipophilicity and solubility of the Th-L II aggregate in the nonpolar media.

Critical fluctuations in liquid–liquid extraction organic phases controlled by extractant and diluent molecular structure

“…45 The resulting three-phase system is incompatible with processing equipment, resulting in a limit to the per-contact loading capacity of the organic phase. While organic phase structure has long been linked to its phase behavior, 20,39,43,[46][47][48][49][50][51][52][53][54][55][56][57][58] we recently proposed that organic phase structure over a wide range of binary extractant/diluent mixtures is consistent with critical fluctuations originating from the critical point associated with the third phase formation phase transition. 59,60 That is, organic phase structuring is consistent with what would be expected from the concentration fluctuations induced by the critical point associated with the liquid-liquid phase transition.…”

“…7,14–18 To that end, LLE systems are often modeled as microemulsions, where organic phase structure is treated as colloidal particles representing reverse micellar aggregates. 19–43…”

Sequestration of Np(IV) and Pu(IV) with Hexa-n-octyl nitrilotriacetamide (HONTA) in an Ionic Liquid: Unusual Species vis-à-vis a Molecular Diluent

“…There is no clarity in the literature as to why there should be a decrease in the distribution ratio with an increase in the phase modifier concentration. However, one would assume the formation of aggregates (of HONTA) in the absence of the phase modifier, which could break in its presence . It can also be interpreted as an interaction between the phase modifier and the ligand molecules, which will eventually reduce the ligand concentration .…”

“…However, one would assume the formation of aggregates (of HONTA) in the absence of the phase modifier, which could break in its presence. 53 It can also be interpreted as an interaction between the phase modifier and the ligand molecules, which will eventually reduce the ligand concentration. 54 A better explanation is based on higher extraction of acid in the organic phase facilitated by the presence of a phase modifier.…”

Critical Fluctuations in Liquid-Liquid Extraction Organic Phases Controlled by Extractant and Diluent Molecular Structure

“…While organic phase structure has long been linked to its phase behavior, 20,39,43,[46][47][48][49][50][51][52][53][54][55][56][57][58] we recently proposed that organic phase structure over a wide range of binary extractant/diluent mixtures is consistent with critical fluctuations originating from the critical point associated with the third phase formation phase transition. 59,60 This could explain certain limitations to common microemulsion descriptions of LLE systems, such as the presence of structure in the organic phase even in the absence of extracted polar solutes needed to induce the formation of water-in-oil aggregates.…”