Presence of a Molecular Ligand alters the Solvent Behavior of an Ionic Liquid: A Combined Physicochemical Properties, FTIR Spectroscopy and Dynamic Light Scattering Investigation

Abstract

The physicochemical properties of an ionic liquid (IL) based extraction system depends on the type of cation and anion present and the type of extractant dissolved in it. In this paper, we have investigated the solvent behavior of a ionic liquid based system comprising of a diglycolamide based extractant: T2EHDGA (N,N,N’,N’‐tetra(2‐ethylhexyl) diglycolamide) dissolved in an imidazolium based ionic liquid: [C4mim][NTf2] (1‐butyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl) imide). The properties of IL systems such as dynamic viscosity (η) and density (ρ) were measured as a function concentration of T2EHDGA, nitric acid and the experimental temperature using a rolling ball viscosmeter and a U‐tube densitometer. Viscosity and density of IL phase increase and decrease respectively in the presence of ligand in it. Dynamic Light Scattering (DLS) results obtained by DLS spectrometer inferred the aggregation behavior of IL phase and the average aggregate size of IL increases in presence of T2EHDGA. The water uptake by IL phase upon acid equilibration inferred from FTIR spectroscopy specified a decrease in visocisty and density of the solvent. The evaluation of molecular volume (Vm) and standard entropy (So) offered higher values for T2EHDGA as compared to [C4mim][NTf2] whereas, the lattice potential energy (UPot) showed reverse trend. The refractive index (n) of T2EHDGA/IL mixture measured by a refractometer suggested the considerable changes in presence of T2EHDGA and nitric acid.

Ligand Effect on Physicochemical Properties of Ionic Liquid

“…[92] To this when an extractant is added the average size increase drastically implying the self-aggregation of the extractant as well as the intermolecular interaction between ligand and ionic liquid. [48,49,52] From Figure 7A, it is understood that the average aggregate size increases with a rise in the concentration of extractant and is sharp for T2EHDGA (value increases remarkably within the range of very less concentration difference, 0 M to 0.1 M in [C 4 mim][NTf 2 ] phase) as compared to TBP (0 M to 1.6 M). As explained elsewhere, T2EHDGA undergoes strong self-aggregation due to its structure and the functional entities present in it and this is a perfect indication of the associative interaction of T2EHDGA with [C 4 mim][NTf 2 ] as a result of which the distribution of aggregation increases in IL phase.…”

“…The ionicity of IL phase gets perturbed when a molecular ligand is added to it and accordingly the physicochemical properties change. [52] It must be highlighted here that while considering the literature reported viscosity of [C 4 mim][NTf 2 ], there is a number of reports citing different values at room temperature. [29,[74][75][76][77][78] This is due to the variance in purity and source of the precursor chemicals as well as the slight variation in the property between a laboratory synthesized and a commercially available product.…”

“…Among these, the viscosity of TBP, D2EHPA, T2EHDGA, HDEHDGA and DHOA have been reported by various research groups with marginal differences in the value. [41,[47][48][49][50][51][52][68][69][70][71][72][73] This could be possibly due to different measurement tools and variance in the source of the precursor reagents for making the compound as well as the precise temperature setting. The order of viscosity for these ligands follow: HDEHDGA > T2EHDGA > D2EHPA > DHOA > TBP and the value exponentially decrease with an increase in temperature (for bringing all the data points in one platform, the logarithmic value has been taken in Figure 2A).…”

“…In this context, the viscosity of IL phase containing different extractants has been reported elsewhere. [48][49][50][51][52] The authors have reported that 'η' value decreases drastically with an increase in feed acidity and the decrease is more prominent in presence of ligands in IL phase except for DHOA/IL mixture (vide infra). [49] The reason is attributed to the ample amount of nitric acid (and hence water) extraction which essentially improves the mobility of ions by decreasing the intermolecular force of attraction and the presence of ligands enables strong 5B).…”

“…[42][43][44][45][46] As far as the detailed understanding of the physicochemical properties of ILs used as the diluent in conjunction with different extractants used for the reprocessing of SNFs is concerned, there are interesting reports those mainly deal with the understanding of different physicochemical properties of the most extensively used imidazolium based ionic liquid: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C 4 mim][NTf 2 ]) containing a different class of nuclear extractants (extractants used for the separation of actinides and fission products). [47][48][49][50][51][52] Since the research in this direction is at its growing stage and is certainly an interesting research field, the accumulation of all the published results on various physicochemical properties of aforementioned solvent systems indeed would bring out the overall knowledge of an IL solvent for SNF reprocessing application.…”