Improving understanding of solvent effects on intermolecular interactions in reactive liquid–liquid extraction with Isothermal Titration Calorimetry and molecular modeling

Solvent effects on extractant conformational energetics in liquid-liquid extraction: a simulation study of molecular solvents and ionic liquids

Extractant design in liquid-liquid extraction (LLE) is a research frontier of metal ion separations that typically focuses on the direct extractant-metal interactions. However, a more detailed understanding of energetic drivers of separations beyond primary metal coordination is often lacking, including the role of solvent in the extractant phase. In this work, we propose a new mechanism for enhancing metal-complexant energetics with nanostructured solvents. Using molecular dynamics simulations with umbrella sampling, we find that the organic solvent can reshape the energetics of the extractant's intramolecular conformational landscape. We calculate free energy profiles of different conformations of a representative bidentate extractant, n-octyl(phenyl)-N,N-diisobutyl carbamoyl methyl phosphinoxide (CMPO), in four different solvents: dodecane, tributyl phosphate (TBP), and dry and wet ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]). By promoting reorganization of the extractant molecule into its binding conformation, our findings reveal how particular solvents can ameliorate this unfavorable step of the metal separation process. In particular, the charge alternating nanodomains formed in ILs substantially reduce the free energy penalty associated with extractant reorganization. Importantly, using alchemical free energy calculations, we find that this stabilization persists even when we explicitly include the extracted cation. These findings provide insight into the energetic drivers of metal ion separations and potentially suggest a new approach to designing effective separations using a molecular-level understanding of solvent effects.

Applications of isothermal titration calorimetry in pure and applied research from 2016 to 2020

The last 5 years have seen a series of advances in the application of isothermal titration microcalorimetry (ITC) and interpretation of ITC data. ITC has played an invaluable role in understanding multiprotein complex formation including proteolysis-targeting chimeras (PROTACS), and mitochondrial autophagy receptor Nix interaction with LC3 and GABARAP. It has also helped elucidate complex allosteric communication in protein complexes like trp RNA-binding attenuation protein (TRAP) complex. Advances in kinetics analysis have enabled the calculation of kinetic rate constants from pre-existing ITC data sets. Diverse strategies have also been developed to study enzyme kinetics and enzyme-inhibitor interactions. ITC has also been applied to study small molecule solvent and solute interactions involved in extraction, separation, and purification applications including liquid-liquid separation and extractive distillation. Diverse applications of ITC have been developed from the analysis of protein instability at different temperatures, determination of enzyme kinetics in suspensions of living cells to the adsorption of uremic toxins from aqueous streams.

Can the Isothermal Calorimetric Curve Shapes Suggest the Structural Changes in Micellar Aggregates?

Inspired by the unusual shapes of the titration curve observed for many surfactants and mixed colloidal systems, we decided to extend the analysis to isothermal titration calorimetric curves (ITC) by paying special attention to potential structural changes in micellar aggregates. In this paper, we used isothermal titration calorimetry in conjunction with Scanning Transmission Electron Microscopy (STEM), Small-Angle Neutron Scattering (SANS) and X-ray Scattering (SAXS) methods support by Monte Carlo and semiempirical quantum chemistry simulations to confirm if the isothermal calorimetric curve shape can reflect micelle transition phenomena. For that purpose, we analysed, from the thermodynamic point of view, a group of cationic gemini surfactants, alkanediyl-α,ω-bis(dimethylalkylammonium) bromides. We proposed the shape of aggregates created by surfactant molecules in aqueous solutions and changes thereof within a wide temperature range. The results provide evidence for the reorganization processes and the relationship (dependence) between the morphology of the created aggregates and the conditions such as temperature, surfactant concentration and spacer chain length which affect the processes.

Evaluation of Distribution of Succinic Acid between Binary Phase System with Biodiesel + N,N-Dioctyloctan-1-amine

The present study is aimed at using one of the most promising methods called reactive extraction to extract succinic acid from aqueous solution by using N,N-dioctyloctan-1-amine in biodiesel as diluent made from sunflower oil, rice bran oil, sesame oil, and karanji oil. The results of extraction studies with the diluents (physical) showed their inability to recover any acid by themselves. In reactive extraction, the organic phase extracting power solely depends on tri-n-octylamine. The ranges of the distribution coefficient are found as 7.62–18.12 for sunflower oil biodiesel, 8.33–17.45 for rice bran oil biodiesel, 7.0–17.67 for sesame oil biodiesel, and 9.85–21.36 for karanji oil biodiesel. The ranges of the loading ratio are 0.1–3.0 for sunflower oil biodiesel, 0.1–2.9 for rice bran oil biodiesel, 0.2–2.9 for sesame oil biodiesel, and 0.1–2.9 for karanji oil biodiesel. The karanji and sunflower oil showed higher values of distribution coefficient (KD) over rice bran oil and sesame oil which might be due to presence of both C20 and special fatty acids. The results show that biogenous diluents along with N,N-dioctyloctan-1-amine as extractant form a nontoxic and viable option for the extraction of succinic acid in the binary phase system.