Binary Electrolyte Mixtures Research Articles

Diffusion coefficients in binary electrolyte mixtures by dynamic light scattering and molecular dynamics simulations

In this work, three binary electrolyte mixtures represented by different molecular solvents and dissolved lithium bis(trifluoromethylsulfonyl)imide ([Li][NTf2]) are characterized by determining their Fick diffusion coefficient D11. For this, dynamic light scattering (DLS) and molecular dynamics (MD) simulations were applied in the temperature range between (293.15 and 398.15) K at a solute mole fraction x[Li][NTf2] of 0.05. DLS is a contactless measurement technique that requires no calibration and allows to investigate multiple thermophysical properties, including D11, in macroscopic thermodynamic equilibrium. To get reliable thermophysical properties for the binary mixtures from MD simulations, force fields (FFs), describing inter- and intramolecular interactions between atoms, for the pure solvents were taken from the literature and reparametrized. The objective for the reparameterization was to accurately predict the static dielectric constant ε, density ρ, and viscosity η of the solvents. Using the optimized FFs, D11 for the binary electrolyte mixtures could be predicted via MD simulations and the results agree with those from DLS within an average absolute relative deviation of 23%. Most data points agree within combined uncertainties, considering an average expanded statistical uncertainty (coverage factor k = 2) of 18% for D11 from MD simulations and an average expanded experimental uncertainty (k = 2) of 10% for D11 from DLS. η and ε of the solvent were found to correlate with D11 and, thus, the diffusive mass transport in electrolyte mixtures. A large η leads to small D11, due to the slower dynamics in the system. Also a small solvent ε leads to small D11, which is due to the formation of ion pairs or clusters, which hinders the free movement.

Cooperative sorption of weak and strong electrolytes in microporous adsorbents

Sorption of pure electrolytes and binary electrolyte mixtures in non-ionic microporous adsorbents was studied in batch and fixed-bed systems. Hypercrosslinked polystyrene (HCPS) and densely crosslinked dextran gel (G10) were used as adsorbents and the studied model systems were HCl/CaCl2/HCPS at 25 °C and Na2-tartrate/NaOH/G10 at 50 °C. The results were explained in terms of cooperative sorption and the experimental data were correlated using a generalized Donnan equilibrium model together with Pitzer model for evaluation of the ion-ion and ion-adsorbent interactions. Moreover, a model was included for partial ion dehydration at high concentrations.Non-linear sorption observed at very low and high concentrations of pure electrolytes was explained by electrostatic exclusion and partial ion dehydration. In binary mixtures, a more complex behavior was observed because of cooperativity and good separation of the electrolyte mixtures was obtained in the fixed-bed experiments involving pulse feed and elution with pure water. In the HCl/CaCl2/HCPS system, selective uptake of HCl was enhanced by increasing the total electrolyte concentration. This trend can be explained by taking into account both size exclusion of CaCl2 and the non-ideal behavior of the electrolyte mixture. Separation of Na2-tartrate and NaOH was dominated by size exclusion of the tartrate anion and well-defined separation is possible even at very high column loading. Moreover, the observed splitting of Na2-tartrate to NaH-tartrate and NaOH can be explained on the basis of cooperativity. The results suggest that the proposed model applies equally well to materials having rigid microporous structure and to densely cross-linked gels.

Measurement of Setschenow constants for six hydrophobic compounds in simulated brines and use in predictive modeling for oil and gas systems.

Treatment and reuse of brines, produced from energy extraction activities, requires aqueous solubility data for organic compounds in saline solutions. The presence of salts decreases the aqueous solubility of organic compounds (i.e. salting-out effect) and can be modeled using the Setschenow Equation, the validity of which has not been assessed in high salt concentrations. In this study, we used solid-phase microextraction to determine Setschenow constants for selected organic compounds in aqueous solutions up to 2-5 M NaCl, 1.5-2 M CaCl2, and in Na-Ca binary electrolyte solutions to assess additivity of the constants. These compounds exhibited log-linear behavior up to these high NaCl concentrations. Log-linear decreases in solubility with increasing salt concentration were observed up to 1.5-2 M CaCl2 for all compounds, and added to a sparse database of CaCl2 Setschenow constants. Setschenow constants were additive in binary electrolyte mixtures. New models to predict CaCl2 and KCl Setschenow constants from NaCl Setschenow constants were developed, which successfully predicted the solubility of the compounds measured in this study. Overall, data show that the Setschenow Equation is valid for a wide range of salinity conditions typically found in energy-related technologies.

Density functional theory calculations for ethylene carbonate-based binary electrolyte mixtures in lithium ion batteries

The density functional theory (DFT) calculations have been performed to investigate the interaction of Li+ with various organic solvents widely used as Li ion rechargeable battery electrolytes such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC); and their EC-based binary mixtures at the level of B3LYP/6-31G (d). The interaction of Li+ with these solvents has been calculated in terms of electronic structures of clusters of the mixtures of organic solvents including a lithium ion. The main objective of our investigation is to help in understanding a stable and enhancing ionic transfer at graphite/electrolyte interface assisted by the mixtures of the solvents. The calculated results favor the stability of EC-based binary mixtures and high EC-content binary mixture systems. In infrared (IR) vibrational spectra, the IR active modes of the solvent show significant changes due to the cation-solvent interaction.

Synthesis of Pyrrolidinium-Based Poly(ionic liquid) Electrolytes with Poly(ethylene glycol) Side Chains

The synthesis and characterization of a new family of pyrrolidinium based poly(ionic liquid) (PIL) electrolytes with poly(ethylene glycol) (PEG) pendant groups is reported. The PILs were synthesized from a diallyl methyl amine hydrochloride monomer, which was obtained in large quantities using a modified Eschweiler-Clarke reaction. As additional plasticizers for the PILs, pyrrolidinium ionic liquids (ILs), also with PEG groups, were synthesized. All obtained PILs and ILs revealed excellent thermal stabilities to greater than 300 °C. Binary electrolyte mixtures were prepared by blending the PILs and ILs in different weight ratios. In addition, a ternary mixture of the best performing PIL and IL and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) was prepared. The obtained electrolyte blends showed very good ionic conductivities in the best case up to 2.4 mS cm–1 at 25 °C and 10.2 mS cm–1 at 100 °C and outperformed their pyrrolidinium counterparts with alkyl side chain that were synthesized as a refe...

(Invited) LiFSI-PYR1AFSI Binary Electrolyte Mixtures for Lithium Batteries

A physicochemical and electrochemical investigation on lithium batteries binary electrolyte mixtures, based on N-alkyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquids (PYR1AFSI) and lithium bis(fluorosulfonyl)imide (LiFSI) as salt, is presented and discussed. The melting point of PYR1AFSI decreases with the addition of LiFSI, this allowing to obtain low-temperature (-20{degree sign}C) highly conductive (0.5 mScm-1) and non volatile electrolytes. The ionic transport properties were found well correlated with the thermal behavior. The incorporation of LiFSI resulted in a decreased thermal stability and the appearance of minor parasitic cathodic reactions with respect the neat PYR1AFSI materials, likely due to impurities in the LiFSI salt. Despite this issue, the electrochemical stability window was found to exceed 5 Volt.

Critical point of electrolyte mixtures

The critical behavior of electrolyte mixtures was studied using grand canonical Monte Carlo simulations. Mixtures consist of large multivalent macroions and small monovalent co- and counterions. The system can be viewed as a binary mixture of macroions (with their counterions) and salt (co- and counterion pair). The primitive model description was used, in which the ions are point charges with a hard core and the solvent is treated as a uniform dielectric continuum. The grand canonical simulations are based on insertions and removals of neutral molecules: macroion with its counterions or coions and a counterion. We propose a distance biasing method that enables direct grand canonical simulations up to charge asymmetry of 10:1. We calculated the critical loci that connect the salt-free state, which consists of only macroions and counterions, with the pure salt state using mixed-field finite-size scaling with no pressure mixing. The critical parameters are determined for macroion to counterion charge asymmetries of 2:1, 3:1, and 10:1. Our results suggest that binary electrolyte mixtures are type-I mixtures, where the two components mix continuously.

Electrodialysis of binary electrolyte mixtures with membranes modified by organic species

The modification of anion-exchange membranes MA-40 by lauric acid and sodium polystyrenesulfonate is studied. By measuring the wetting angle and using IR spectroscopy, the mechanism of sorption of the modifiers by the membranes is established. The membrane modification is accompanied by hydrophobization of a surface layer and the formation of strong bonds between functional groups of membranes and modifiers. The concentration dependence of the ion separation coefficients in electrodialysis of binary electrolyte mixtures with modified and nonmodified membranes is studied. The selectivity of modified membranes to less-hydrated ions increases and the smaller the solution concentration, the larger the ions.

Electrochemical synthesis of cosin from fluorescein

Electrosynthesis of cosin has been carried out by brominating fluorescein in different media such as NaHCO3, CH3COONa, NH3, etc., and also in several binary electrolyte mixtures at stationary/rotating anodes at different current densities and temperatures. Based on the results of the yield and dye content obtained in different media, the best conditions for the electrosynthesis of eosin have been selected. The yield of eosin was extremely sensitive to electrolyte pH. NaHCO3 and CH3COONa were found to be suitable media. TSTΔ(Ti/TiO2, RuO2) was found to be most suitable in the production of cosin. An attempt at reuse of electrolyte was successful in NaHCO3 medium with the dye being isolated as its sodium salt. A plausible mechanism for the bromination of fluorescein has been discussed. Scale-up studies on eosin are in progress.

Activity coefficients in binary electrolyte mixtures HCl + MgCl2 + H2O at 298.15�K

Activity coefficients of hydrochloric acid in aqueous mixed solutions with magnesium chloride have been determined at 298.15°K from electromotiveforce measurements of the cell Pt, H2(g, 1 atm.)⋎HCl(mA), MgCl2(mB)⋎AgCl⋎Ag at constant total ionic strengths of 0.1, 0.5, 1.0, 2.0, and 3.0 moles-kg−1. The data were interpreted in terms of Scatchard's and Pitzer's equations whereupon it was found that the former gave a better fit of the experimental data but the latter were reasonably adequate. Activity coefficients for magnesium chloride in the mixtures were derived using Pitzer's equations.