At present, the mechanism of the influence of salinity on wettability of reservoir rocks is still insufficient. In this paper, molecular dynamics simulation is used to study the wetting behavior of ...water droplets on the surface of sandstone under different salinities, and the system equilibrium configuration is used to analyze the interaction among the components of the system. To clarify the mechanism of the influence of ions on the wetting of water droplets on the sandstone surface, the distribution characteristics of anions and cations, ion hydration and the association between anions and cations in the equilibrium system were analyzed. The influence of salinity on the wettability of water droplets on the sandstone surface is comprehensively evaluated by calculating the average interaction potential between ions and water molecules, the change in cohesive energy density among particles in the brine system and the change in the number of hydrogen bonds among water molecules.
Display omitted
•A Stern double-electric layer is used to explain the distribution characteristics.•The structural characteristics of the ion hydration layer were clarified.•The form of existence of ion pairs in solution was studied.
The formation of an ion-association complex (IA) between sulfonephthalein dye and basic nitrogen-containing compound in an organic solvent medium has been for the first time used to develop an ...automated SIA method. In highly polar aprotic solvents, the tautomeric equilibrium for such dyes is strongly shifted towards the colorless lactonic form. The addition of a basic nitrogen-containing substance leads to the formation of IA with a highly colored quinonoid form, which is accompanied by an increase in the absorbance of the dye band at approximately 400 nm. Protonation of pyridine nitrogen in loratadine, structure and binding places of IA were shown using quantum-chemical calculations. The very simple, direct and non-extraction spectrophotometric SIA method with high throughput of 43 h−1 was developed based on the formation of IA between loratadine and bromocresol purple in the medium of acetonitrile used both as solvent and carrier. The calibration graph was linear in the concentration range from 1.0 to 20 mg L−1 with correlation coefficient of 0.9992. The developed method was successfully applied to the analysis of pharmaceutical formulations.
Display omitted
•Interaction of sulfonephthalein dyes and nitrogen-containing compounds.•Shift of tautomeric equilibrium in an organic solvent medium.•Non-extraction spectrophotometric SIA method for loratadine determination.•Loratadine was determined using its ion associate with bromocresol purple.
Block copolymer electrolytes (BCE) such as polystyrene-block-poly(ethylene oxide) (SEO) blended with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and composed of mechanically robust ...insulating and rubbery conducting nanodomains are promising solid-state electrolytes for Li batteries. Here, we compare ionic solvation, association, distribution, and conductivity in SEO-LiTFSI BCEs and their homopolymer PEO-LiTFSI analogs toward a fundamental understanding of the maximum in conductivity and transport mechanisms as a function of salt concentration. Ionic conductivity measurements reveal that SEO-LiTFSI and PEO-LiTFSI exhibit similar behaviors up to a Li/EO ratio of 1/12, where roughly half of the available solvation sites in the system are filled, and conductivity is maximized. As the Li/EO ratios increase to 1/5 the conductivity, of the PEO-LiTFSI drops nearly 3-fold, while the conductivity of SEO-LiTFSI remains constant. FTIR spectroscopy reveals that additional Li cations in the homopolymer electrolyte are complexed by additional EO units when the Li/EO ratio exceeds 1/12, while in the BCE, the proportion of complexed and uncomplexed EO units remains constant; Raman spectroscopy data at the same concentrations show that Li cations in the SEO-LiTFSI samples tend to coordinate more to their counteranions. Atomistic-scale molecular dynamics simulations corroborate these results and further show that associated ions tend to segregate to the SEO-LiTFSI domain interfaces. The opportunity for “excess” salt to be sequestered at BCE interfaces results in the retention of an optimum ratio of uncompleted and complexed PEO solvation sites in the middle of the conductive nanodomains of the BCE and maximized conductivity over a broad range of salt concentrations.
Understanding ion transport in membrane materials is key to engineering and development of desalination and water purification technologies as well as electro-membrane applications. To date, modeling ...of ion transport has mainly relied on mean-field approaches, originally intended for weak inter-ionic interactions, i.e., high reduced temperature T*. This condition is violated in many membranes, which could explain disagreement between predicted trends and experiments. The paper highlights observed discrepancies and develops a new approach based on the concept of ion association, more adequate in the low-T⁎ limit. The new model addresses ion binding and mobility consistently within the same physical picture, applied to different types of single and mixed salts. The resulting relations show a significantly weaker connection between ion partitioning and permeability than the standard ones. Estimates using primitive model (PM) of ions in a homogeneous dielectric suggest that non-PM mechanisms, originating from the molecular structure of the ion-solvating environment, might enhance ion association in membranes. PM analysis also predicts that ion solvation and association must be rigidly related, yet non-PM effects may decouple these phenomena and allow a crossover to non-trivial regimes consistent with experiments and simulations. Despite the crude nature of the presented approach and some questions remaining open, it appears to explain most available experimental data and presents a step towards predictive modeling of ion-selective membrane separations in water-, environment- and energy-related applications.
Display omitted
•Membrane characteristics in RO, NF and electro-separations represent low T* regime.•Mean-field high-T* models are inadequate for ion transport in low-T* membranes.•Appropriate approach introduces ion association as key element and explains observed trends.•Association decouples permeability from partitioning and modifies mobility.•Primitive electrostatics is insufficient for predicting ion-specific solvation and association.
Predicting ion uptake and selectivity in ion-exchange membranes is desired for many applications, yet a suitable physical description defining the most appropriate ion-specific parameters is still ...challenging. Here, we systematically develop an ion-association-based approach to modeling ion uptake in ion-exchange membranes from solutions of symmetric and non-symmetric salts. The model treats association in an ion-specific manner, self-consistently accounting for equilibria between free ions in solution and within the membrane phase (salt injection) and between free and associated species within the membrane (association equilibria), subjects to overall membrane electroneutrality. The resulting models, including different possible association equilibria, were employed to fit the reported data for Nafion 117 and CR61 cation-exchange membranes in equilibrium with NaCl, MgCl2, CaCl2, and Na2SO4 single-salt solutions. The results are compared with the previously reported fits to the Manning condensation model, which shows that both models produce similarly good fits for NaCl, MgCl2, and CaCl2 solutions in the 0.01 to 1 M range. However, the greater flexibility and specificity of the association model allow addressing deviations observed for Na2SO4 solutions and for CaCl2 above 1 M as free-ion paring and possible formation of charged NaSO4- and CaCl+ pairs, respectively. The results demonstrate the present model may be a sound non-mean-field alternative to the Manning condensation model, capable of addressing ion-specificity and multiple modes of association.
Display omitted
•Ion association model proposed, used to analyze salt uptake in Nafion 117, CR61.•Model relations developed for different salt types.•The present and Manning models perform similarly for 1:1 salt.•The association model shows advantages for multivalent ions and solutions >1 M.•Issues of limited data range and redundancy of parameters discussed.
The origin of dolomite has been an issue for hundreds of years, and its kinetic inhibition is a critical aspect of this issue. Dissolved sulfate is regarded as an inhibitor for dolomite formation ...because it can bind Mg2+ to form tight ion pairs and thus prevent the incorporation of Mg2+ into dolomite. Using Raman spectroscopy, we investigated the Mg2+–SO42− association in vapor-saturated aqueous MgSO4/MgCl2/NaCl solutions at temperatures of 25 to 200°C. The Mg2+–SO42− association is highly temperature and concentration dependent: the fractions of contact ion pairs (CIPs) and triple ion pairs (TIs) increase with increasing temperature and MgSO4 concentration. The presence of MgCl2 increases the Mg2+/SO42− ratio and favors Mg2+–SO42− interactions to produce CIPs and TIs, whereas the presence of NaCl exerts a negative effect on Mg2+–SO42− interactions, particularly at high temperatures (i.e., ≥150°C). The primary sulfate species in concentrated MgSO4 solutions at high temperatures (i.e., ≥2mol/kg, 200°C) are various contact ion pairs, whereas those in diluted solutions at Earth surface temperature appear to be unassociated SO42− and weakly associated solvent-separated and solvent-shared ion pairs. We propose that dissolved sulfate can inhibit the incorporation of Mg2+ into dolomite crystals by attracting Mg2+ to form tight contact ion pairs under hydrothermal conditions. However, thermochemical sulfate reduction (TSR) can effectively remove sulfate and free Mg2+ to enhance the precipitation of hydrothermal dolomite from sulfate-bearing fluids. The inhibiting effect of dissolved sulfate on the formation of massive low-temperature dolomite appears to have been overestimated. Removal of sulfate by anaerobic bacterial sulfate reduction (BSR) may not be responsible for the formation of microbial dolomite at surface temperatures. These new understandings also have implications for the study of thermochemical sulfate reduction because the formation of CIPs can increase the activity of sulfate in reactions with hydrocarbons.
•In situ Raman spectroscopy was used to study Mg2+–SO42− ion pairing at 25–200°C.•Tight contact ion pairs are favored in concentrated and high Mg2+/SO42− ratio solutions.•Mg2+–SO42− contact ion pairing increases with increasing temperature.•TSR can free Mg2+ by removing sulfate to promote hydrothermal dolomite formation.•Inhibition effect of SO42− on low-temperature dolomite formation was overestimated.
Mammalian glutamate transporters are essential for the most aspects of normal brain functioning including cognition, memory and learning. Its structure and functional properties are vigorously ...investigated now. However, there is no compliance amongst researchers with the elementary events sequence of glutamate transporter cycle. The sequence of elementary events was analysed using stochastic simulation of the processes. It was shown that in the case of invariant equilibrium constants of reactions of substrate and co-transported ions associations transposition in binding events sequence leads to alteration of protein affinity to substrate, but has insufficient impact on maximal transport velocity. Moreover, it was indicated that glutamate should be bound to the protonated form of transporter after the second sodium ion association.
We use molecular dynamics simulations to examine the role of polymerization and changing monomer carbon spacer length on the hydration and ion association properties of carboxybetaine and ...sulfobetaine zwitterionic homopolymers with LiCl and KCl salts in aqueous solution. Our simulation results suggest that polymerization leads to increased water and ion effective residence time compared to previous studies while varying carbon spacer length causes a change in the partial charge of zwitterionic functional groups. This change in partial charge affects the distribution and quality of water and ion interactions. Additionally, investigating the ionic conductivity by use of the Nernst-Einstein equation, we found that KCl had higher conductivity than LiCl for both polymer types, which is explained by the shorter effective residence time of K+ ions to the polymer than that of Li+.
•Zwitterion polymerization increases water and ion effective residence time.•Neighboring monomers to some extent inhibit water molecules and ions from associating to zwitterions.•Ionic conductivity is inversely proportional to cation effective residence time.
Liquid–liquid phase separation (LLPS) is an intermediate step during the precipitation of calcium carbonate, and is assumed to play a key role in biomineralization processes. Here, we have developed ...a model where ion association thermodynamics in homogeneous phases determine the liquid–liquid miscibility gap of the aqueous calcium carbonate system, verified experimentally using potentiometric titrations, and kinetic studies based on stopped‐flow ATR‐FTIR spectroscopy. The proposed mechanism explains the variable solubilities of solid amorphous calcium carbonates, reconciling previously inconsistent literature values. Accounting for liquid–liquid amorphous polymorphism, the model also provides clues to the mechanism of polymorph selection. It is general and should be tested for systems other than calcium carbonate to provide a new perspective on the physical chemistry of LLPS mechanisms based on stable prenucleation clusters rather than un‐/metastable fluctuations in biomineralization, and beyond.
A quantitative, general model for liquid–liquid phase separation is introduced based on “nonclassical” nucleation via prenucleation clusters. Potentiometric titrations and stopped‐flow ATR‐FTIR spectroscopy verify the predictive power of the model for the aqueous calcium carbonate system. This model reconciles the previously inconsistent literature values for the solubilities of amorphous calcium carbonates.
PDF
