Equilibria In Aqueous Solutions Research Articles

Aqueous solubility of Zn incorporated into Mg-Al-layered double hydroxides

Hydrotalcite minerals are layered double hydroxides (LDH) which play an important role in immobilizing hazardous compounds to decontaminate industrial wastewaters. The stability of an LDH is mostly evaluated in terms of its low solubility in water. However, the solubility of divalent trace metals immobilized by Mg-Al-type LDHs is not well known. Hydrotalcites containing Zn in solid solution, (Mg+Zn)3-Al-LDH, were synthesized by alkaline co-precipitation. A series of eleven LDH phases with Zn mole fractions XZn = Zn/(Mg+Zn) of 0–1 were characterized by powder X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, thermogravimetry (TGA), scanning and transmission electron microscopy (SEM/TEM), Brunauer-Emmett-Teller (BET) surface area analysis, and inductively coupled plasma mass (ICP-MS) spectrometry. The XRD analysis provided sharp characteristic spacings for d003 and d006 which occurred for all samples, confirming a layered LDH structure. Cell parameters (a, c) obeyed Vegard’s law and confirmed the formation of a regular solid-solution series without a mixing gap. An aqueous equilibrium time was determined by kinetic dissolution experiments. Steady-state solubility occurred after 120 days, but the experiments continued up to 240 days. The XRD and SEM/TEM analyses indicated no phase changes during the long-term dissolution experiments; neither were phase impurities detected after 240 days. The solubility products of the Mg- and Zn-bearing endmember compositions were calculated from experimentally determined total cation and anion concentrations using the Visual Minteq code for considering element speciation and ion pairing. The solubility product decreased as the Zn mole fraction increased, suggesting that the Zn-bearing LDH phases were more stable than the pure Mg3-Al-LDHs. Solid-solution aqueous-solution thermodynamic equilibrium modeling using the Lippmann “total solubility product” approach and applying Lippmann diagrams with logarithmic x-axes revealed a log-linear decrease in aqueous Zn solubility. The results are promising for remediation of metal-bearing liquid wastes because the metals that co-precipitated with the LDH were more strongly retained and, therefore, less soluble than the hydroxides or carbonates of the trace metal.

Aqueous Solution Equilibria and Spectral Features of Copper Complexes with Tripeptides Containing Glycine or Sarcosine and Leucine or Phenylalanine

Copper(II) complexes of glycyl-L-leucyl-L-histidine (GLH), sarcosyl-L-leucyl-L-histidine (Sar-LH), glycyl-L-phenylalanyl-L-histidine (GFH) and sarcosyl-L-phenylalanyl-L-histidine (Sar-FH) have potential anti-inflammatory activity, which can help to alleviate the symptoms associated with rheumatoid arthritis (RA). From pH 2–11, the MLH, ML, MLH-1 and MLH-2 species formed. The combination of species for each ligand was different, except at the physiological pH, where CuLH-2 predominated for all ligands. The prevalence of this species was supported by EPR, ultraviolet-visible spectrophotometry, and mass spectrometry, which suggested a square planar CuN4 coordination. All ligands have the same basicity for the amine and imidazole-N, but the methyl group of sarcosine decreased the stability of MLH and MLH-2 by 0.1–0.34 and 0.46–0.48 log units, respectively. Phenylalanine increased the stability of MLH and MLH-2 by 0.05–0.29 and 1.19–1.21 log units, respectively. For all ligands, 1H NMR identified two coordination modes for MLH, where copper(II) coordinates via the amine-N and neighboring carbonyl-O, as well as via the imidazole-N and carboxyl-O. EPR spectroscopy identified the MLH, ML and MLH-2 species for Cu-Sar-LH and suggested a CuN2O2 chromophore for ML. DFT calculations with water as a solvent confirmed the proposed coordination modes of each species at the B3LYP level combined with 6-31++G**.

Molecular dynamics of liquid-liquid equilibrium and interfacial properties of aqueous solutions of methyl esters.

In this work, the liquid-liquid phase equilibria and interfacial properties of methyl ester + water binary mixtures are determined at atmospheric pressure and from 278 to 358 K combining the direct coexistence technique and molecular dynamics simulations. Methyl esters are modelled using new parametrizations based on the united atom TraPPE model force field proposed recently by us [E. Feria, J. Algaba, J. M. Míguez, A. Mejía, P. Gómez-Álvarez and F. J. Blas, Phys. Chem. Chem. Phys., 2019, 22, 4974-4983] that are able to predict the vapour-liquid interfacial properties of pure methyl esters with high accuracy. In the case of water, we consider the well-known TIP4P/2005 model, the most popular rigid and non-polarizable model to describe the interfacial properties of pure water. The simulations are performed using the direct coexistence technique in the isothermal-isobaric or NPzT ensemble in combination with molecular dynamics. We obtain density profiles, temperature-densities and temperature-composition projections of the phase diagrams, and interfacial tensions. The liquid-liquid interfacial tension is calculated from the normal and tangential components of the pressure tensor according to the mechanical virial route. We pay attention particularly to the ability of the molecular models in predicting the experimental behavior of the systems. Simulation results are able to account for the liquid-liquid phase equilibria of these binary mixtures, in good agreement with the experimental data taken from the literature. Unfortunately, experimental values for interfacial tensions are substantially overestimated by predictions from computer simulations in all cases. To our knowledge, this is the first time that the liquid-liquid phase equilibrium and interfacial properties of methyl ester + water mixtures have been predicted from computer simulations.

Aggregates of Ethoxylated Surfactant with Added n-Octanol: Details of Corona Structure from a Molecular-Thermodynamic Model

Polar heads of many nonionic surfactants have complex chemical structure. In the micellar aggregates these heads are engaged in specific and nonspecific interactions with one another and with the hydration water. We present here a molecular-thermodynamic aggregation model that takes into account explicitly the hydrogen bonding and other specific interactions within the hydrated corona of a mixed nonionic aggregate. Apart from the aggregation characteristics that are usually obtained from the classical aggregation models – critical micelle concentration (CMC), the distribution of the aggregates over the size and shape and their composition - our model helps to establish a number of likely details of structure, including the corona thickness, hydration, and the partial penetration of the micelle components in the corona region. In this work the model is applied to describe the aggregation equilibrium in aqueous solutions of nonionic surfactants of the polyoxyethylene-p-(1,1,3,3-tetramethylbutyl)phenyl ether family that is closely related to Tritons, the commercial surfactants of an extensive practical use. The model gives reasonable estimates of the CMCs and predicts correctly the trends in CMC and in the aggregation numbers vs the number of oxyethylene groups in the surfactant molecule. The hydration numbers and the fraction of water engaged in hydrogen bonds are calculated. For the hydration of the aggregate coronae, the predicted trend is in line with that found for other ethoxylated nonionic surfactants: poly(ethylene oxide) alkyl ethers and neonols. For surfactant solutions with added n-octanol, the model suggests that the tails of the octanol molecules are deeply buried in the cores of the mixed micelles with only one methylene group penetrating the coronae. We predict co-surfactant effect of added n-octanol: the depression of the CMCs is progressively stronger for more hydrophilic surfactants with larger EO heads.

PRECIPITATION OF METAL HYDROXIDES FROM AQUEOUS SOLUTIONS AS A RESULT OF SPONTANEOUS CONDENSATION OF POLYNUCLEAR HYDROXOCOMPLEXES

Background. Generally, it is assumed that the formation of a solid phase (precipitate) happens when the activities of the involved ions would exceed those defined by the thermodynamic solubility product. However, in case of precipitation of metal hydroxides, this is a simplification, and the real pattern is more complicated, since metal ions form strong mono- and polynuclear hydroxocomplexes in a solution. Formation of such complexes, especially those with zero charge, should result in the deviation from the common solubility product rule. Objective. The aim of this paper is to develop a precipitation model, which takes into account the effect of the formation of the hydroxocomplexes on the solubility of a metal hydroxide. Eventually, this solubility includes the sum of the concentrations of metal ions, and all neutral, positively and negatively charged hydroxocomplexes involved in all equilibria in aqueous solution. Methods. We assume that formation of the solid precipitate is the result of spontaneous condensation of polynuclear neutral hydroxocomplexes when their concentrations in a solution exceed a certain critical value. These critical concentrations can be estimated from the consideration of all equilibria with the assumption that the equilibrium constants for the formation of neutral polynuclear complexes when their nuclearity increases by one are approximately equal and do not significantly depend on the size of the particle. Results. Using this approach, we developed the model, which predicts spontaneous condensation with formation of a precipitate. Also, we calculated the dependencies of pH-logCM for precipitation of various divalent cation hydroxides. It was shown that there exist minimal concentrations, below which no precipitates are formed at any pH value. Conclusions. Such approach also explains the nature of linear correlations between logarithms of solubility products and stability constants of neutral complexes described in literature. These results are important for the development and optimization of industrial wastewater treatment processes.

Conformational Heterogeneity and Cooperative Effects of Mammalian ALOX15.

Arachidonic acid lipoxygenases (ALOXs) have been suggested to function as monomeric enzymes, but more recent data on rabbit ALOX15 indicated that there is a dynamic monomer-dimer equilibrium in aqueous solution. In the presence of an active site ligand (the ALOX15 inhibitor RS7) rabbit ALOX15 was crystalized as heterodimer and the X-ray coordinates of the two monomers within the dimer exhibit subtle structural differences. Using native polyacrylamide electrophoresis, we here observed that highly purified and predominantly monomeric rabbit ALOX15 and human ALOX15B are present in two conformers with distinct electrophoretic mobilities. In silico docking studies, molecular dynamics simulations, site directed mutagenesis experiments and kinetic measurements suggested that in aqueous solutions the two enzymes exhibit motional flexibility, which may impact the enzymatic properties.

Thermodynamic Characteristics of Protolytic Equilibria of L-Alanayl-L-isoleucine in Aqueous Solutions

Potentiometry and calorimetry are used to study protolytic equilibria in aqueous solutions of L‑alanyl-L-isoleucine. The equilibrium constants and heat effects of the dipeptide dissociation reactions are determined. The standard thermodynamic characteristics of the investigated equilibria are calculated. The obtained results are compared to the corresponding data for related compounds.

Solid–Liquid Equilibria in Aqueous Solutions of Tris, Tris-NaCl, Tris-TrisHCl, and Tris-(TrisH)2SO4 at Temperatures from 5 to 45 °C

The substance Tris (or THAM, 2-amino-2-hydroxymethyl-1,3-propanediol) is used in the preparation of pH buffer solutions for applications in natural water chemistry, including seawater. The developm...

Alcohol Dehydration by Extractive Distillation with Use of Aminoethers of Boric Acid

Aminoethers of boric acid (AEBA) were studied as potential extractants for the separation of aqueous–alcoholic azeotropic mixtures by extractive distillation. The conditions of vapor–liquid equilibrium in aqueous solutions of ethanol and isopropanol in the presence of AEBA were studied. The division of AEBA molecules into group components was proposed, and previously unknown geometric parameters of the boron group and the energetic pair parameters of the boron group with the alkane group, ether group, amine-3d group, and alcohol group were determined within the framework of the Universal Functional Group Activity Coefficient (UNIFAC) model. The modeling of the extractive rectification process of an ethanol–water mixture with AEBA as extractant has been carried out. The dependences of the cost function on the extractant flow rate, the residual water content in it and the number of theoretical trays were obtained. A technological scheme for ethanol dehydration has been proposed, and its technological characteristics have been calculated.

Selective Recovery of Molybdenum over Rhenium from Molybdenite Flue Dust Leaching Solution Using PC88A Extractant

Selective solvent extraction of molybdenum over rhenium from molybdenite (MoS2) flue dust leaching solution was studied. In the present work, thermodynamic calculations of the chemical equilibria in aqueous solution were first performed, and the potential–pH diagram for the Mo–Re–SO42−–H2O system was constructed. With the gained insight on the system, 2-ethylhexyl phosphonic acid mono-(2-ethylhexyl)-ester (PC88A) diluted in kerosene was used as the extractant agent. Keeping constant the reaction temperature and aqueous-to-organic phase ratio (1:1), organic phase concentration and pH were the studied experimental variables. It was observed that by increasing the acidity of the solution and extractant concentration, selectivity towards Mo extraction increased, while the opposite was true for Re extraction. Selective Mo removal (+95%) from leach solution containing ca. 9 g/L Mo and 0.5 g/L Re was achieved when using an organic phase of 5% PC88A at pH = 0. No rhenium was coextracted during 10 min of extraction time at room temperature. Density functional theory (DFT) calculations were performed in order to study the interactions of organic extractants with Mo and Re ions, permitting a direct comparison of calculation results with the experimental data to estimate selectivity factors in Mo–Re separation. For this aim, PC88A and D2EHPA (di-(2-ethylhexyl) phosphoric acid) were simulated. The interaction energies of D2EHPA were shown to be higher than those of PC88A, which could be due to its stronger capability for complex formation. Besides, it was found that the interaction energies of both extractants follow this trend considering Mo species: MoO22+ > MoO42−. It was also demonstrated through DFT calculations that the interaction energies of D2EHPA and PC88A with species are based on these trends, respectively: MoO22+ > MoO42− > ReO4− and MoO22+ > ReO4− > MoO42−, in qualitative agreement with the experimental findings.

Characterizing Polymer Hydration Shell Compressibilities with the Small-System Method.

The small-system method (SSM) exploits the unique feature of finite-sized open systems, whose thermodynamic quantities scale with the inverse system size. This scaling enables the calculation of properties in the thermodynamic limit of macroscopic systems based on computer simulations of finite-sized systems. We herein extend the SSM to characterize the hydration shell compressibility of a generic hydrophobic polymer in water. By systematically increasing the strength of polymer-water repulsion, we find that the excess inverse thermodynamic correction factor () and compressibility () of the first hydration shell change sign from negative to positive. This occurs with a concurrent decrease in water hydrogen bonding and local tetrahedral order of the hydration shell water. The crossover lengthscale corresponds to an effective polymer bead diameter of 0.7 nm and is consistent with previous works on hydration of small and large hydrophobic solutes. The crossover lengthscale in polymer hydration shell compressibility, herein identified with the SSM approach, relates to hydrophobic interactions and macromolecular conformational equilibria in aqueous solution. The SSM approach may further be applied to study thermodynamic properties of polymer solvation shells in mixed solvents.

Solution chemistry in the surface region of aqueous solutions

Abstract Solution chemistry is commonly regarded as the physical chemistry of reactions and chemical equilibria taking place in the bulk of a solvent, and between solutes in solution, and solids or gases in contact with the solution. Our knowledge about such reactions and equilibria in aqueous solution is very detailed such as their physico–chemical constants at varying temperature, pressure, ionic medium and strength. In this paper the solution chemistry in the surface region of aqueous solutions, down to ca. 10 Å below the water–air interface, will be discussed. In this region, the density and relative permittivity are significantly smaller than in the aqueous bulk strongly affecting the chemical behaviour of solutes. Surface sensitive X-ray spectroscopic methods have recently been applicable on liquids and solutions by use of liquid jets. This allows the investigation of the speciation of compounds present in the water–air interface and the surface region, a region hardly studied before. Speciation studies show overwhelmingly that neutral molecules are accumulated in the surface region, while charged species are depleted from it. It has been shown that the equilibria between aqueous bulk, surface region, solids and/or air are very fast allowing effective transport of chemicals over the aqueous surface region.

Measurement and Correlational Study of Phase Equilibria in Aqueous Solutions of N,N-Dimethylacetamide with 1-Pentanol and 1-Hexanol at 298.15 and 308.15 K

Tie-line data were determined for the systems (water + N,N-dimethylacetamide + 1-pentanol) and (water + N,N-dimethylacetamide + 1-hexanol) at the temperatures of 298.15 and 308.15 K under 101.1 KPa. The phase diagrams for these systems are of type I according to the Treyball classification. The thermodynamic consistency of the measured tie-line data was verified using the Hand and Othmer–Tobias equations. The distribution ratios (D) and selectivity (S) values were computed according to the measured tie-line data to assess the extracting behaviors of the selected solvents. The experimental data were correlated using the nonrandom two liquid and the universal quasichemical activity coefficient equations and the Elephant Herding Optimization Algorithm (EHA). EHA is an emerging nature-inspired stochastic global optimization method, and it has been used for liquid–liquid equilibrium (LLE) parameter estimation of local composition models in multicomponent mixtures. The performance of this new swarm-based metaheuristic search method for LLE modeling with and without the closure equation has been analyzed.

Acid-Base Properties of 5-Hydroxy-1,3,6-trimethyluracil in Aqueous Solutions

5-Hydroxy-1,3,6-trimethyluracil has been studied using 1H, 13C, and 15N NMR as well as UV spectroscopy. The constants and thermodynamic characteristics of its acid-base equilibrium in aqueous solutions have been determined. The stepwise mechanism of 5-hydroxy-6-methyluracil dissociation in aqueous solutions has been elucidated.

Magnetic solid-phase extraction as a novel method for the prediction of the bioaccessibility of polycyclic aromatic hydrocarbons

Chemical methods used to predict the bioaccessibility of hydrophobic organic compounds (HOCs) still need further development and improvement. In this work, magnetic solid-phase extraction (MSPE) based on poly(β-cyclodextrin)-coated magnetic polydopamine (Fe3O4@PDA@PCD) was first introduced to assess the bioaccessibility of polycyclic aromatic hydrocarbons (PAHs) in soils. Due to its good hydrophilicity and submicrometer scale, Fe3O4@PDA@PCD displayed a higher extraction rate for PAHs in an aqueous solution (equilibrium time < 5 min) than Tenax resin, which had an equilibrium time longer than 30 min. The merits of Fe3O4@PDA@PCD are beneficial to accelerate the desorption of PAHs from soil, especially for high molecular weight PAHs, in which the amounts extracted by Fe3O4@PDA@PCD were 1.2–2.8 times higher than those extracted by Tenax resin. The desorption kinetics data were well fitted with a two- or three-fraction model. The fitting results indicated that the MSPE method can be used to predict the bioaccessible fractions of PAHs. By comparing the prediction results obtained from the MSPE method with bioassays using earthworms, a significant linear correlation (R2 = 0.98) with a slope statistically close to 1 was obtained. These results suggested that the MSPE method can act as a simple and efficient method to measure the bioaccessibility of PAHs in soil.

Synthesis, protonation constants and biological activity determination of amino acid-salicylaldehyde-derived Schiff bases.

Schiff bases represent a class of molecules widely studied for their importance in organic and coordination chemistry. Despite the large amount of studies on the chemical and biological properties of the Schiff bases, the different experimental conditions prevent a useful comparison to search for a correlation structure-activity. Moreover, literature is lacking in comprehensive data on the spectroscopic characterization of these compounds. For this reason, six Schiff bases, derived from salicylaldehyde and natural amino acids were fully characterized by nuclear magnetic resonance and infrared spectroscopy, and their aqueous solution equilibria, antiproliferative activity and DNA-binding activity were examined. All experimental conditions were kept constants to achieve comparable information and useful insights about their structure-activity correlation. The synthesized compounds showed DNA binding constants in the 101-102M-1 range, depending on the substituent present in the amino acid side-chain, and resulted devoid of significant cytotoxic activity against the different human tumor cell lines showing IC50 values higher than 100µM.

Predicting phase equilibrium of aqueous sugar solutions and industrial juices using COSMO-SAC

COSMO-SAC model was used to predict bubble point pressure, bubble point temperature, water activity, freezing point depression and solubility of sugars in aqueous solutions. COSMO-SAC is based on quantum chemistry computations and has a high predictive character, not requiring a specific parameter calibration for each application. For most systems studied, the results obtained were close to experimental data, with relative mean errors varying from 0.1% to 6.4% for vapor–liquid equilibrium. Solubility results with COSMO-SAC presented the same tendency observed in experimental data, but with higher deviations, probably due to the fact that the loss of crystalline structure in sugars is a kinetic process more than a thermodynamic melting, and does not occur at a single temperature with a constant enthalpy value. COSMO-SAC model was also used to predict industrial juices water activity, bubble point temperature, and freezing point depression. Very low deviations from experimental data were observed for COSMO-SAC calculations, similar to those found for simpler sugar solutions.

Experimental studies and thermodynamic modeling on vapor-liquid equilibrium of aqueous solutions containing 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ionic liquid, (d+)-Galactose, (d-)-Fructose, (d+)-Lactose and sucrose at 298.15 K

Vapor-liquid equilibrium for aqueous solutions of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([bmpl][otf]) + (d+)-galactose, [bmpl][otf] + (d-)-fructose, [bmpl][otf] + (d+)-lactose and [bmpl][otf] + sucrose was investigates at 298.15 K. For this purpose, water activity in these systems was measured using an improved isopiestic method at T = 298.15 K. Osmotic coefficient of ionic liquid and vapor pressure of solutions were calculated from measured water activity data. NRTL, NRF-NRTL, mNRTL, Wilson and TNRF-mNRTL local composition based models were used to correlate the water activity data for binary and ternary aqueous solutions containing ionic liquid and saccharides considered in this work. All of these models predicted satisfactorily water activity data for the systems investigated. Water activity data and vapor pressure decreasing were utilized to highlight the solute-solvent interactions.

A head-to-toe dimerization has physiological relevance for ligand-induced inactivation of protein tyrosine receptor type Z

Protein-tyrosine phosphatase (PTPase) receptor type Z (PTPRZ) has two receptor isoforms, PTPRZ-A and -B, containing tandem intracellular PTP-D1 and -D2 domains, with only D1 being active. Pleiotrophin (PTN) binding to the extracellular PTPRZ region leads to inactivation of its PTPase activity, thereby facilitating oligodendrocyte precursor cell (OPC) differentiation and myelination in the central nervous system. However, the mechanisms responsible for PTN-induced PTPRZ inactivation remain unclear. We herein report that the crystal structure of the intracellular region of PTPRZ (PTPRZ-ICR) shows a "head-to-toe"-type dimer conformation, with D2 masking the catalytic site of D1. MS analyses revealed that PTPRZ-ICR proteins remain in monomer-dimer equilibrium in aqueous solution and that a substrate-derived inhibitory peptide or competitive inhibitor (SCB4380) specifically bind to the monomer form in a 1:1 ratio. A D2 deletion (ΔD2) or dimer interface mutation (DDKK) disrupted dimer formation, but SCB4380 binding was maintained. Similar to WT PTPRZ-B, monomer-biased PTPRZ-B-ΔD2 and PTPRZ-B-DDKK variants efficiently dephosphorylated p190RhoGAP at Tyr-1105 when co-expressed in BHK-21 cells. The catalytic activities of these variants were not suppressed by PTN treatment, but were inhibited by the cell-permeable PTPase inhibitor NAZ2329. Of note, the PTN treatment did not enhance OPC differentiation in primary cultured glial cells from ΔD2 or PTPase-inactive PTPRZ-B (CS) mutant knock-in mice. Our results thus indicate that PTN-induced PTPRZ inactivation results from dimer formation of the intracellular tandem PTP domains in a head-to-toe configuration, which is physiologically relevant to the control of OPC differentiation in vivo.

ACID-BASE EQUILIBRIA IN SOLUTIONS OF NEW 4-HYDROXYSTYRYL DYES BASED ON 1-OCTILPIRIDINIUM

In this paper, the acid-base properties of a number of new 4-hydroxystyryl dyes based on 1-octylpyridinium in solutions have been studied by tristimulus colorimetry and spectrophotometry. The studied dyes are obtained by the interaction of equivalent amounts of corresponding heterocyclic methylene bases salts with 4-hydroxyaldehydes in acetonitrile. The advantages of the tristimulus colorimetric method in the study of acid-base equilibria in dyes solutions were noted. It was shown that the use of data on the change in the values of colorimetric functions (specific color discrimination), depending on the medium acidity allows us to determine the constants of ionization of the functional groups of 4-hydroxystyryl dyes. It was shown that the quantitative description of the color of objects of different nature by calculating their color coordinates on the basis of spectrophotometric data allows obtaining information about their state in solutions. Using the values of the colorimetric functions of the ionic-molecular forms of 4-hydroxystyryl dyes as an analytical signal allows us to obtain a coherent picture of existing acid-base equilibria in a wide range of the medium acidity. Based on the obtained experimental data, a probable scheme of acid-base equilibrium in aqueous solutions of 4-hydroxystyryl dyes was proposed. It was shown that in the acidic medium 4-hydroxystyryl dyes exist in a protonated single-charged cationic form, and with pH rise dissociation of phenolic hydroxyl and tautomerization with the formation of a merocyanine form occurs. The corresponding diagrams for the distribution of ion-molecular forms of 4-hydroxystyryl dyes have been constructed, depending on the acidity of the medium.