Special features of structure formation of sodium chloride concentrated aqueous solution on its passing to supercritical state

Structural properties of aqueous 4.37 M solution of NaCl under sub-and supercritical conditions (p 25 MPa, T 450–750 K) were studied by the method of integral equations in the atom-atom approximation. The passing of the system to the supercritical state is accompanied by a substantial rearrangements in local structural fragments with hydrogen bonds and in the nearest surrounding of the ions. In this case a transition is observed from the solution structure with hydrated ions and ionic pairs (subcritical region) to the structure with free molecules and low-molecular formations, probably, in the form of clusters (supercritical region).     

Structural parameters of RbBr aqueous solution under nearcritical and supercritical conditions with close densities (～0.65 g/cm3)

The results of a theoretical investigation for structural properties of diluted RbBr aqueous solution under nearcritical (T = 638 K, p = 38.9 MPa, ρ = 0.65 g/cm³) and supercritical conditions (T = 697 K, p = 63.3 MPa, ρ = 0.64 g/cm³) are given. The features of structure formation of the system at a transition to near-critical and supercritical conditions were determined. The data were compared with the structural parameters of the solution under standard conditions.     

Bulk properties and hydration numbers of components of water–salt,water–urea,and water–urea–salt systems

With an increase in the concentration of additives, the hydration numbers of compounds decrease. Thus, in a saturated 54.6% solution, urea loses approximately 3/4 of the initial amount of water, forming an aquacomplex of the composition (NH₂)₂CO?H₂O. In a supersaturated 44% solution, the sodium chloride aquacomplex is dehydrated by 2/3, and in a supersaturated 67% solution, sodium sulfate is dehydrated by 5/6. The density of these solutions is 1.354÷1.360 g/cm³ (44% NaCl) and 1.800÷1.849 g/cm³ (67% Na₂SO₄). In a saturated urea solution, NaNO₃, NaCl, and Na₂SO₄ complexes lose 53÷55% of hydration water. It is shown that the interactions in the binary water–urea system somewhat increase the hydration number of the salts (structural hydration). The hydration water density, a structurally important characteristic, increases in the series of solutions of urea, NaNO₃, NaCl, and Na₂SO₄. In the same series of additives, the excess volume of binary water–urea and water–salt systems becomes more negative.     

Binding of 1,5-bis(p-sulfonatophenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane with tetra(methyl viologen) calix[4]resorcinol

Binding of an amphiphilic dianion 1,5-bis(p-sulfonatophenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane (APCO^2?) with an amphiphilic octacation tetra(methyl viologen) calix[4]resorcinol (MVCA⁸⁺) in media containing different amounts of water and DMSO using NaClO₄ or NaCl as supporting electrolytes was shown for the first time by cyclic voltammetry and spectrophotometry. The stoichiometry of the complex depends on the MVCA⁸⁺: APCO^2? ratio, medium, and supporting electrolyte. A 1: 1 charge-transfer complex is mainly formed (λ_max = 480 nm) in 30% DMSO at a ratio of the compounds of 1: 1. A similar 1: 1 complex of APCO^2? with a model compound methyl viologen MV²⁺ (λ_max = 482 nm) is formed under these conditions. A donor-acceptor interaction occurs between the acceptor viologen units and nitrogen-centered electron-donating fragments of the APCO^2? dianion. An increase in the content of APCO^2? in the solution leads to an additional binding of one (30 vol.% DMSO, water, NaClO₄) or two (30 vol.% DMSO, water, NaCl) particles of APCO^2? with the hydrophobic fragments of MVCA⁸⁺. The complexes aggregate to form insoluble precipitates in aqueous and water-DMSO media. A selective reversible electroswitching from the bound to free state of one of the three bound APCO^2? particles was performed when reducing MVCA⁸⁺ to MVCA^4·+ in a 30 vol.% DMSO/NaCl medium.     

Calculation of partial molar volume of components in supercritical ammonia synthesis system

The partial molar volumes of components in supercritical ammonia synthesis system are calculated in detail by the calculation formula of partial molar volume derived from the R-K equation of state under different conditions. The objectives are to comprehend phase behavior of components and to provide the theoretic explanation and guidance for probing novel processes of ammonia synthesis under supercritical conditions. The conditions of calculation are H₂/N₂ = 3, at a concentration of NH₃ in synthesis gas ranging from 2% to 15%, concentration of medium in supercritical ammonia synthesis system ranging from 20% to 50%, temperature ranging from 243 K to 699 K and pressure ranging from 0.1 MPa to 187 MPa. The results show that the ammonia synthesis system can reach supercritical state by adding a suitable supercritical medium and then controlling the reaction conditions. It is helpful for the supercritical ammonia synthesis that medium reaches supercritical state under the conditions of the corresponding total pressure and components near the normal temperature or near the critical temperature of medium or in the range of temperature of industrialized ammonia synthesis. __________ Translated from Journal of Chemical Industry and Engineering, 57(7):1503–1507 [译自: 化工学报]     

超临界水中醋酸锌水解反应的分子动力学模拟

氧化锌(ZnO)是一种重要的化工原料, 超临界水热合成法制备纳米ZnO的第一步是锌盐与碱或水发生水解反应生成Zn(OH)₂, 后者接着脱水生成ZnO. 以Zn(CH₃COO)₂为原料, 直接和超临界水(SCW)反应能够制备纳米级的ZnO颗粒, 但对反应机理的探讨较少. 本研究利用分子动力学模拟超临界条件下Zn(CH₃COO)₂水解反应过程中的结构和能量变化, 发现Zn(CH₃COO)₂在SCW中容易聚集成无定形的团簇, 1个Zn²⁺平均和5个CH₃COO^-和1个H₂O配位, 形成6配位的八面体结构. 处于Zn(CH₃COO)₂团簇和SCW界面的Zn²⁺能够和更多的H₂O配位. 水解反应后系统的势能降低, 同时伴随Zn(CH₃COO)₂团簇结构的改变. 反应产物OH^-分布在Zn(CH₃COO)₂团簇内部, 富集Zn²⁺, 而CH₃COOH则分布在SCW中. 本文的工作为超临界水热合成的反应过程提供了基本的理论依据.     

Synthesis of nanometric chromium (III) oxide powders in supercritical alcohol

A new semi-continuous process has been developed for the synthesis of Cr₂O₃ powders starting with Cr(III) acetylacetonate or acetate hydroxide dissolved in methanol. The precursor is first decomposed in the solvent under supercritical state (T = 325 to 450 °C, P = 10 MPa) and the formed solid is dried under either nitrogen or air flow. For various reaction temperatures and drying conditions, nanometric particles of pure amorphous or crystalline Cr₂O₃ powders are obtained with surface area between 350 and 30 m².g⁻¹ according to their crystallization state.     

Features of ion hydration and ion association in sub- and supercritical aqueous solutions of rubidium bromide

Results of investigations by the integral equation method in the XRISM approximation of features of structure formation in sub- and supercritical 0.5 M aqueous solution of RbBr. As the system passes from the sub- to supercritical state, the tetrahedral network of the solvent is destroyed, which is typical of water and aqueous solutions; thermal dehydration of the cation and anion are also observed. Dehydration is accompanied by contraction of the first hydration sphere of Rb⁺ and extension of the first hydration sphere of Br⁻. Both in sub- and supercritical solutions conditions a minor amount of contact ion pairs is present whose fraction is Higher by 15% compared to that in standard conditions.     

Hydrothermal synthesis of metal oxide nano- and microparticles in supercritical water

Hydrothermal syntheses of nano- and microparticles of metal oxides of two types, LiMO _n (LiCoO₂, LiNiO₂, LiZnO₂, and LiCuO₂) and MO _n (Ga₂O₃, CeO₂) were performed under continuous conditions in a tubular reactor with the use of supercritical water. An important role in the synthesis of nanoparticles and the reproducibility of the results was played by the conditions of mixing of supercritical water and precursor solution flows. The morphology and composition of synthesized compounds were studied by scanning electron microscopy and X-ray diffraction. The syntheses of LiCoO₂, LiNiO₂, LiZnO₂, LiCuO₂, Ga₂O₃, and CeO₂ were most successful.     

The Unexpected Helical Supramolecular Assembly of a Simple Achiral Acetamide Tecton Generates Selective Water Channels

Achiral 2-hydroxy-N-(diphenylmethyl)acetamide (HNDPA) crystallizes in the P6₁ chiral space group as a hydrate, building up permeable chiral crystalline helical water channels. The crystallization-driven chiral self-resolution process is highly robust, with the same air-stable crystalline form readily obtained under a variety of conditions. Interestingly, the HNDPA supramolecular helix inner pore is filled by a helical water wire. The whole edifice is mainly stabilized by robust hydrogen bonds involving the HNDPA amide bonds and CH^…π interactions between the HNDPA phenyl groups. The crystalline structure shows breathing behavior, with completely reversible release and re-uptake of water inside the chiral channel under ambient conditions. Importantly, the HNDPA channel is able to transport water very efficiently and selectively under biomimetic conditions. With a permeability per channel of 3.3 million water molecules per second in large unilamellar vesicles (LUV) and total selectivity against NaCl, the HNDPA channel is a very promising functional nanomaterial for future applications.     

Drawing Out the Structural Information About the First Hydration Layer of the Isolated Cl− Anion Through the FTIR-ATR Difference Spectra

The Fourier transform infrared-attenuated total reflectance (FTIR-ATR) difference spectra of aqueous MgSO₄, Na₂SO₄, NaCl and MgCl₂ solutions against pure water were obtained at various concentrations. The difference spectra of the solutions showed distinct positive bands and negative bands in the O–H stretching region, indicating the influences of salts on structures of hydrogen-bonds between water molecules. Furthermore the difference spectra of MgCl₂ solutions against NaCl solutions and those of MgSO₄ solutions against Na₂SO₄ solutions with the same concentrations of anions (Cl^? or SO ₄ ^2? , respectively) allowed extracting the structural difference of the first hydration layer between Mg²⁺ and Na⁺. Using SO ₄ ^2? as a reference ion, structural information of the first hydration layer of the Cl^? anion was obtained according to the difference spectra of MgCl₂ solutions against MgSO₄ solutions and those of NaCl solutions against Na₂SO₄ solutions containing the same concentrations of cations (Mg²⁺ or Na⁺, respectively). The positive peak at ~3,407 cm^?1 and negative peak at ~3,168 cm^?1 in these spectra indicated that adding Cl^? decreased the strongest hydrogen-bond component and increased the relatively weaker one.     

Chitosan‐modified graphene oxide as a modifier for improving the structure and performance of forward osmosis membranes

Chitosan (CS) with good hydrophilicity and charged property was used to modify graphene oxide (GO), the obtained GO‐CS was used as a novel modifier to fabricate thin film composite forward osmosis (FO) membranes. The results revealed that the amino groups on CS reacted with carboxyl groups on GO, and the lamellar structure of the GO nanosheets was peeled off by CS, resulting in the reducing of their thicknesses. The GO‐CS improved the hydrophilicity of polyethersulfone (PES) substrate, and their contact angles decreased to 64° with the addition of GO‐CS in the substrate. GO‐CS also increased the porosity of the substrate and surface roughness of FO membrane, thereby optimizing the water flux and reverse salt flux of FO membrane. The average water flux of the FO membrane reached the optimal flux of 21.34 L/(m² h) when GO‐CS addition was 0.5 wt%, and further addition of GO‐CS to the substrate would decrease the water flux of FO membrane, and the reverse salt flux also decreased to the lowest value of 2.26 g/(m² h). However, the salt rejection of the membrane increased from 91.4% to 95.1% when GO‐CS addition increased from 0.5 to 1.0 wt% under FO mode using 1 mol/L sodium chloride (NaCl) solution as draw solution (DS). In addition, high osmotic pressure favored water permeation, and at the same concentration of DS, magnesium chloride (MgCl₂) exhibited better properties than NaCl. These results all suggested that GO‐CS was a good modifier to fabricate FO membrane, and MgCl₂ was a good DS candidate.     

Molecular dynamics simulation of the key characteristics of the supercritical CO<Subscript>2</Subscript>–pentaerythritol tetraacetate system

Supercritical CO₂ is widely used in many fields of industry. Investigation of statistical mechanics of CO₂ fluid under quasi critical and supercritical state has great significance. Equilibrium molecular dynamics (EMD) simulations are carried out to investigate the statistical mechanics and macroscopic performance of CO₂ fluid under the quasi critical and supercritical state. The results show that the bond length and bond angle distributions for supercritical CO₂ are Gaussian distribution basically. The dimers’ proportion of supercritical CO₂ system changes with pressure increasing. T-type dimer has high share within the system when pressure is higher than 9MPa. It can be inferred that T-type dimer leads to CO₂ physical properties changing tempestuously under supercritical state. The effect that lubricating oil has on microstructure and heat transfer of supercritical CO₂ is also investigated in the present work. The results show the lubricating oil produces significant effect on the dimers’ structure under low pressure.     

Synthesis and aqueous solution properties of hydrophobically modified anionic acrylamide copolymers

In this investigation, hydrophobically modified polyacrylamide with low amounts of anionic long‐chain alkyl was synthesized by the free radical polymerization in deionized water. This water‐soluble copolymerization method is more convenient compared with the traditional micellar copolymerization methods. The copolymers were characterized using Fourier transform infrared, ¹H NMR, and the molecular weight and polydispersity were determined using gel permeation chromatography. The solution behavior of the copolymers was studied as a function of composition, pH, and added electrolytes. As NaCl was added to solutions of AM/C₁₁AM copolymers or pH was lowered, the shielding or elimination of electrostatic repulsions between carboxylate groups of the C₁₁AM unit lead to coil shrinkage. The steady shear viscosity and dynamic shear viscoelastic properties in semidilute, salt‐free aqueous solutions were conducted to examine the concentration effects on copolymers. In addition, the shear superimposed oscillation technique was used to probe the structural changes of the network under various stresses or shear conditions. We prepared hydrophobically modified polyacrylamide with N‐alkyl groups in the aqueous medium. The advantage of this method is that the production is pure without surfactants. These results suggest that the unique aqueous solution behavior of the copolymers is different from conventional hydrophobically associating acrylamide. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2465–2474, 2008     

Oil-in-water analysis using supercritical fluid extraction interfaced with fixed wavelength infrared detection

The combination of a direct aqueous supercritical fluid extraction system interfaced to a fixed wavelength infrared detector; measuring CH₂ (ν_asymmetric) absorbance at 2930 cm^?1, has been successfully developed for the analysis of oil-in-water. Using an optional, in-line silica gel treatment procedure, method accuracy for determining Brent Delta crude oil in spiked 500 mL water samples was 92.0% to 94.5% with RSD 4.7% to 6.5%. The supercritical fluid extraction-infrared method enables a second analysis of the same water sample without silica gel treatment. For second sets of analyses without silica gel treatment, method accuracy for determining Brent Delta crude oil in spiked 500 mL water samples was 87% to 96.0% with RSD 7.5% to 9.5%. Results of this study indicate that the silica gel treatment procedure reduces the calculated level of Brent Delta crude oil-in-water by 6.6–12.4% relative to samples analysed without silica gel treatment. The results of a study involving Fourier transform infrared spectroscopy indicate a limit of detection for n-decane of approximately 0.5 mg L^?1 by measuring CH₂ (ν_asymmetric) absorbance using the supercritical fluid extraction-infrared method. Sample preparation using direct aqueous supercritical fluid carbon dioxide extraction provides an indefinite means for the use of infrared techniques to measure oil-in-water.     

Extraction of PCDD/PCDF from soil with supercritical CO2: Optimization by a three-level factorial design approach

A highly sensitive and selective voltammetric procedure is described for the simultaneous determination of eleven elements (Cd, Pb, Cu, Sb, Bi, Se, Zn, Mn, Ni, Co and Fe) in water samples. Firstly, differential pulse anodic stripping voltammetry (DPASV) with a hanging mercury drop electrode (HMDE) is used for the direct simultaneous determination of Cd, Pb, Cu, Sb and Bi in 0.1 M HCl solution (pH = 1) containing 2 M NaCl. Then, differential pulse cathodic stripping voltammetry (DPCSV) is used for the determination of Se in the same solution. Zn is subsequently determined by DPASV after raising the pH of the same solution to pH 4. Next, the pH of the medium is raised to pH 8.5 by adding NH₃/NH₄Cl buffer solution for the determination of Mn by DPASV. Ni and Co are determined in the same solution by differential pulse adsorptive stripping voltammetry (DPAdSV) after adding DMG (1 × 10^–4 M). Finally, 1 × 10^–5 M 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) is added to the solution for the determination of Fe by DPAdSV. The optimal conditions are described. Relative standard deviations and relative errors are calculated for the eleven elements at three different concentration levels. The lower detection limits for the investigated elements range from 1.11 × 10^–10 to 1.05 × 10^–9 M, depending on the element determined. The proposed analysis scheme was applied for the determination of these eleven elements in some ground water samples.     

Utilization of N,N‐Dialkylcarbamic Acid Derived from Secondary Amines and Supercritical Carbon Dioxide: Stereoselective Synthesis of Z Alkenyl Carbamates with a CO2‐Soluble Ruthenium–P(OC2H5)3 Catalyst

Reversible transformation of diethylamine ( 1 ) and supercritical carbon dioxide (scCO₂) into N,N‐diethylcarbamic acid ( 2 ) was confirmed by direct acquisition of ¹H and ¹³C{¹H} NMR spectra. The equilibrium between 1 +CO₂ and 2 is strongly influenced by conditions of the supercritical state. Low temperature favors formation of carbamic acid, whereas high temperature causes decarboxylation. On the basis of the spectroscopic results of carbamic acid formation under scCO₂ conditions, the ruthenium‐catalyzed formation of alkenyl carbamates from terminal alkynes, 1 , and carbon dioxide was investigated to demonstrate the useful transformation of elusive carbamic acids. Selectivity toward the CO₂‐fixation products over enynes obtained by dimerization of the alkyne substrates was improved by the use of scCO₂ as a reaction medium. In particular, a CO₂‐soluble ruthenium complex, trans‐[RuCl₂{P(OC₂H₅)₃}₄], was found to be effective in affording Z alkenyl carbamates with high stereoselectivity.     

Potentiometric and Theoretical Studies of (2Z, 3Z)-2H-benzo[b][1,4]thiazine-2,3(4H)-dionedioxime with Some Divalent Transition Metal Ions

The protonation equilibria of (2Z, 3Z)-2H-benzo[b][1,4]thiazine-2,3(4H)-dionedioxime (BTDH₂) together with the equilibria of its bis- binary complexes of Co(II), Ni(II), Cu(II) and Zn(II) were investigated potentiometrically. The investigation was carried out at 25 ± 0.1 °C, in aqueous solution, with a constant ionic strength of 0.100 mol·dm^?3 NaCl. The protonation constants of the ligand together with the stability constants of a variety of complexes were determined potentiometrically in 10 % ethanol–water mixed solution using the SUPERQUAD computer program. Theoretical calculations were set up to assist in understanding the protonation sequence in the ligand molecule via the semi-empirical molecule orbital method of parameterized model number 3. Results are discussed in connection to the basicity of the donor atoms and structural arrangement of the ligand. Although BTDH₂ has two dissociable protons, four protonation constants can be measured under the experimental conditions presented. These four protonation constants (as log₁₀ βs) are 10.245, 19.397, 22.414 and 25.176.     

A Comparative Investigation of Mixing Rules for Property Prediction in Multicomponent Electrolyte Solutions

A mathematical technique is developed to investigate physicochemical property prediction of solution mixtures from the corresponding properties of the pure dissolved systems, as is often expressed in empirical ‘mixing rules’ such as those of Young and of Zdanovskii. A systematic method to distinguish between the inherent characteristics of such rules is needed because experimental studies have proved indecisive. Sound mixing rules must be found to support current efforts in thermodynamic modelling where conventional approaches like the Pitzer equations lack robustness. Density differences relative to pure water, osmotic coefficients and heat capacities are investigated with mixtures including {NaCl + MgCl₂}(aq) and {NaCl + Na₂SO₄}(aq) as specific examples representing common-anion and common-cation asymmetric strong electrolyte solutions respectively. Water activity curves for hydrochloric acid and the alkali metal chloride solutions are also considered. The results confirm that, at the present state of the art, differences between mixing rules are for the most part insignificant at 25 °C, being about the same or less than would be expected from experimental uncertainty. As the predicted differences are even smaller at higher temperature, it can be posited that all reasonably well-established mixing rules in the literature will give approximately equivalent and satisfactory predictions of solution properties under superambient conditions. This is particularly important since the effects of temperature on the magnitude of ternary interactions are not well known from experiment.     

Investigation of AOT reverse microemulsions in supercritical carbon dioxide

Bis(2-ethylhexyl) sodium sulphosuccinate (AOT) was successfully solubilised in supercritical carbon dioxide (scCO₂), with ethanol or pentanol as co-solvent. Three molecular spectroscopic probes: methyl orange (MO), 8-hydroxy-1,3,6-pyrenetrisulphonic acid trisodium salt (HPTS), and riboflavin (RF) were used to examine the solubilisation characteristics of the water/scCO₂ microemulsions formed with AOT. MO was extracted at various operating conditions, although the wavelength of its solvatochromic absorption maximum was not indicative of bulk water properties. Instead, the spectral results imply that MO may be located at the surfactant/water interface. The highly water-soluble dye HPTS was unable to be extracted into scCO₂/AOT/water systems, suggesting that the water in the reverse micelle core was not as polar under supercritical conditions as those at ambient conditions. Finally, RF was extracted into the supercritical phase (40°C, 175 bar) with pentanol co-solvent, with an apparent enhanced uptake compared with the value at 40°C and ambient pressure in bulk water. This appears to be due to the presence of microcrystals dispersed in the supercritical phase.