Hydrophobic interaction in water-ethanol mixtures

The solubility, free energy, entropy, and enthalpy of solutions of methane and ethane were determined in various mixtures of water and ethanol. The hydrophobic interaction for the pair methane-methane was computed, in these mixtures, using an approximate expression derived previously. Also the entropy and the enthalpy associated with the hydrophobic interaction were evaluated from the experimental data. The main finding is the steep decrease in the strength of the hydrophobic interaction as the mixture becomes richer in alcohol. The variation of the corresponding entropy and enthalpy also shows an abrupt change at the composition ofx _ethanol0.2. Some interpretations of these findings in terms of structural changes in the solvent are discussed.     

Thermodynamic parameters for the protonation of urea at different temperatures and ionic strengths

Potentiometric measurements of aqueous urea solutions (less than2.5 mol-dm⁻³ of urea) were performed at different temperature and ionic strengths (10≤T≤45°C; 0.02≤I≤0.6 mol-dm⁻³) to determine thermodynamic protonation parameters. The formation of the species U₂H⁺ was hypothesized to explain the concentration dependence of the protonation constant.     

Structral Effects on the Mutual Solubility of Solution Components and Formation of Zinc and Cadmium Chloride Complexes in Zn(Cd)(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-LiCl-H<Subscript>2</Subscript>O Systems

Solubility isotherms have been determined for Zn(Cd)(NO₃)₂-LiCl-H₂O systems at 25°C, and Raman spectra in the range of M-Cl and nitrate ion vibrations have been measured. A single bond system is realized over the whole range of concentrations in the Zn(NO₃)₂-LiCl-H₂O system because the cybotactic groups are structurally compatible. In the Cd(NO₃)₂-LiCl-H₂O system, the cybotactic groups are incompatible, which leads to structural microheterogeneities in post-eutectic concentrations.Original Russian Text Copyright © 2004 by M. K. Khripun, A. A. Kiselev, A. Yu. Efimov, L. A. Mund, and V. P. Petranovskii__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 1027–1033, November–December, 2004.     

Hydrophobic Effect: Solubility of Non-polar Substances in Water,Protein Denaturation and Micelle Formation

The 'hydrophobic effect' of the dissolution process of non-polar substances in water has been analysed under the light of a statistical thermodynamic molecular model. The model, based on the distinction between non-reacting and reacting systems explains the changes of the thermodynamic functions with temperature in aqueous systems. In the dissolution of non-polar substances in water, it follows from the model that the enthalpy change can be expressed as a linear function of the temperature (ΔH _app =ΔH ^ø +n _w C _p,w T ). Experimental solubility and calorimetric data of a large number of non-polar substances nicely agree with the expected function. The specific contribution of n _w solvent molecules depends on the size of solute and is related to destructuring (n _w >0) of water molecules around the solute. Then the study of 'hydrophobic effect' has been extended to the protein denaturation and micelle formation. Denaturation enthalpy either obtained by van't Hoff equation or by calorimetric determinations again depends linearly upon denaturation temperature, with denaturation enthalpy, ΔH _den , increasing with T . A portion of reaction enthalpy is absorbed by a number n _w of water molecules (n _w >0) relaxed in space around the denatured moieties. In micellization, an opposite process takes place with negative number of restructured water molecules (n _w <0) because the hydrophobic moieties of the molecules joined by hydrophobic affinity occupy a smaller cavity.     

Effects of Metal Ion Adduction on the Gas-Phase Conformations of Protein Ions

Changes in protein ion conformation as a result of nonspecific adduction of metal ions to the protein during electrospray ionization (ESI) from aqueous solutions were investigated using traveling wave ion mobility spectrometry (TWIMS). For all proteins examined, protein cations (and in most cases anions) with nonspecific metal ion adducts are more compact than the fully protonated (or deprotonated) ions with the same charge state. Compaction of protein cations upon nonspecific metal ion binding is most significant for intermediate charge state ions, and there is a greater reduction in collisional cross section with increasing number of metal ion adducts and increasing ion valency, consistent with an electrostatic interaction between the ions and the protein. Protein cations with the greatest number of adducted metal ions are no more compact than the lowest protonated ions formed from aqueous solutions. These results show that smaller collisional cross sections for metal-attached protein ions are not a good indicator of a specific metal–protein interaction in solution because nonspecific metal ion adduction also results in smaller gaseous protein cation cross sections. In contrast, the collisional cross section of α-lactalbumin, which specifically binds one Ca²⁺, is larger for the holo-form compared with the apo-form, in agreement with solution-phase measurements. Because compaction of protein cations occurs when metal ion adduction is nonspecific, elongation of a protein cation may be a more reliable indicator that a specific metal ion–protein interaction occurs in solution.      

Enthalpies of solution of glycine in aqueous solutions of 1,2-diols and glycerol at 25°C

Dissolution enthalpies of glycine in mixtures of water with 1,2-ethanediol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol, 1,3-propanediol and glycerol have been measured at 25°C. The enthalpic pair interaction coefficients of the glycine zwitterion with the polyalcohol molecule have been determined by using the standard solution enthalpies of glycine in water and in aqueous solutions of the polyalcohols. The values of the resultant enthalpic interaction coefficients are interpreted assuming a criterion in the form of the effect of hydrophobic alkyl radicals on the interactions between the hydroxyl groups of polyalcohols and the zwitterion of glycine.     

The Study of Water Structure in Aqueous Solutions of Butoxyethanol by Enthalpy of Mixing Measurements

In order to confirm the existence of regions I and II in aqueous solutions of butoxyethanol(BE), the concentration and temperature dependences of enthalpies of mixing of aqueous BE solutions with some organic solvents were measured. It has been found that the increments of apparent enthalpies of mixing per mole of water with respect to the mole fraction of BE change irregularly at a certain concentration. This concentration nearly corresponds to the reported boundary between regions I and II. Although similar behavior has also been observed in aqueous solutions of iso-butoxyethanol, aqueous solutions oftert-butoxyethanol have shown no anomalous behaviors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Volumetric properties of mixtures of water and methanol H/D-isotopomers between 5 and 45°C

Densities of H/D-isotopomers mixtures of water (H₂O, D₂O) and methanol (CH₃OH, CD₃OH, CH₃OD, and CD₃OD) over the full range of compositions were measured at 5, 15, 25, 35, and 45°C. Results have been used to calculate molar volumes, excess molar volumes, apparent molar volumes, and isotope effects of the mixtures. The volumetric properties are discussed in terms of the structural changes in water-methanol solutions under the influence of isotope substitution.     

Nickel(II) Oxide Solubility and Phase Stability in High Temperature Aqueous Solutions

A platinum-lined, flowing autoclave facility was used to investigate the solubility behavior of nickel(II) oxide (NiO) in deoxygenated ammonium and sodium hydroxide solutions, between 21 and 315°C. Solubilities were found to vary between 0.4 and 400 nmol-kg^–1. The measured nickel ion solubilities were interpreted via a Ni(II) ion hydroxo- and ammino-complexing model, and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. Two solid phase transformations were observed: at temperatures below 149°C, the activity of Ni(II) ions in aqueous solution was controlled by a hydrous Ni(II) oxide (theophrastite) solid phase rather than by anhydrous NiO (bunsenite); above 247°C, Ni(II) activities were controlled by cubic rather than by rhombohedral bunsenite.     

Structural properties of dilute aqueous solutions of dimethylformamide and acetone based on computer simulation

The Monte Carlo simulation of molecular configurations for aqueous solutions ofN,N-dimethylformamide and acetone was carried out. The atom-atom radial distribution functions were determined. The topological properties of the H-bond system were investigated. The concentrations of closed H-bond cycles and the radial distribution functions of their geometric centers were found. It was shown that the local arrangement of molecules and supermolecular assemblies typical of the H-bond network in neat water is retained in the solutions studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 592–599, April, 1998.     

溶剂气浮法去除水中腐殖酸的动力学和热力学初步研究

研究了溶剂气浮法去除水中的腐殖酸,该技术可以作为生物处理后的填埋渗出液的后处理方法。腐殖酸通过与表面活性剂十六烷基三甲基溴化铵生成离子缔合物气浮进入有机溶剂异戊醇而从水中去除。大于计量比的表面活性剂的量在10 m in之内可达92%的去除率。气浮速率在一定程序上随着气流速率的增加而增加,基本上与有机溶剂的体积无关。同时,还考察了电解质、乙醇及溶液的pH值对水中腐殖酸的溶剂气浮过程的影响。研究表明,该溶剂气浮过程遵从一级动力学。气浮过程的表观活化能作为一个特征参数被提出来,其值为2.87 kJ/mol。     

Influence of the size of fixed-rigidity spheres on the structural and energy characteristics of hydrophobic hydration

Monte Carlo simulations of molecular configurations of aqueous solutions of spherical particles with a special potential of solute—water interaction were carried out. The influence of the particle size on the properties of hydration shells was investigated. Two regimes of hydrophobic hydration with a crossover point at 0.4 nm were found. Hydration of smaller particles causes insignificant changes in the properties of water. Particles larger than 0.4 nm break the liquid water structure. Breaking effects are more pronounced in the first hydration shell of particles. Dewetting of hard sphere surfaces predicted by the LCW phenomenological theory has peculiarities in the case of hydration of fixed-rigidity spheres. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1777–1786, September, 2008.     

Zinc(II) oxide solubility and phase behavior in aqueous sodium phosphate solutions at elevated temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility/phase behavior of zinc(II) oxide in aqueous sodium phosphate solutions at temperatures between 17 and 287°C. ZnO solubilities are observed to increase continuously with temperature and phosphate concentration. At higher phosphate concentrations, a solid phase transformation to NaZnPO₄ is observed. NaZnPO₄ solubilities are retrograde with temperature. The measured solubility behavior is examined via a Zn(II) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria are obtained from a least-squares analysis of the data. The existence of two new zinc(II) ion complexes, Zn(OH)₂(HPO₄)^2– and Zn(OH)₃(H₂PO₄)^2–, is reported for the first time. A summary of thermochemical properties for species in the systems ZnO–H₂O and ZnO–Na₂O–P₂O₅–H₂O is also provided.     

Models having the thermodynamic properties of aqueous solutions of tetraalkylammonium halides

The hypernetted-chain integral-equation-approximation method is used to calculate the ion-ion pair correlation functions and the thermodynamic properties of models of the kind previously studied,⁽¹⁾ which are based on an ion-ion pair potential having four terms: the usual Coulomb term, a core repulsion term, a term to represent a well-known dielectric repulsion effect, and a Gurney term to represent the effect of the overlap of the structure-modified regions, solvation shells or cospheres, when the ions come close together. The coefficientA _ij of the last term for each pairi,j of ionic species is the only parameter that is adjusted to fit the solution data. It is determined by fitting excess-free-energy (osmotic-coefficient) data. It is scaled to represent the molar free-energy change of water displaced from the cospheres when they overlap. The corresponding entropy changeS _ij and volume changeV _ij are determined by fitting, respectively, excessenthalpy and excess-volume data. Problems of finding a uniquely best set of parameters are discussed together with many examples of variations of the model, most of which require further investigation. A consistent set of these parameters, which represents much of what is known about the thermodynamic excess functions of these solutions at concentrations up to about 0.5M, is interpreted as far as possible in terms of the data for thermodynamic solvation functions for the same systems.This study was aided by a grant from the Office of Saline Water, U.S. Department of Interior.     

Solubility and phase behavior of nickel oxide in aqueous sodium phosphate solutions at elevated temperatures

A platinum-lined, flowing autoclave facility was used to investigate the solubility/phase behavior of nickel oxide (NiO) in aqueous sodium phosphate solutions between 290 and 560 K. A layer of hydrous nickel oxide was concluded to exist on the nickel oxide surface below 468 K; only at higher temperatures did the anhydrous nickel oxide phase control the nickel ion solubility behavior. The measured solubility behavior was examined via a nickel(II) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from a least-squares analysis of the data. The existence of two new nickel ion complexes are reported for the first time: Ni(OH)₂(HPO₄)⁼ and Ni(OH)₃(H₂PO₄)⁼. The positive entropy change associated with the formation of Ni(OH)₃(H₂PO₄)⁼ leads to its dominance in alkaline phosphate solutions at elevated temperatures.     

温度对水的离子积与溶液酸碱性的pH判据的影响

讨论了温度对水的离子积Kw和溶液酸碱性的pH判据的影响,分析了容易产生的模糊认识.Kw随温度升高显著增大,使溶液呈中性的pH(中性pH = pKw/2)在高温(>25 ℃)和低温区(<25 ℃)分别出现小于和大于7.0的情况.     

Enthalpies of interaction of glycine with some amides in aqueous solutions at 298.15 K

The dissolution enthalpies of glycine in aqueous solutions of acetamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylformamide, N,N-diethylformamide and N,N-diethylacetamide were measured at 298.15 K. The enthalpic pair interaction coefficients of glycine zwitterion-amide molecules were determined by using standard solution enthalpies of glycine in water and aqueous solutions of amides. The additivity of groups concept of Savage and Wood was used to estimate the contribution of each of the functional groups of the studied amides.     

Heats of solution and the influence of solutes on the temperature of maximum density of water

The influence of N,N-dimethylformamide, N,N-dimethylacetamide, 2-pyrrolidinone, N-methyl-2-pyrrolidinone, and N-t-butyl-2-pyrrolidinone on the temperature of maximum density (TMD) of water has been investigated. Heats of solution for these compounds and for diethylsulfoxide, tetramethylene sulfoxide, tetramethylene sulfone, dimethyl sulfoxide, and tetrahydrofuran in water at 23°C are also reported. These data, together with published TMD and heat of solution data for other solutes, are used to examine the relationship between the enthalpy change on dissolution of a solute in water and the effect of the dissolved substance on the structure of water as reflected by changes in the temperature of maximum density of the system.     

Near-infrared spectra of water and aqueous electrolyte solutions at high pressures

The near-infrared spectra (9500 to 11000 cm^–1) of pure water and aqueous solutions of alkali halides, MgCl₂, NaClO₄, and R₄NBr were measured at temperatures between 10 and 55°C and pressures up to 500 MPa. From the analysis of the absorption spectra the following conclusions are drawn. (1) The ice I-like open structure is destroyed and the packed structure is formed as the pressure is increased. (2) The open structure of water is destroyed by the addition of alkali halides and MgCl₂ and water molecules are restricted around the ions by ion-dipole interactions. This results in a loosening of the O–H bond. (3) The perchlorate ion destroys the open structure of water and the ion-dipole interaction with water is insignificant. (4) The Bu₄N⁺ ion forms water structure around the ion similar to that of the clathrate open structure.     

静电和疏水效应对SARS冠状病毒3CL蛋白酶二聚体稳定性的影响

从TGEV3CL蛋白酶二聚体结构出发,研究了TGEV3CL蛋白酶二聚体单体之间的静电和疏水相互作用.蛋白质的静电相互作用通过有限差分方法求解Poisson-Boltzmann方程得到,疏水相互作用通过分析溶剂可及性表面模型得到.考察了不同pH值对SARS3CL蛋白酶二聚体静电和疏水相互作用的影响,在pH=5.5～8.5时,二聚体静电相互作用能、静电去溶剂化能和疏水自由能都具有较小的数值,表明在该条件下静电和疏水相互作用有利于二聚体的稳定存在.由于SARS3CL蛋白酶活性模式为二聚体,因此,在该pH值范围内,有利于蛋白酶保持活性.在pH=7.0条件下,蛋白酶单体之间具有最强的静电和疏水相互作用,从而使蛋白酶具有最强的活性,这与实验结果相一致.pH值对静电去溶剂化能的影响大于疏水自由能,表明静电作用是造成强酸或强碱条件下二聚体不能稳定存在的主要原因.