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.     

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.     

Liquid-Liquid Equilibria for Water + Nitromethane + (1-Propanol,or + Acetone,or + p-Dioxane) Ternary Systems at 303.15 K

Abstract

Liquid-liquid equilibria, distribution coefficients, and selectivities of ternary systems of the type: (water + K + nitromethane), where Kis 1-propanol, acetone, or p-dioxane, have been determined at (303.15 ± 0.05) K, in order to evaluate the suitability of nitromethane for extracting preferentially the second components from their aqueous solutions. The line data were satisfactorily correlated by the Othmer and obias method, and the plait point coordinates for the three systems were estimated. The experimental data were compared with values calculated using the NRTL and UNIQUAC models, and with those predicted by the UNIFAC group contribution method. This last method predicts qualitative and quantitative behaviour which are in disagreement with experimental results, while the values calculated using the other two models are in agreement but only when the concentration of component K is low. The three ternary systems studied have distribution coefficients higher than unity, and high selectivities. Therefore, nitromethane could be considered as a potential solvent for the extraction of K from its aqueous solutions     

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.     

Ionic Liquids and Water: Hydrophobicity vs. Hydrophilicity

Many chemical processes rely extensively on organic solvents posing safety and environmental concerns. For a successful transfer of some of those chemical processes and reactions to aqueous media, agents acting as solubilizers, or phase-modifiers, are of central importance. In the present work, the structure of aqueous solutions of several ionic liquid systems capable of forming multiple solubilizing environments were modeled by molecular dynamics simulations. The effect of small aliphatic chains on solutions of hydrophobic 1-alkyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ionic liquids (with alkyl = propyl [C₃C₁im][NTf₂], butyl [C₄C₁im][NTf₂] and isobutyl [iC₄C₁im][NTf₂]) are covered first. Next, we focus on the interactions of sulphonate- and carboxylate-based anions with different hydrogenated and perfluorinated alkyl side chains in solutions of [C₂C₁im][C_nF_2n+1SO₃], [C₂C₁im][C_nH_2n+1SO₃], [C₂C₁im][CF₃CO₂] and [C₂C₁im][CH₃CO₂] (n = 1, 4, 8). The last system considered is an ionic liquid completely miscible with water that combines the cation N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium [N_{1 2OH 2OH 2OH}]⁺, with high hydrogen-bonding capability, and the hydrophobic anion [NTf₂]^–. The interplay between short- and long-range interactions, clustering of alkyl and perfluoroalkyl tails, and hydrogen bonding enables a wealth of possibilities in tailoring an ionic liquid solution according to the needs.     

Anomalies in Noble-Metal-Based Liquid Alloys

Experimental data on liquid noble-metal-based alloys were analysed in terms of a pseudopotential approach. The anomalies in the heat of mixing, the electrical resistivity and the magnetic susceptibility were found to be reflected by the theory in a qualitative manner even if compound formation or short range order is not taken into account. The emphasis was placed on liquid Ag-In, but other examples of this particular class of alloys were also included in the discussion.     

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.     

Is there a region of highly structured water around a nonpolar solute molecule?

Published measurements of water proton chemical shifts for dilute solutions of alcohols and other hydrocarbon derivatives surprisingly seem to imply that hydrophobic groups enhance water structure near 0°C but disrupt it at elevated temperatures. A model is presented which allows these observations to be rationalized and is consistent with experimental values of enthalpies and heat capacities of solution of hydrocarbon gases. It requires the assumption that hydration-shell H-bonds have higher bond-breaking enthalpies and entropies than those in bulk water. These quantities are evaluated from available thermochemical data. Using the corresponding free energies of bond breaking, it is then calculated that the fraction of broken H-bonds is larger in the hydration shell than in the bulk liquid even at temperatures near the freezing point. The Model does not invoke formation of extended ordered regions that could be described as icebergs and that melt when the solutions are heated.     

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.     

低能氮离子与水的化学合成作用

化学改性是离子与物质相互作用的一种重要效应，即外来离子的介入引起材料中元素或基团间化学反应，更突出的是离子直接参与化合形成新的物质，从而改变材料的性质＊’1．近年来离子注入生物体产生独特的诱变效应引起了广泛关注问．注入离子在生物材料中引起的化学合成较好地解     

LC Determination of Mono-Substituted Phenols in Water Using Liquid–Liquid–Liquid Phase Microextraction

A simple liquid–liquid–liquid microextraction device of new design was used to pre-concentrate phenols from water samples before liquid chromatographic (LC) analysis. Extraction was induced by the pH difference inside and outside an organic phase located at the interface. The pH of the donor phase outside the organic phase was adjusted to 1 with HCl whereas the acceptor phase was a basic solution at pH 13. On stirring neutral phenols were extracted into the organic solvent then back-extracted into 1 μL of basic acceptor solution suspended from the tip of a micro syringe. The acceptor phase was then withdrawn into the micro syringe and injected directly into the LC. The technique uses a low-cost disposable extraction ‘device’ and is very convenient to operate. Up to 230-fold enrichment of analytes could be achieved. This procedure could also serve as a sample clean-up step because neutral and basic compounds were not extracted into the acceptor phase. The RSD (n = 5) was better than 6.2% and the linear calibration range was from 1 to 1000 µg–L⁻¹ with r ² ≥ 0.992.Optimization of experimental conditions (rate of stirring, ionic strength of the sample solution, concentration of reagents, time of extraction, and organic solvent volume) were also examined. The method was applied to the determination of phenols in tap and well waters.Revised: 14 February and 29 March 2005     

Aqueous Brønsted–Lowry Chemistry of Ionic Liquid Ions

Ionic liquids have become commonplace materials found in research laboratories the world over, and are increasingly utilised in studies featuring water as co‐solvent. It is reported herein that proton activities, a_H⁺, originating from auto‐protolysis of H₂O molecules, are significantly altered in mixtures with common ionic liquids comprised of Cl^?, [HSO₄]^?, [CH₃SO₄]^?, [CH₃COO]^?, [BF₄]^?, relative to pure water. pa_H⁺ values, recorded in partially aqueous media as ?log(a_H⁺), are observed over a wide range (～0–13) as a result of hydrolysis (or acid dissociation) of liquid salt ions to their associated parent molecules (or conjugate bases). Brønsted–Lowry acid–base character of ionic liquid ions observed is rooted in equilibria known to govern the highly developed aqueous chemistry of classical organic and inorganic salts, as their well‐known aqueous pKs dictate. Classical salt behaviour observed for both protic and aprotic ions in the presence of water suggests appropriate attention need be given to relevant chemical systems in order to exploit, or avoid, the nature of the medium formed.     

On the properties of surfactant aqueous solutions

Summary From studies of aqueous solutions of dodecylammoniumnitrate an association mechanism has been proposed involving multiequilibrium. In the concentration range considered we can differentiate between two aspects: ranges of marked qualitative and quantitative changes and formation of differently structured surfactant species.

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Zusammenfassung Aus der Untersuchung wäßriger Lösungen von Dodecylammoniumnitrat würde ein Assoziationsmechanismus abgeleitet, welcher ein Multiequilibrium beinhaltet. Im untersuchten Konzentrationsbereich können zwei Bereiche unterschieden werden: Bereiche mit ausgeprägten qualitativen und quantitativen Änderungen und Bereiche mit der Bildung von verschieden strukturierten oberflächenaktiven Spezies.

     

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.     

Identification of Free OH and its Implication on Structural Changes of Liquid Water

The molecular structure of liquid water has been an outstanding issue for many years. The identification of free -OH holds the key in differentiating structure models for liquid water. By analyzing the relative changes of the intensity and depolarization ratio in temperature dependent Raman spectra, the occurrence of free -OH in liquid water is unambiguously de-termined. Furthermore, upon the increase of temperature from 5 oC to 85 oC, the structure of liquid water undergoes significant change, but the relative proportion of free -OH is con-siderably small and remains almost unchanged. This implies that the breaking of hydrogen bond from the tetrahedral structure prefers to occur at the site of the hydrogen acceptor. The energetic favoring of the structural change for liquid water is thus clearly revealed from experiments.     

Complications in the Use of the Taylor Dispersion Method for Ternary Diffusion Measurements: Methanol + Acetone + Water Mixtures

The Taylor dispersion technique is used to measure the ternary mutual diffusion coefficients of aqueous nonelectrolyte solutions at 25°C. The dispersion of the injected solutes is recorded by a differential refractometer and an ultraviolet-visible detector. The diffusion coefficients are calculated directly by fitting the theoretical dispersion equations to about six experimental curves simultaneously. If the ternary diffusion effects in the measured dispersion profiles are not confused by the inaccuracy of the experimental method or an unfavorable relative detector sensitivity, the diffusion coefficients are precise. For the system methanol + acetone + water, it is shown that the Taylor dispersion method is unsuitable for the determination of all the diffusion coefficients if the methanol mole fraction is less than 0.45 or the acetone mole fraction if more than 0.001.     

Enthalpies of Solution and Dilution of Some L-α-Amino Acids in Water at 298.15 K

The enthalpies of solution and dilution of glycine, L-α-alanine, L-valine, L-α-leucine, L-α-serine and L-α-threonine in water were measured at 298.15 K. The results were applied to calculate the enthalpic coefficients of homogeneous interactions between zwitterions of L-α-amino acids in aqueous solutions. This revised version was published online in August 2006 with corrections to the Cover Date.     

高效液相色谱-串联质谱法同时检测水中17种抗生素

建立了高效液相色谱-串联质谱(HPLC-MS/MS)同时检测水中大环内酯类、磺胺类、甲氧苄啶、四环素类、氟喹诺酮类五类共17种抗生素含量的分析方法。水样经Oasis HLB固相萃取小柱富集净化后,以CORTECSTM C18色谱柱(100×4.6mm,2.7μm)分离,2%甲酸+2mmol/L乙酸铵溶液-乙腈为流动相,在电喷雾离子源正离子模式下采用质谱多反应监测模式(MRM)检测。结果表明,各目标化合物在0.50~250ng/mL范围内,线性关系良好(R20.990),检测限为0.01~2.50ng/L;在10和100ng/L添加水平下,17种抗生素的加标回收率为60.6%~125%,相对标准偏差(RSDs,n=4)为0.311%~14.8%。应用该方法测定广州市某河流的水样,除三乙酸竹桃霉素、氯四环素、洛美沙星、恩氟沙星上游未被检出,下游微量检出外,其余13种抗生素均有检出,其中氧四环素的浓度最高,最大值达560ng/L,四环素次之,为540ng/L。     

水、甲醇和乙醇液体微结构性质的Car-Parrinello 分子动力学模拟

采用Car-Parrinello 分子动力学(CPMD)方法分别研究了水、甲醇和乙醇的液体微结构性质.研究结果显示:在水、甲醇和乙醇三个体系中O…O径向分布函数曲线的第一个峰位置分别为0.278、0.276 和0.275nm; O…H径向分布函数曲线的第一个峰位置分别为0.178、0.176和0.177 nm.表明基团(氢原子、甲基、乙基)的差异对O…O第一个峰的位置影响很小.但基团的差异对径向分布函数峰高的影响却很显著,由水到乙醇第一个峰的高度逐渐变高.空间分布函数表明氧原子和氢原子在溶剂分子周围有取向地分布,这与径向分布函数所表现出尖锐的第一个峰相一致.氢键分布分析显示,水、甲醇和乙醇的平均氢键数分别为3.62、1.99 和1.87,表明水形成了网状氢键结构,而甲醇、乙醇形成链状氢键结构.     

Activity Coefficients of Electrolytes from Liquid Membrane Cells. XI. The Nonsulfate 1:2 Salts,Potassium Oxalate,and Sodium 1,5-Naphthalenedisulfonate

Activity coefficients of K₂C₂O₄ and Na₂ds, ds^2– = 1,5-naphthalenedisulfonate anion, which were previously unavailable, are examined at 25.0°C using liquid membrane cells. The results for both salts are approximated satisfactorily by the primitive model in spite of the fact that ds^2–, a planar ion with electric charges distant from one another, does not resemble a charged hard sphere. Data are compared with those of K₂SO₄ and bivalent metal perchlorates. The Pitzer ion-interaction parameters are reported.