Distribution of carboxylic acids between water and nonpolar organic solvents

Equations which describe the distribution of carboxylic acids between water and nonpolar organic solvents, and which make allowance for their dimerization in the organic solvent, their association with water molecules, and the nonideal nature of the organic phase, have been examined. It has been shown that it is necessary to study the distribution of the carboxylic acid and water between the phases in order to determine the constants for the distribution, hydration, and dimerization of the carboxylic acids.     

Role of water structure in thermodynamics of nonpolar solvation

The SSOZ (site-site Ornstein-Zernike) equation with an original closure condition for liquid molecular systems is used to calculate thermodynamic functions of noble gas solvation in water. Water is modeled by two close sets of atom-atomic potential functions. The calculations indicate that the chemical solvation potential is strongly sensitive to water structure. A comparison with experiment is given. Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 736–741, July–August, 1996. Translated by L. Smolina     

Solution thermodynamics of valnemulin hydrogen fumarate in different pure solvents

Solubility of valnemulin hydrogen fumarate in five pure solvents was determined within temperature range of (278.15 to 323.15) K by a gravimetric method. The results show that the solubility of valnemulin hydrogen fumarate in tested pure solvents increases with the increasing temperature. The solubility values were correlated by the Wilson model, NRTL model and UNIQUAC model. The UNIQUAC volume parameter, area parameter, and Wilson liquid molar volume parameter of valnemulin hydrogen fumarate were estimated by the group contribution method. It was found that the correlated results are in good agreement with the experimental results. Furthermore, the mixing thermodynamic properties of valnemulin hydrogen fumarate in solutions, including the mixing Gibbs energy, the mixing enthalpy and entropy, were determined by using the Wilson model and the experimental solubility results.     

Solubility of hydrogen sulfide in organic solvents

The solubilities of hydrogen sulfide expressed in mole fractions may be related, through multiparameter equations, to parameters that account for the specific and nonspecific solvation ability of the solvent and for the cohesion energy. The key factors are the self-association of the solvent, which reduces the solubility, and the complexing ability with hydrogen sulfide, which increases the solubility.     

Phase behavior and self-organized structures of diglycerol monolaurate in different nonpolar organic solvents

Nonaqueous phase behavior and reverse micellar structures of diglycerol monolaurate (DGL) in different nonpolar organic solvents, such as n-decane, n-tetradecane, and n-hexadecane, have been studied over a wide range of compositions and temperatures. The equilibrium phases are identified by means of visual observation and small-angle X-ray scattering (SAXS). A solid phase present at lower temperature swells small amount of oils and transforms into a lamellar liquid crystalline structure at higher temperature. The melting temperature of the solid phase is virtually constant at all mixing ratios of the surfactant and oil. With the further increase of temperature, the liquid crystal transforms into an isotropic single-liquid phase near the surfactant axis, whereas there is a coexistence region of two isotropic phases near the solvent axis. The area of the two-liquid (II) phase region depends largely on the hydrocarbon chain length of the oils, the longer chain leading to the wider II area. Accordingly, the DGL surfactant is most miscible with decane, exhibiting a reduced miscibility with increasing solvent hydrocarbon chain length. Increasing temperature enhances the dissolution tendency of the surfactant in oil, where the two-liquid phase transforms into an isotropic single phase. SAXS analysis based on the GIFT technique is used to characterize the structure of the reverse micellar aggregates in the isotropic single-phase liquids. We have demonstrated that instead of changing polarity or a functional group of the solvent molecules, if we optimize the hydrophilic nature of the surfactant head group, the alkyl chain length of the solvent oils can serve as a tunable parameter of the micellar geometry. The hydrophilic surfactant DGL interestingly forms cylindrical micelles in nonpolar oils, decane, and tetradecane in the dilute region above the II phase region. The micellar size shows temperature dependence behavior, and the micellar length goes on increasing with decreasing temperature; eventually we found a signature of the onset of critical fluctuations in the deduced pair-distance distribution function near the phase separation line. The signature of the attractive interaction between the cylindrical reverse aggregates when a phase separation line is approached is likely to be a precursor of critical phenomenon. Doping with a trace of water results in a similar but more pronounced structural enhancement. The transfer free energy of diglycerol moiety from a hydrophilic environment to different hydrocarbon oils may account for these phenomena.     

Solubility and solution thermodynamics of sorbic acid in eight pure organic solvents

By the gravimetric method, the solubility of sorbic acid in eight solvents including ethanol, 2-propanol, methanol, 1-butanol, ethyl acetate, methyl tert-butyl ether, acetone and acetonitrile was determined over a temperature range from 285.15 to K at atmospheric pressure. For the temperature range investigated, the solubility of sorbic acid in the solvents increased with increasing temperature. The experimental values were correlated with the linear solvation energy relationship, modified Apelblat equation, λh equation, non-random two-liquid (NRTL) model, and Wilson model. On the other hand, the enthalpy, entropy and Gibbs free energy of dissolution were obtained from these solubility values by using the van’t Hoff and Gibbs equations. The excess enthalpy of solution was estimated on the basis of λh equation. Furthermore, the a priori predictive model COSMO-RS was employed to predict the solubility of sorbic acid in selected solvents and reasonable agreement with experimental values is achieved.     

Lifetimes and the dynamics of hydrogen bonds in liquid water

Based on Raman spectra of light, heavy, and half-heavy water in the region of O-H and O-D stretching vibrations and on the independent-oscillator model, it is shown that a peculiarity of the liquid state of water is the nonequivalence of O-H groups of water molecules in hydrogen bonding. The structure of liquid water and the mechanism of its molecular mobility are considered in this context.     

Proper and improper hydrogen bonds in liquid water

The total lifetime distributions for hydrogen bonds in snapshots of molecular dynamics simulations of water serve as a basis to identify a class of proper hydrogen bonds. Proper bonds emerge and break up when restructuring the surrounding area of the hydrogen bond networkwhich weakly depend on the properties of this individual bond, i.e., almost randomly. Therefore, the distribution of the bond lifetimes is described by an exponential function similar to the distribution of the mean free path time in gas. It is shown that proper hydrogen bonds are strong, long-lived, and tetrahedrally oriented bonds. They account for about 80% of the bonds in each snapshot. Thus, these bonds form the basis or framework of the hydrogen bond network of water. The other, improper bonds have a substantially shorter lifetime; these are weak, bifurcated, and quickly switching bonds.     

Local structure of solvation of nonpolar chains in nonpolar solvents at infinite dilution

The SSOZ (site-site Ornstein-Zernike) equation is used to study the local structure of solvation of linear nonpolar molecules in nonpolar solvents. The atom-atomic interaction potentials are described by the Lennard-Jones potential. The chain-solvent atom-atomic pair correlation functions are calculated in relation to the chain length (number of atoms), solvent density, and the ratio of the geometrical parameters of solvent and chain atoms. Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 742–749, July–August, 1996. Translated by L. Smolina     

Properties of the network of the hydrogen bonds of water

Networks of the hydrogen bonds and those consisting of lines connecting nearby molecules were constructed using configurations of water molecules obtained by the Monte-Carlo method. The concentrations of closed cycles of hydrogen bonds were established to be determined only by the probability of hydrogen bond formation. Characteristics of a model ideal water network were determined. Topological properties of the Polk model and those of the network of nearest neighbors substantially differ from the properties of the ideal network. The totality of the hydrogen bonds in pure water was proposed to be considered as a hierarchical system. Three topologically different structures of water associates were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 928–931, May, 1997.     

A method for detecting water in organic solvents

The potent dehydrating ability of difluoro(aryl)-lambda (3)-iodanes is exploited to develop a convenient (19)F-NMR-based aquametry method that is more sensitive than coulometric Karl Fischer titration. The key difluoro(aryl)-lambda (3)-iodane reagents are synthesized readily from commercially available and inexpensive precursors.     

Solvation of ions in liquid solvents and proteins

Despite their well-known drawbacks, the approaches of continuum electrostatics are widely used at the analysis of the energies of solvation and reorganization. We propose a method to check the applicability of these approaches in the determination of the solvation energy, which is based on measuring the difference of redox potentials ΔE of two consecutive redox reactions, e.g. for the pairs Co(Cp) ₂ ⁺ /Co(Cp)₂/Co(Cp) ₂ ^? (here, Cp is cyclopentadienyl). In this difference, the solvophobic effects and the liquid junction potential between the working and reference electrodes, which is impossible to measure, cancel out. From the difference of ΔE in two different solvents, the sum of the electrostatic components of the cation-and anion-transfer energies is determined. It is shown that, for large low-charged ions in aprotic media, the continuum electrostatics proves to be true in a wide range of dielectric permittivities including those typical for proteins. The Stokes shift of fluorescence spectra for proflavine (PF) showed that the water reorganization energy and, hence, the energy of the static dielectric response are anomalously high. To study this effect on the solvation energy, we determined the redox potentials of the Co(Cp) ₂ ⁺ /Co(Cp)₂ pair in a number of water-organic media. The organic cosolvent breaks the water structure and reduces the reorganization energy. Accordingly, the redox potential turns more positive. This allowed us to determine the energy of transfer of Co(Cp) ₂ ⁺ ions (and, hence, of other ions) nonviolated by the water structure specifics. The experimental energies of the acetate transfer exceed those calculated by an order of magnitude. This demonstrates the incorrectness of the widely used semicontinuum calculations of the pK of ionogenic groups of proteins. A new algorithm, which permits overcoming this discrepancy, is proposed, namely, the short-range interactions are taken into account based on the experimental energies of the transfer to a model DMF solvent, while the transfer energy from this solvent to the protein is calculated electrostatically. The energy of the ion charging in a protein consists of two physically different components, namely, the charging energy in the pre-existing field of protein dipoles and charges and the energy of the dielectric response of the medium. The former energy is determined by the electronic polarization of the protein (its optical dielectric permittivity), while the latter is determined by all kinds of polarization (static permittivity). Taking into account all the aforementioned peculiarities leads to reasonable agreement with the experiment when estimating the pK of certain groups in α-chymotrypsin. These calculations as well as experimental data (both our and taken form the literature (molecular dynamics)) point to the enhanced dielectric permittivity of the outer layers of proteins.