Ignat Yu. Shilov

Long range order and hydrogen bonding in liquid methanol: A Monte Carlo simulation

Abstract A Monte Carlo simulation of liquid methanol was performed in NVT ensemble at 298 K using a cubic simulation box containing 500 molecules. Long-range correlations in the liquid are discussed on the basis of site–site radial distribution functions. Hydrogen bonding and topological structure of the methanol aggregates were evaluated in detail, namely the number of linked molecules, formation of branches and cyclic structures. The necessity of larger simulation boxes for a full structural description and thermodynamic characterization of hydrogen-bonded liquids is clearly established by the results.

Supramolecular structure and physicochemical properties of the mixture tetrachloromethane-methanol

Abstract The supramolecular structure and macroscopic properties over the whole concentration range and wide temperature interval of the mixture tetrachloromethane-methanol was studied. A structural model including the chainlike alcohol aggregates of various length and cyclic tetramers due to H-bonding OH…O as well as complexes of alcohol aggregates with a solvent molecule due to H-bonding OH…Cl was proved to describe thermodynamic excess functions (Gibbs energy, enthalpy), dielectric permittivity and Rayleigh light scattering ratios of the mixture tetrachloromethane-methanol. Thermodynamic (equilibrium constants, enthalpies and entropies of association processes) and structural parameters of the aggregates were obtained. Integral and differential parameters of aggregation in mixtures were evaluated. The long-range supramolecular ordering due to specific bonding was established. The unified analysis of the thermodynamic, dielectric and optic properties in terms of generalized quasichemical models was carried out. Supramolecular structure was found to play a significant role in determining the physicochemical properties of the mixture tetrachloromethane-methanol.

Molecular structure and physicochemical properties of acetone–chloroform mixtures

The molecular structure of acetone–chloroform mixtures has been studied. Thermodynamic mixing functions, permittivity, and Rayleigh ratios of isotropic and anisotropic light scattering are considered. A molecular model including the formation of the 1 : 1 and 1 : 2 molecular complexes is substantiated. A set of thermodynamic and structural parameters of the complex formation was obtained. The thermodynamic parameters of complex formation defined by the minimisation procedure for excess Gibbs energy of mixture was applied for calculation of excess enthalpy and entropy as well as dielectric and optical properties of mixtures. For the first time the unified representation and analysis of the thermodynamic, dielectric and optical properties of acetone–chloroform mixtures in terms of generalized quasichemical models on the molecular-structural level was realised.

The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye–Hückel Theory

The Debye-Hückel theory has been extended to allow for arbitrary concentration dependence of the electrolyte solution static permittivity. The theory follows the lines advanced by Erich Hückel ( Hückel, E. Phys. Z. 1925, 26, 93) but gives rise to more general and lucid results. New theoretical expressions have been obtained for the excess free energy of solution, activity coefficient of water and mean ionic activity coefficient. The thermodynamic functions contain two terms representing interionic interactions and ion-water (solvation) interactions. The theory has been applied to calculate the activity coefficients of components in the aqueous solutions of alkali metal chlorides from LiCl to CsCl at ambient conditions making use of permittivities taken from experimental dielectric relaxation studies. Calculations without parameter adjustment have demonstrated a semiquantitative agreement with experimental data, reproducing both the nonmonotonic concentration dependence of the activity coefficients and the ordering of activity coefficients for the salts with different cations. A good agreement with experimental data is obtained for the aqueous solutions of LiCl in the concentration range up to 10 mol/kg. The nonmonotonic concentration dependence of activity coefficients is explained as a result of a balance between the effect of interionic interactions and the solvation contribution which appears quite naturally in the framework of the Debye-Hückel approach after incorporation of variable permittivity of solution.

Modeling of supramolecular structure and dielectric properties of butanols from melting point to supercritical state.

The supramolecular structure and dielectric properties of pure fluid butanols have been modeled in the temperature range from the melting point to the vicinity of the critical point using the quasichemical model of nonideal associated solution (QCNAS). The model of supramolecular structure takes into account both chain-like and cyclic aggregates. Dielectric permittivity and dipole correlation factor of n-, sec-, and tert-butanol are calculated in the temperature intervals 191-553, 166-533, and 298-503 K, respectively, using parameters obtained earlier for ambient conditions. The thermodynamic and structural parameters of supramolecular aggregation are reported. Size and structure distribution functions of aggregates are evaluated in the entire temperature range. Mean degree of aggregation of n-butanol decreases from hundreds at low temperatures to about 1.3 around critical point. The structure of n-butanol and s-butanol is characterized mainly by chain-like aggregation. tert-Butanols degree of aggregation is about 4 at ambient conditions and falls to 1.4 approaching the critical point. The fractions of molecules in chain-like and cyclic aggregates are comparable for tert-butanol. This conclusion refutes the frequently expressed opinion that tert-butanol consists mostly of cyclic species.

A quasichemical model for comb-like aggregation of monohydric alcohols: supramolecular structure and macroscopic properties

A new quasichemical model of the supramolecular structure of the liquid consisting of chain-like and comb-like hydrogen-bonded aggregates with branches of unit length has been developed. Analytical expressions for structural characteristics (size and structure distributions of aggregates), dielectric (permittivity, dipole correlation factor), optic (mean molecular anisotropy in liquid, optic correlation factor, anisotropic Rayleigh light scattering ratio) and thermodynamic (energy of intermolecular interactions in liquid, vaporisation enthalpy) properties of the liquid as functions of structural and thermodynamic aggregation parameters have been derived. The analytical model developed creates the foundation for studying the role of branched aggregates in the supramolecular structure of liquids and various macroscopic properties determined by different molecular parameters. The model is applied to pure methanol at ambient conditions. The dependence of structural characteristics of liquid, dipole correlation factor, permittivity, mean molecular anisotropy in liquid, vaporisation enthalpy on the equilibrium constants of chain-like and branched aggregation is studied. The results are compared with experimental data and computer simulation. The influence of branching degree of aggregates on different physicochemical properties of liquid is revealed and discussed.

Molecular dynamics simulation of dielectric constant and cluster structure of liquid methanol: the role of cluster–cluster dipole correlations

A molecular dynamics simulation of liquid methanol at ambient conditions with two different three-site potential models was performed and the evaluated dielectric constant was discussed in the light of the cluster structure of the liquid. The distribution of the pair interaction energy of molecules and the cluster size distribution were calculated. An aggregation contribution to dielectric constant was defined and calculated as a function of the threshold H-bond energy using energetic criterion of H-bond. The structural information on dipole–dipole correlations of molecules incorporated in the size and structure distribution of aggregates proved to cover about 80% of the calculated dielectric constant of methanol. The other 20% should be attributed to the cluster–cluster dipole correlations.