Hartmut Krienke

The solvation of ions in acetonitrile and acetone: A molecular Ornstein–Zernike study

The solvation of alkali and halide ions in acetonitrile and acetone has been investigated via the molecular Ornstein–Zernike theory using the hypernetted chain approximation. Theoretical Gibbs solvation energies and solvation numbers are compared with experiments and numerical simulations. The calculated single-ion solvation energies are used to check the hypotheses serving to split-up the measured solvation energies of salts into their single-ion components. The solvation structure around the ions is discussed in detail and shown to be strongly influenced by the solvent–solvent spatial correlations. The calculated interionic potentials of mean force are presented and used to compute ion-ion association constants which are compared with experiment. The influence of the Lennard-Jones parameters of the ions upon the calculated properties is emphasized.

Dielectric constants of liquid formamide, N-methylformamide and dimethylformamide via molecular Ornstein-Zernike theory

Abstract Kirkwood factors, yielding dielectric constants, are calculated from pair correlation functions, which are numerical solutions of the hypernetted-chain approximation of molecular Ornstein-Zernike (MOZ) theory. The combined influence of the molecular polarizability and the hydrogen bond strength is investigated. Using a reasonable diameter for the hydrogen size in the amide group, the MOZ Kirkwood factors and dielectric constants are in good agreement with the experimental values. This is explained by the statistical correlations between the orientations of two near molecules. This is consistent with hydrogen bonds, forming networks in formamide and chains in N-methylformamide.

Solution structure of NaNO3 in Water: Diffraction and molecular dynamics simulation study

The structure of a series of aqueous sodium nitrate solutions (1.9-7.6 M) was studied using a combination of experimental and theoretical methods. The results obtained from diffraction (X-ray, neutron) and molecular dynamics simulation have been compared and the capabilities and limitations of the methods in describing solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in description of hydration spheres of the sodium ion but do not yield detailed structural information on the anions hydration structure. Molecular dynamics simulations proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions, ion pair formation, and bulk structure of solutions.

Hydrogen bonded network properties in liquid formamide.

Molecular dynamics simulations have been performed for liquid formamide using two different types of potential model (OPLS, Cordeiro). The structural results obtained from simulation were compared to experimental (x-ray and neutron diffraction measurements) outcomes. A generally good agreement for both models examined has been found, but in the hydrogen bonded region (2.9 A) the Cordeiro model shows a slightly better fit. Besides the evaluation of partial radial distribution functions, orientational correlation functions and energy distribution functions, describing the hydrogen bonded structure, have been calculated based on the statistical analysis of configurations, resulting into a new insight in the clustering properties and topology of hydrogen bonded network. It has been shown that in liquid formamide exists a continuous hydrogen bonded network and from the analysis of the distribution of small rings revealed the ring size distribution in liquid formamide. Our study resulted that the ring size distribution of the hydrogen bonded liquid formamide shows a broad distribution with a maximum around 11. It has been found that the topology in formamide is significantly different than in water.

The solvation of ions in acetonitrile and acetone. II. Monte Carlo simulations using polarizable solvent models

Structural properties and energies of solvation are simulated for alkali and halide ions. The solvation structure is discussed in terms of various site–site distribution functions, of solvation numbers, and of orientational correlation functions of the solvent molecules around the ions. The solvent polarizability has notable effects which cannot be intuitively predicted. In particular, it is necessary to reproduce the experimental solvation numbers of small ions. The changes of solvation properties are investigated along the alkali and halide series. By comparing the solvation of ions in acetone to that in acetonitrile, it is shown that the spatial correlations among the solvent molecules around an ion result in a strong screening of the ion–solvent direct intermolecular potential and are essential to understand the changes in the solvation structures and energies between different solvents. The solvation properties derived from the simulations are compared to earlier predictions of the hypernetted chain ...

Structure and thermodynamics of liquid acetonitrile via Monte Carlo simulation and Ornstein-Zernike theories

Abstract Acetonitrile is modeled by a system of three sites, which are the centres of short-range Lennard-Jones interactions and which carry partial electric charges. Site-site distribution functions, internal energies, and dielectric constants are computed, using three theoretical methods: Monte Carlo simulation, molecular Ornstein-Zernike theory and site-site Ornstein-Zernike theory. The results of these calculations are compared and discussed in the light of the approximations inherent in the various approaches. The molecular pair distribution function is analysed to find pair configurations occuring in the liquid with a high probability. The effects of these configurations on the thermodynamic and structural properties are studied.

Modeling Tetraalkylammonium Halide Salts in Water: How Hydrophobic and Electrostatic Interactions Shape the Thermodynamic Properties

The explicit water molecular dynamics simulation was used to study tetramethylammonium and tetraethylammonium chloride and bromide solutions in water at 298 K. The outcome of the simulations in the form of various distribution functions was used to construct the solvent-averaged potentials between interacting molecules. In the next step, which involved the Ornstein-Zernike integral equation theory in the hypernetted chain approximation, these potentials were used to calculate the osmotic coefficients. We showed that this approach is able to explain the experimental results for the osmotic pressure of these salts.

A NEW BIASED MONTE-CARLO METHOD FOR COMPUTING COEFFICIENTS OF THE BRIDGE FUNCTIONS OF LIQUIDS

We describe an efficient biased Monte-Carlo method for calculating the diagrams appearing in the coefficients of the so-called bridge function of the integral equation theory of liquids. These diagrams represent multi-dimensional integrals of products of ‘bond’ functions of the intermolecular distances. The method rests on the generation of independent Markov chains and is well adapted to highly parallel computation. It can be used for systems with any pair potential. The feasibility and efficiency of the method are demonstrated for the second and third order coefficients of the bridge functions of fluids of hard and Lennard-Jones spheres. For these systems there are analytical expressions of the bridge function deduced from computer simulations to which we compare our bridge function approximations which include the second and third order coefficients with h as the bond function. Our new approximations of the bridge function are used in the closure of the Ornstein-Zernike relation. The obtained structura...

A singlet reference interation site model theory for solid/liquid interfaces Part II: Electrical double layers.

The previously established singlet reference interaction site model (SRISM) theory for the calculation of the fluid structure in the vicinity of a plane impenetrable interface is renormalized for the application to electrical double layers. In combination with the HNC and KH closures, the equations are solved numerically for a 1 M electrolyte solution adjacent to a charged wall with varying surface charge densities. The wall-solvent and wall-ion density distributions as well as the profiles of the electrical field and the electrical potential are compared to computer simulation results. Reasonable agreement is obtained.

Influence of the intermolecular electrostatic potential on properties of polar polarizable aprotic solvents

The liquid properties of models of acetonitrile, acetone and chloroform are calculated within the framework of the hypernetted chain approximation of the molecular Ornstein—Zernike theory. The shape of a molecule is described by a set of Lennard-Jones sites. Its electrostatic properties are modelled either by the first multipole moments up to the octopole or by partial charges, and by a point polarizability tensor. The multipole moments and the partial charges are computed by ab initio molecular orbital methods. In the liquid phase, the polarizability is taken into account by calculating an effective induced point dipole moment using a self-consistent mean-field approximation. While the Lennard-Jones part of the internal excess energy is nearly independent of the description of the electrostatic interaction and of the polarizability, the electrostatic part and the dielectric constant change notably. The models with the partial charges lead to dielectric constants which are in good agreement with the exper...