M. G. Kiselev

Prediction of cosolvent effect on solvation free energies and solubilities of organic compounds in supercritical carbon dioxide based on fully atomistic molecular simulations.

The solubility of organic compounds in supercritical fluids can be dramatically affected by addition of a suitable cosolvent (entrainer) at small concentrations. This makes the screening of the best-suited cosolvent an important task for the supercritical technology. The present study aims to improve our fundamental understanding of solvation in supercritical CO2 with cosolvents. We address the following questions: (1) How does the solvation free energy depend on the chemical class of an organic solute and the chemical nature of co-solvents? (2) Which intermolecular interactions determine the effect of a cosolvent on the solubility of organic compounds? We performed extensive calculations of solvation free energies of monofunctional organic molecules at infinite dilution in supercritical media by the Bennetts acceptance ratio method based on fully atomistic molecular dynamics sampling. Sixteen monofunctional organic molecules were solvated in pure sc-CO2 and sc-CO2 with addition of 6 molar % of cosolvents of different chemical nature: ethanol, acetone, and n-hexane. Cosolvent-induced solubility enhancement (CISE) factors were also calculated. It was found that formation of significant number of hydrogen bonds between a solute and cosolvent molecules leads to a profound solubility enhancement. The cosolvent effect is proportional to the number of hydrogen bonds. When polar cosolvents do not form hydrogen bonds with solutes, the CISE correlates with the dipole moment of solute molecules. However, the electrostatic interactions have a small impact on the solubility enhancement compared to hydrogen bonding. Addition of a nonpolar cosolvent, n-hexane, has a very little effect on the solvation Gibbs free energy of studied small organic molecules. The observed trends were discussed in line with available experimental data.

Structure of methanol-methanol associates in dilute methanol-water mixtures from molecular dynamics simulation

Abstract Molecular dynamics simulations have been performed for 8 methanol-water solutions using rigid and flexible potential models. The heat capacity, the radial distribution functions and potential mean force obtained by MD simulations were compared to previous simulations and experimental results. Special attention has been paid to the anomalous behaviour of the heat capacity of dilute aqueous solutions of methanol. This behaviour can be attributed to a cooperative effect resulting from methanol-methanol associations.

A statistical theory of cosolvent-induced coil-globule transitions in dilute polymer solution

We present a statistical model of a dilute polymer solution in good solvent in the presence of low-molecular weight cosolvent. We investigate the conformational changes of the polymer induced by a change of the cosolvent concentration and the type of interaction between the cosolvent and the polymer. We describe the polymer in solution by the Edwards model, where the partition function of the polymer chain with a fixed radius of gyration is described in the framework of the mean-field approximation. The contributions of polymer-cosolvent and the cosolvent-cosolvent interactions in the total free energy are treated also within the mean-field approximation. For convenience we separate the system volume on two parts: the volume occupied by the polymer chain expressed through its gyration volume and the bulk solution. Considering the equilibrium between the two subvolumes we obtain the total free energy of the solution as a function of radius of gyration and the cosolvent concentration within gyration volume. After minimization of the total free energy with respect to its arguments we obtain a system of coupled equations with respect to the radius of gyration of the polymer chain and the cosolvent concentration within the gyration volume. Varying the interaction strength between polymer and cosolvent we show that the polymer collapse occurs in two cases--either when the interaction between polymer and cosolvent is repulsive or when the interaction is attractive. The reported effects could be relevant for different disciplines where conformational transitions of macromolecules in the presence of a cosolvent are of interest, in particular in biology, chemistry, and material science.

Detailed insight into the hydrogen bonding interactions in acetone–methanol mixtures. A molecular dynamics simulation and Voronoi polyhedra analysis study

Voronoi polyhedra (VP) analysis of mixtures of acetone and methanol is reported on the basis of molecular dynamics computer simulations, performed at 300 K and 1 bar. The composition of the systems investigated covers the entire range from neat acetone to neat methanol. Distribution of the volume, reciprocal volume and asphericity parameter of the VP as well as that of the area of the individual VP faces and of the radius of the empty voids located between the molecules are calculated. To investigate the tendency of the like molecules to self-associate the analyses are repeated by disregarding one of the two components. The self-aggregates of the disregarded component thus turn into large empty voids, which are easily detectable in VP analysis. The obtained results reveal that both molecules show self-association, but this behavior is considerably stronger among the acetone than among the methanol molecules. The strongest self-association of the acetone and methanol molecules is found in their mole fraction ranges of 02-0.5 and 0.5-0.6, respectively. The caging effect around the methanol molecules is found to be stronger than around acetones. Finally, the local environment of the acetone molecules turns out to be more spherical than that of the methanols, not only in the respective neat liquids, but also in their mixtures.

Investigation of the Local Structure in Sub and Supercritical Ammonia Using the Nearest Neighbor Approach: A Molecular Dynamics Analysis

Molecular dynamics simulations of ammonia were performed in the (N,P,T) ensemble along the isobar 135 bar and in the temperature range between 250 and 500 K that encompasses the sub and supercritical states of ammonia. Six simple interaction potential models (Lennard-Jones pair potential between the atomic sites, plus a Coulomb interaction between atomic partial charges) of ammonia reported in the literature were analyzed. Liquid-gas coexistence curve, critical temperature, and structural data (radial distribution functions) have been calculated for all models and compared with the corresponding experimental data. After choosing the appropriate potential model, we have investigated the local structure by analyzing the nearest neighbor radial, mutual orientation, and interaction energy distributions. The change in the local structure was traced back to the change of the nonlinear behavior (which is more pronounced at low temperatures) of the average distance between a reference ammonia molecule and its subsequent nearest neighbor. Our results suggest to use the position of the maximum in the fluctuation of the average distance to define the border of the first solvation shell (particularly at high temperature when the minimum of the radial distribution is not well-defined). Indeed, the effect of the temperature on the position of this maximum shows clearly that the spatial extent of the solvation shell increases with a concomitant decrease of the involved number of ammonia molecules. Furthermore, our results show that the signature of the hydrogen bonding is mainly observed for temperature below 300 K. This signature is quantified by a short distance contribution to the closest radial nearest neighbor distribution, by a strong mutual orientation (defined by the angles between the axis joining the nitrogen atoms and the molecular axes) and by a strong attractive character of the total interaction energy.

A modified Poisson-Boltzmann theory: Effects of co-solvent polarizability

In this paper within a field-theoretical approach taking into account explicitly a co-solvent with a nonzero dipole and a polarizability tensor, we derive a modified Poisson-Boltzmann equation. Applying the modified Poisson-Boltzmann equation, we formulate a generalized Gouy-Chapman theory for the case when an electrolyte solution is mixed with a polar co-solvent having a large polarizability. We show that an increase of the co-solvent concentration as well as the co-solvent polarizability lead to a significant increase of differential capacitance at sufficiently high surface potentials of the electrode, whereas the profile of the electrostatic potential becomes considerably more long-ranged. On the contrary, an increase in the permanent dipole of the co-solvent only weakly affects the differential capacitance.

Heterogeneity of the Local Structure in Sub- and Supercritical Ammonia: A Voronoi Polyhedra Analysis

We report results of molecular dynamics simulations and detailed analysis of the local structure of sub- and supercritical ammonia in the range of temperature between 250 and 500 K along the 135 bar isobar. This analysis is based on the behavior of distributions of metric and topological properties of the Voronoi polyhedra (VP). We show that by increasing the temperature, the volume, surface, and face area distributions of the Voronoi polyhedra as well as the vacancy radius distribution broaden, particularly near the temperature T(α), where the calculated thermal expansion coefficient has its maximum. Furthermore, the rate of increase of the corresponding mean values and fluctuations increases drastically when approaching T(α). This behavior clearly indicates that the local structure, as described by the VP, becomes increasingly heterogeneous upon approaching this temperature. This heterogeneous distribution of ammonia molecules is traced back to the increase of the large voids with increasing temperature, and is also clearly seen in the behavior of the fluctuation of the local density, as measured by the VP. More interestingly, the maximum in the heterogeneity coincides with the maximum of the fluctuation in the density of the VP.

Structure of orthophosphoric Acid-N,N-dimethylformamide complexes

Ab initio calculations weere used to obtain the characteristics of the most stable configurations of the (H3PO4)2, DMFA-H3PO4, and DMFA-(H3PO4)2 complexes in a vacuum. The changes in the geometric parameters in this series of complexes were analyzed and the energies of intermolecular interactions were estimated. The DMFA-(H3PO4)2 complex was found to be most stable. It was established that the hydrogen bond between the DMFA and H3PO4 molecules is formed with the participation of both lone pairs of the oxygen atom of the DMFA molecule.

On the theory of electric double layer with explicit account of a polarizable co-solvent

We present a continuation of our theoretical research into the influence of co-solvent polarizability on a differential capacitance of the electric double layer. We formulate a modified Poisson-Boltzmann theory, using the formalism of density functional approach on the level of local density approximation taking into account the electrostatic interactions of ions and co-solvent molecules as well as their excluded volume. We derive the modified Poisson-Boltzmann equation, considering the three-component symmetric lattice gas model as a reference system and minimizing the grand thermodynamic potential with respect to the electrostatic potential. We apply present modified Poisson-Boltzmann equation to the electric double layer theory, showing that accounting for the excluded volume of co-solvent molecules and ions slightly changes the main result of our previous simplified theory. Namely, in the case of small co-solvent polarizability with its increase under the enough small surface potentials of electrode, the differential capacitance undergoes the significant growth. Oppositely, when the surface potential exceeds some threshold value (which is slightly smaller than the saturation potential), the increase in the co-solvent polarizability results in a differential capacitance decrease. However, when the co-solvent polarizability exceeds some threshold value, its increase generates a considerable enhancement of the differential capacitance in a wide range of surface potentials. We demonstrate that two qualitatively different behaviors of the differential capacitance are related to the depletion and adsorption of co-solvent molecules at the charged electrode. We show that an additive of the strongly polarizable co-solvent to an electrolyte solution can shift significantly the saturation potential in two qualitatively different manners. Namely, a small additive of strongly polarizable co-solvent results in a shift of saturation potential to higher surface potentials. On the contrary, a sufficiently large additive of co-solvent shifts the saturation potential to lower surface potentials. We obtain that an increase in the co-solvent polarizability makes the electrostatic potential profile longer-ranged. However, increase in the co-solvent concentration in the bulk leads to non-monotonic behavior of the electrostatic potential profile. An increase in the co-solvent concentration in the bulk at its sufficiently small values makes the electrostatic potential profile longer-ranged. Oppositely, when the co-solvent concentration in the bulk exceeds some threshold value, its further increase leads to decrease in electrostatic potential at all distances from the electrode.

Quantum-chemical studies of dimethylformamide 1 : 1 complexes with phosphoric acid

The structures of two phosphoric acid conformations, dimethylformamide (DMFA), four protonated DMFA forms, and nine DMFA-H3PO4 complexes in which the proton acceptor is a oxygen or nitrogen atom of the DMFA molecule are optimized by DFT/B3LYP using the 6-31++G(d, p) basis set. The structural changes in DMFA that occur upon its protonation are discussed. The stabilization energy and transferred charge values upon the formation of a hydrogen bond are calculated for all of the studied complexes by means of NBO analysis. The potential energy surface is scanned to study the possibility of proton transfer.