D. L. Gurina

Structure of solvates of o-hydroxybenzoic acid in supercritical CO2-cosolvent media, according to molecular dynamics data

Three-component supercritical carbon dioxide-cosolvent (methanol, ethanol, water)-o-hydroxybenzoic acid (o-HBA) mixtures at a density of 0.7 g/cm3 and temperatures of 318 and 348 K are simulated by means of molecular dynamics. The solvate structures are investigated. It is shown that the solvation mechanism of o-HBA (particularly the o-HBA molecule forming a stable solvate complex with one molecule of a cosolvent via a hydrogen bond through the carboxyl group) does not depend on the temperature or the cosolvent. It is noted that the form of the cosolvent in a supercritical fluid varies: alcohols are distributed in the bulk in the form of monomers and hydrogen-bonded dimers, and water molecules tend to form microclusters along with chained and spatially branched structures by means of hydrogen bonds. It is established that the local molar fraction of cosolvent around the solvate complexes grows. It is concluded that the solvation of o-HBA is determined by the behavior of cosolvent in media of supercritical CO2.

Hydrogen-bonded clusters and solvate structures in the supercritical CO2-water-o-hydroxybenzoic acid system: the car-parrinello molecular dynamics

Hydrogen-bonded clusters and solvate structures formed by o-hydroxybenzoic acid (o-HBA) and water in supercritical CO2 were studied (T = 318 K, 348 K, ρ = 0.7 g/cm3). The atom-atom radial distribution functions, coordination numbers, average numbers of hydrogen bonds for individual atomic groups, and power spectrum were calculated by the Car-Parrinello molecular dynamics. Despite the high polarity of the cosolvent, the hydroxyl group of o-HBA predominantly forms intramolecular hydrogen bond, while hydrogen bonds with water involve only the atoms of carboxyl groups. The temperature effect on the stability of these bonds showed itself in different ways. The intermolecular interactions of o-HBA with carbon dioxide were found to be weaker than those with water. It was established that the Lewis acid-Lewis base interactions between CO2 and the hydroxyl group of the solute increase with increasing temperature. Instantaneous configurations illustrating the temperature effects on the molecular structures were obtained.

Structural features of binary mixtures of supercritical CO2 with polar entrainers by molecular dynamics simulation

Computer simulations of supercritical carbon dioxide and its mixtures with polar cosolvents: water, methanol, and ethanol (concentration, 0.125 mole fractions) at T = 318 K and ρ = 0.7 g/cm3 are performed. Atom-atom radial distribution functions are calculated by classical molecular dynamics, while the probability distributions of relative orientation of CO2 molecules in the first and second coordination spheres describing the geometry of the nearest environment of CO2 molecules and the trajectories of cosolvent molecules are found using Car-Parrinello molecular dynamics. Based on the latter, the conclusions regarding structure and interactions of polar entrainers in their mixtures with supercritical CO2 are made. It is shown that the microstructure of carbon dioxide varies only slightly upon the introduction of cosolvents.

Formation of molecular complexes of salicylic acid, acetylsalicylic acid, and methyl salicylate in a mixture of supercritical carbon dioxide with a polar cosolvent

The solvate structures formed by salicylic acid, acetylsalicylic acid, and methyl salicylate in supercritical (SC) carbon dioxide with a polar cosolvent (methanol, 0.03 mole fractions) at a density of 0.7 g/cm3 and a temperature of 318 K were studied by the molecular dynamics method. Salicylic and acetylsalicylic acids were found to form highly stable hydrogen-bonded complexes with methanol via the hydrogen atom of the carboxyl group. For methyl salicylate in which the carboxyl hydrogen is substituted by a methyl radical, the formation of stable hydrogen bonds with methanol was not revealed. The contribution of other functional groups of the solute to the interactions with the cosolvent was much smaller. An analysis of correlations between the obtained data and the literature data on the cosolvent effect on the solubility of the compounds in SC CO2 showed that the dissolving ability of SC CO2 with respect to a polar organic substance in the presence of a cosolvent increased only when stable hydrogen-bonded complexes are formed between this substance and the cosolvent.

Numerical simulation of the solvate structures of acetylsalicylic acid in supercritical carbon dioxide containing polar co-solvents

Hydrogen-bonded complexes of acetylsalicylic acid with polar co-solvents in supercritical carbon dioxide, modified by methanol, ethanol, and acetone of 0.03 mole fraction concentration, are studied by numerical methods of classical molecular dynamics simulation and quantum chemical calculations. The structure, energy of formation, and lifetime of hydrogen-bonded complexes are determined, along with their temperature dependences (from 318 to 388 K at constant density of 0.7 g cm−3). It is shown that the hydrogen bonds between acetylsalicylic acid and methanol are most stable at 318 K and are characterized by the highest value of absolute energy. At higher supercritical temperatures, however, the longest lifetime is observed for acetylsalicylic acid–ethanol complexes. These results correlate with the known literature experimental data showing that the maximum solubility of acetylsalicylic acid at density values close to those considered in this work and at temperatures of 318 and 328 K is achieved when using methanol and ethanol as co-solvents, respectively.

Solvation of o-hydroxybenzoic acid in pure and modified supercritical carbon dioxide, according to numerical modeling data

The dissolution of an elementary fragment of crystal structure (an o-hydroxybenzoic acid (o-HBA) dimer) in both pure and modified supercritical (SC) carbon dioxide by adding methanol (molar fraction, 0.035) at T = 318 K, ρ = 0.7 g/cm3 is simulated. Features of the solvation mechanism in each solvent are revealed. The solvation of o-HBA in pure SC CO2 is shown to occur via electron donor-acceptor interactions. o-HBA forms a solvate complex in modified SC CO2 through hydrogen bonds between the carboxyl group and methanol. The hydroxyl group of o-HBA participates in the formation of an intramolecular hydrogen bond, and not in interactions with the solvent. It is concluded that the o-HBA-methanol complex is a stable molecular structure, and its lifetime is one order of magnitude higher than those of other hydrogen bonds in fluids.

Structure of hydrogen-bonded associates in supercritical water under low and high pressures

The character and structural features of hydrogen-bonded associates in sub- and supercritical water are studied by analyzing distributions of the dipole moments of water molecules at P = 40, 80, and 100 MPa and T = 373–773 K, calculated using Car-Parrinello molecular dynamics. The main types of hydrogen-bonded structures and their changes upon isobaric heating are determined. It is shown that clusters with tetrahedral configurations exist in supercritical water only under high pressure.

Hydrogen bond and dipole moment in sub- and supercritical water close to the saturation curve

The Car-Parrinello molecular dynamics method was used to determine the distributions of water dipole moments under normal conditions, at the critical point, and in six thermodynamic states in sub- and supercritical phase diagram regions. Dipole moment changes along the saturation curve, the 650 K isotherm, and the 30 MPa isobar were analyzed.

Effect of pressure on the structure and dynamics of hydrogen bonds in ethylene glycol–water mixtures: Numerical simulation data

Water−ethylene glycol mixtures containing from 0.002 to 0.998 mole fractions of ethylene glycol at T = 298.15 K and P = 0.1 and 100 MPa are simulated by means of classical molecular dynamics. Such structural and dynamic characteristics of hydrogen bonds as the average number and lifetime, along with the distribution of molecules over the number of hydrogen bonds, are calculated; their changes are analyzed, depending on the mixture’s composition and pressure. It is shown that the components are characterized by a high degree of interpenetration and form a uniform infinite hydrogen-bonded cluster over the range of concentrations. It is found that the higher the concentration of ethylene glycol, the greater the stability of all hydrogen bonds. It is concluded that an increase in pressure lowers the number of hydrogen bonds, while the average lifetime of the remaining hydrogen bonds grows.

Calculating the radial distribution functions of supercritical methanol by means of Car-Parrinello and classical molecular dynamics

Radial distribution functions and the average number of hydrogen bonds per methanol molecule under standard, subcritical, and supercritical conditions are obtained via classical molecular dynamics and Car-Parrinello nonempirical molecular dynamics. It is shown that independent methods of modeling yield close results. It is noted that the calculated radial distribution functions agree well with the experimental data only at T = 298 K and P = 0.1 MPa, while at high temperatures and pressures, considerable divergence from the experimental functions known from the literature is observed. It is concluded that both modeling methods reproduce the degree of hydrogen bonding in methanol and its variations depending on the state parameters and correspond closely to the experimental results.