Prediction and experimental verification of the salt effect on the vapour–liquid equilibrium of water–ethanol–2-propanol mixture

Experimental vapour–liquid equilibria of water–ethanol–2-propanol saturated with NaNO₃, NaCl, KCl and containing 0.05 mol CH₃COOK/mol total solvent compared well with those predicted by Tan–Wilson and Tan–non-random two liquid (NRTL) models for multicomponent solvent–solute mixture using a set of solvent–solvent interaction parameters obtained from the regression of the vapour–liquid equilibrium of the solvent mixture without the dissolved solute and a set of solute–solvent interaction parameters calculated from the bubble points of the individual solvent components saturated or containing the same molar ratio of solute/total solvents as the mixture. The results also showed that a solvent component i is salted-in or out of the liquid phase relatively more than solvent component j would depend on whether A_sj/A_si (Tan–Wilson model) or exp(τ_is−τ_js) (Tan–NRTL model) is less or greater than 1. This is consistent with earlier publications on the effect of dissolved solutes (electrolytes and non-electrolytes) on the binary solvents mixtures. These findings confirmed that Tan–Wilson and Tan–NRTL models for multicomponent solvent–solute system can provide an accurate and rapid screening of electrolytes and non-electrolytes for their suitability in facilitating solvent separation by salt distillation of ternary solvent mixtures simply by determining the relative ratios of the solute–solvent interaction parameters from the respective bubble points of the solvent components containing the dissolved solute. The results also suggest that this may also be extended to other multicomponent solvent mixtures.     

Solvent–solute and solute–solute interactions from NMR in nematic phases

Abstract

The use of NMR spectroscopy of molecules oriented in liquid-crystalline media to study solvent–solute and solute–solute interactions in π-systems such as benzene–chloroform and in charge transfer complexes, for example pyridineiodine, is illustrated. Changes in molecular order and chemical shifts as a result of complexation are employed in such studies. The extraordinary symmetry of C₆₀ has also been investigated by using a mixture of liquid crystals of opposite diamagnetic anisotropies indicating, thereby, negligible solvent–solute/solute–solute interactions.     

Energy components of aqueous solution: Insight from hybrid QM/MM simulations using a polarizable solvent model

An energy decomposition method is present in statistical Monte Carlo simulations of aqueous solutions of a series of organic solutes, making use of a hybrid quantum mechanical and polarizable molecular mechanical (QM/MM-PIPF) approach. In the hybrid QM/MM-PIPF method, the mutual solute–solvent polarization effect is specifically considered through a coupled iterative procedure that ensures the convergence of solvent induced dipoles and the solute wave function. It should be noted that the method is an approximate approach without specifically considering the electronic correlation effect between solute and solvent electrons, and energetic results have not been verified by free energy calculations. Nevertheless, the energy decomposition analysis provides insight into the details of the molecular polarization effect. Qualitative trends of the energy components from such analyses provide guidance in the understanding of the nature of intermolecular interactions in biomolecular systems, whereas quantitative results on specific terms may be utilized to derive empirical, yet computationally more efficient, force fields. Polarization effects are found to be significant, which contribute 10% to 23% to the total solute–solvent interaction energies. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 :1061–1071, 1997     

Modeling absorption spectra of molecules in solution

The presence of solvent tunes many properties of a molecule, such as its ground and excited state geometry, dipole moment, excitation energy, and absorption spectrum. Because the energy of the system will vary depending on the solvent configuration, explicit solute–solvent interactions are key to understanding solution-phase reactivity and spectroscopy, simulating accurate inhomogeneous broadening, and predicting absorption spectra. In this tutorial review, we give an overview of factors to consider when modeling excited states of molecules interacting with explicit solvent. We provide practical guidelines for sampling solute–solvent configurations, choosing a solvent model, performing the excited state electronic structure calculations, and computing spectral lineshapes. We also present our recent results combining the vertical excitation energies computed from an ensemble of solute–solvent configurations with the vibronic spectra obtained from a small number of frozen solvent configurations, resulting in improved simulation of absorption spectra for molecules in solution.     

Use of Masson's and Jones–Dole equations to study different types of interactions of three pharmacologically important drugs in ethanol

The volumetric and viscometric study of three allopathic drugs (sodium valporate, benzalkonium chloride, and losartan potassium) in ethanol solvent is reported here. This study was carried out at four different temperatures that is, from 288.15 to 318.15 K. The accurately measured density values were used to calculate partial molar volume at infinite dilution, solute–solute interaction parameter, Hepler's constant, partial molar expansivity constant, and isobaric thermal expansion coefficient. The viscosity measurements were carried out for the calculation of constants of Jones–Dole equation and to calculate different thermodynamic parameters of viscous flow which include standard free energy change, standard enthalpy change, and standard entropy change of viscous flow. All these viscometric and volumetric parameters are useful for understanding the different types of interactions of drugs in solution and to study the drug action in body. The results of both volumetric and viscometric studies showed that all drugs had structure promoting effect on solvent, existing of strong solute–solvent interaction, and very weak solute–solute interaction. For all these drugs, solvophobic interaction was found to be dominant over electrostriction. Viscometric studies also showed the existing of stronger solute–solvent interaction in ground state as compared to that in transition state.     

On the separation mechanism of gel permeation chromatography

The separation mechanism of gel permeation chromatography was investigated by static experiments. It was found that the solute molecule is excluded from part of the inner space of the gel particle, which is entirely available to the solvent molecule. The excluded volume, ΔV, increases with increase of molecular size of the solute. A linear relationship was observed between the ΔV and the logarithm of molecular size. Excluded volume was found to be independent of solute concentration as was expected. Absorption effect was negligible with polystyrene gel. However, a strong effect was observed between acids and polydextran gel. The possibility of using absorption effects to increase the separability of GPC is suggested.     

Nonstationary diffusion of polystyrenes through a cellophane membrane

Diffusion of five polystyrene fractions at various concentrations in toluene through cellophane membranes has been observed. The results have been used to calculate friction coefficients between solvent and solute, and between solute and membrane. The calculation requires measurement of the diffusion coefficient and the reflection coefficient of the solute, of the permeability for the solvent, of the pore volume of the membrane, and of the partition coefficient of the solute between membrane and solvent. By comparing the friction coefficient between solvent and solute in the membrane with this coefficient in free solution, the tortuosity factor and the pore diameter of the membrane can be estimated. The dependence of the friction coefficients on molecular weight M₂ of the solute is determined. For large values of M₂, the friction between solute and solvent is the determining factor. The friction coefficient between solute and solvent increases more strongly with M₂ in the membrane than in free solution owing to an entrance effect for the permeating solute at the interface.     

Modeling solvent effects in molecular dynamics simulations of proteins

A series of computer simulations has been carried out on bovine pancreatic trypsin inhibitor using various models to mimic the effects of explicit bulk solvent on the structure of the protein. The solvent properties included are the polarization of the solute by the polar bulk solvent and the restraining effect on the motional freedom of the solute due to frictional drag at the solvent–protein surface interface. The former has been included by using a distance–dependent dielectric permittivity to screen the electrostatic interactions, whereas the latter is simulated by adding a limited number of solvent molecules near the protein surface. To achieve the proper mobility of the water molecules, their motion was restrained by adding a harmonic restraining force. It was found that a very small force constant was sufficient to model the static and dynamical behavior of the fully solvated solute, but that it was necessary to include enough explicit waters to occupy the first solvation shell. © 1992 John Wiley & Sons, Inc.     

Acceleration of convergence in Monte Carlo simulations of aqueous solutions using the metropolis algorithm. Hydrophobic hydration of methane

Convergence problems encountered in the computer simulations of aqueous solutions are discussed. Solute–solvent radial distribution functions are shown to converge very poorly when the standard Metropolis Monte Carlo procedure is applied. To overcome this difficulty, several modifications are made in the Metropolis method. Optimum maximum step sizes are determined for simulations of liquid water. A scheme is employed for preferential sampling of both the solvent and the solute molecules. To test these modifications, a simulation is carried out for pure liquid water, treating a single water molecule as a “solute.” The convergence of the radial distribution functions is found to be accelerated significantly. A further test is made by simulating an aqueous solution of methane, consisting of one methane molecule (using the EPEN /2 potential for methane–water interactions) and 124 water molecules (using the MCY potential for water–water interactions). Again, the convergence of solute–solvent radial distribution functions is found to be accelerated. The computation of partial molar thermodynamic quantities, however, still suffers from convergence difficulties. This problem is discussed in detail. The EPEN /2 potential is found to yield structural and thermodynamic features of hydrophobic hydration that are consistent with available experimental and theoretical results for aqueous solutions of methane.     

Vibrational energy relaxation of a diatomic molecule in a room-temperature ionic liquid

Vibrational energy relaxation (VER) dynamics of a diatomic solute in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are studied via equilibrium and nonequilibrium molecular dynamics simulations. The time scale for VER is found to decrease markedly with the increasing solute dipole moment, consonant with many previous studies in polar solvents. A detailed analysis of nonequilibrium results shows that for a dipolar solute, dissipation of an excess solute vibrational energy occurs almost exclusively via the Lennard-Jones interactions between the solute and solvent, while an oscillatory energy exchange between the two is mainly controlled by their electrostatic interactions. Regardless of the anharmonicity of the solute vibrational potential, VER becomes accelerated as the initial vibrational energy increases. This is attributed primarily to the enhancement in variations of the solvent force on the solute bond, induced by large-amplitude solute vibrations. One interesting finding is that if a time variable scaled with the initial excitation energy is employed, dissipation dynamics of the excess vibrational energy of the dipolar solute tend to show a universal behavior irrespective of its initial vibrational state. Comparison with water and acetonitrile shows that overall characteristics of VER in EMI(+)PF(6) (-) are similar to those in acetonitrile, while relaxation in water is much faster than the two. It is also found that the Landau-Teller theory predictions for VER time scale obtained via equilibrium simulations of the solvent force autocorrelation function are in reasonable agreement with the nonequilibrium results.     

Effet de Solvant en RMN du Carbone-13. III—Cas d'un Soluté Polaire: l'Acétone dans Divers Solvants et en Phase Gazeuse

The chemical shifts of acetone carbons are measured in the gas phase and in nineteen solvents, thus allowing the separation of the screening constant terms arising from the different kinds of solute–solvent interactions. It is shown that for methyl carbons the van der Waals term σ_w, interpreted using Rummens's theory, is more important than the specific solute–solvent interaction term σ_H. In contrast, for the carbonyl carbon, the term σ_H, which is eight times greater than for the methyl carbons, dominates when dipole–dipole interactions and hydrogen bonding occur. No evidence for an electric field term proportional to the Onsager reaction field is found. But when there are dipole–dipole interactions, σ_H is proportional to the electric field of the dipole of a solvent molecule interacting with the dipole of a solute molecule, the two dipoles being antiparallel. The variation of σ_H with the acetone concentration in a non-associating solvent is interpreted as a consequence of the displacement of the acetone in self-association equilibrium, leading to the determination of the equilibrium constant.     

Hybrid supermolecule-polarizable continuum approach to solvation: Application to the mechanism of the Stevens rearrangement

Semiempirical molecular orbital theory has been used to study the effects of solvation by acetonitrile on the Stevens rearrangement of methylammonium formylmethylide to 2-aminopropanal. Three methods of solvation have been used to investigate both the electrostatic and specific solvent–solute effects of solvation: a supermolecule calculation involving the complete geometry optimization of up to six solvent molecules about the solute, the conductor-like screening model (COSMO) polarizable continuum method which allows for geometry optimization of the solute in a solvent defined by its dielectric constant, and a hybrid method in which up to five solvent molecules are incorporated inside the solute cavity and complete geometry optimization of the complex is carried out within the polarizable continuum. A comparison of the calculated geometries, rearrangement activation energies, and enthalpies of solvation from these approaches is presented, and the explicit versus bulk solvation effects are discussed. The overall effect of all methods for incorporating solvation effects is that the radical pair pathway is perferred over the concerted mechanism. © 1996 by John Wiley & Sons, Inc.     

Volumetric and compressibility studies for (L-arginine + D-maltose monohydrate + water) system in the temperature range of (298.15 to 308.15) K

The density and speed of sound of L-arginine (0.025–0.2 mol kg^?1) in aqueous + D-maltose (0–6 mass% of maltose in water) were obtained at temperatures of (298.15, 303.15 and 308.15) K. The apparent molar volume, limiting apparent molar volume, transfer volume, as well as apparent molar compressibility, limiting apparent molar compressibility, transfer compressibility, pair and triple interaction coefficients, partial molar expansibilities, coefficient of thermal expansion and also the hydration number, were calculated using the experimental density and speed of sound values. The results have been discussed in terms of solute–solute and solute–solvent interactions in these systems. Solute–solvent (hydrophilic–ionic group and hydrophilic–hydrophilic group) interactions were found to be dominating over solute–solute (hydrophobic–hydrophilic group) interactions in the solution, which increases with increase in maltose concentration.     

Coupling quantum Monte Carlo to a nonlinear polarizable continuum model for spherical solutes

Starting from the nonlinear dielectric response model of Sandberg and Edholm, we derive an analytical expression of the polarization contribution to the solvation free energy in terms of the electronic density of the solute and the dielectric properties of the solvent. The solvent inhomogeneity is taken into account with the use of a smooth switching function whose spacial variation is established on the basis of how the solvent is arranged around the solute. An explicit form of a local potential representing the solvent effect on the solute is thus obtained by functional analysis. This effective potential can be combined with density functional or quantum chemical methods for the quantum mechanical treatment of the solute. Here, we use quantum Monte Carlo techniques for the solute and apply the method to the hydration of atomic ions finding very good agreement with experimental data.     

Compensation Between the Entropic and Enthalpic Changes Arising from Changes in Solvent–Solvent Interactions in Mixed Solvents

A theorem presented by Professor Ben-Naim (J Phys Chem 82:874–885, 1978) states that the standard state enthalpy and entropy changes arising from changes in the solvent structure that are induced by solvation of a solute cancel exactly in the standard state Gibbs energy. In this paper this is explored by consideration of the thermodynamics of transfer of electrolytes in mixed solvents, using previously developed models of the solvation process. Two cases are considered. One is random solvation, where curvatures in plots of the transfer enthalpies and entropies, which arise from changes in solvent–solvent interactions, exactly compensate in the transfer Gibbs (free) energies, which are sensibly linear with solvent composition. The second type of system are those with strong preferential solvation where it is found that the transfer Gibbs energies can be accounted for quantitatively in terms of changes in the solute–solvent interactions, with no contribution from changes in solvent–solvent interactions. The results are entirely consistent with the Ben-Naim theorem.     

A study of molecular vibrational relaxation mechanism in condensed phase based upon mixed quantum-classical molecular dynamics. II. Noncollisional mechanism for the relaxation of a polar solute in supercritical water

Mixed quantum-classical molecular dynamics method has been applied to vibrational relaxation of a hydrophilic model NO in supercritical water at various densities along an isotherm above the critical temperature. The relaxation rate was determined based on Fermi's golden rule at each state point and showed an inverse S-shaped curve as a function of bulk density. The hydration number was also calculated as a function of bulk density based on the calculated radial distribution function, which showed a good correlation with the relaxation rate. Change of the survival probability of the solute vibrational state was analyzed as a function of time together with the trajectory of the solvent water and the interaction with it. We will show that the solvent molecule resides near the solute molecule for a while and the solvent contributes to the relaxation by the random-noiselike Coulombic interaction only when it stays near the solute. After the solvent leaves the solute, it shows no contribution to the relaxation. The relaxation mechanism for this system is significantly different from the collisional one found for a nonpolar solute in nonpolar solvent in Paper I. Then, the relaxation rate is determined, on average, by the hydration number or local density of the solvent. Thus, the density dependence of the relaxation rate for the polar solute in supercritical water is apparently similar to that found for the nonpolar solute in nonpolar solvent, although the molecular process is quite different from each other.     

Combining the polarizable Drude force field with a continuum electrostatic Poisson–Boltzmann implicit solvation model

In this work, we have combined the polarizable force field based on the classical Drude oscillator with a continuum Poisson–Boltzmann/solvent‐accessible surface area (PB/SASA) model. In practice, the positions of the Drude particles experiencing the solvent reaction field arising from the fixed charges and induced polarization of the solute must be optimized in a self‐consistent manner. Here, we parameterized the model to reproduce experimental solvation free energies of a set of small molecules. The model reproduces well‐experimental solvation free energies of 70 molecules, yielding a root mean square difference of 0.8 kcal/mol versus 2.5 kcal/mol for the CHARMM36 additive force field. The polarization work associated with the solute transfer from the gas‐phase to the polar solvent, a term neglected in the framework of additive force fields, was found to make a large contribution to the total solvation free energy, comparable to the polar solute–solvent solvation contribution. The Drude PB/SASA also reproduces well the electronic polarization from the explicit solvent simulations of a small protein, BPTI. Model validation was based on comparisons with the experimental relative binding free energies of 371 single alanine mutations. With the Drude PB/SASA model the root mean square deviation between the predicted and experimental relative binding free energies is 3.35 kcal/mol, lower than 5.11 kcal/mol computed with the CHARMM36 additive force field. Overall, the results indicate that the main limitation of the Drude PB/SASA model is the inability of the SASA term to accurately capture non‐polar solvation effects. © 2018 Wiley Periodicals, Inc.