Solute-solvent Attractive Interactions Research Articles

How do water-mediated interactions and osmotic second virial coefficients vary with particle size?

We examine quantitatively the solute-size dependences of the effective interactions between nonpolar solutes in water and in a simple liquid. The potential w(r) of mean force and the osmotic second virial coefficients B are calculated with high accuracy from molecular dynamics simulations. As the solute diameter increases from methane's to C60's with the solute-solute and solute-solvent attractive interaction parameters fixed to those for the methane-methane and methane-water interactions, the first minimum of w(r) lowers from -1.1 to -4.7 in units of the thermal energy kT. Correspondingly, the magnitude of B (<0) increases proportional to σα with some power close to 6 or 7, which reinforces the solute-size dependence of B found earlier for a smaller range of σ [H. Naito, R. Okamoto, T. Sumi and K. Koga, J. Chem. Phys., 2022, 156, 221104]. We also demonstrate that the strength of the attractive interactions between solute and solvent molecules can qualitatively change the characteristics of the effective pair interaction between solute particles, both in water and in a simple liquid. If the solute-solvent attractive force is set to be weaker (stronger) than a threshold, the effective interaction becomes increasingly attractive (repulsive) with increasing solute size.

Temperature, Pressure, and Length-Scale Dependence of Solvation in Water-like Solvents. II. Large Solvophovic Solutes.

We use molecular simulation to determine solvation free energies, isochoric solvation energies and entropies, isobaric solvation enthalpies and entropies, partial molecular volumes, and isothermal density derivatives of the solvation free energy as a function of temperature and pressure for hard-sphere solutes with diameters ranging from 4 to 36 Å in TIP4P/2005 and Jagla water-like solvents exhibiting unusual thermodynamics. An important piece of our discussion focuses on the nanometer-sized solutes, for which simulation results are found to be accounted for by the most basic classical thermodynamic treatment contemplating bulk and interfacial contributions to the solvation free energy. Thus, since water's liquid-vapor surface tension is only special inasmuch as it takes unusually large values, solvent's water-like unusual thermodynamics manifests through a term proportional to the pressure in the solvation free energy. As a result, such solvent's unusual thermodynamics is found to be relevant to the temperature and pressure dependence of the isochoric solvation energy and entropy as well as to the isothermal density derivative of the solvation free energy. This sharply contrasts with the findings of the first part of this series indicating that the solvation free energy of small hard spheres responds to temperature and pressure changes as solvent's density does, with such a contrasting picture embodying a "pressure-density dichotomy." As for the length-scale dependence, we find the zero nominal pressure and the solvent's temperature of the maximum density as singular conditions for cavity surface-area size scaling of large solutes to occur for all solvation quantities. We finally argue that the overall study undertaken in this series suggests that water's unusual thermodynamics may be relevant to the thermodynamic stability of clusters of solvophobic units in the temperature-pressure plane. Some comments on the role of solute-solvent attractive interactions are also depicted.

High-frequency shear viscosity and ionic mobility of solutions of polyethylene glycol in ionic liquids

The frequency-dependent complex shear viscosity of the solutions of polyethylene glycol (PEG) in various ionic liquids is measured in the frequency range of 5–205 MHz, and the ionic mobility of the corresponding solutions was also determined. The plateau viscosity in the MHz regime increases on the addition of PEG, and the relaxation in the 100 MHz regime is retarded simultaneously. The amount of the increase in the plateau viscosity correlates with that in the relaxation time, suggesting that the retardation of the relaxation is a reason for the increase in the plateau viscosity. Although the increase in the plateau viscosity is far smaller than that in the zero-frequency viscosity, these two values are correlated with each other, which can be explained as the consequence of the strength of the same kind of the solute-solvent attractive interaction. The decrease in the molar ionic conductivity also correlates with the increase in the plateau viscosity, suggesting that the plateau viscosity can be regarded as the local viscosity of ionic liquids related to the ion conduction. Based on the correlation with the Kamlet-Taft α parameter, the hydrogen bond between the ionic liquid and the oxygen atoms of PEG is suggested to be a possible origin of the increase in the plateau viscosity.

On the solubility of oxygen and xenon in n-hexane and n-perfluorohexane at room temperature

Experimental measurements have shown that, at room temperature, xenon has a good solubility in both n-hexane and n-perfluorohexane, whereas oxygen is poorly soluble in both liquids. However, oxygen proves to be more soluble in n-perfluorohexane than in n-hexane, in contrast with xenon behavior. This difference is investigated by calculating the reversible work of cavity creation in both liquids and estimating the solute–solvent attractive interactions. The magnitude of the reversible work of cavity creation in n-hexane is larger than in n-perfluorohexane, and also the strength of the solute–solvent dispersion attraction is larger in n-hexane. However, it emerges that the dispersion attractions between oxygen and n-perfluorohexane are stronger than expected due to specific interactions between partially negative fluorine atoms and oxygen molecules.

Water anomalous thermodynamics, attraction, repulsion, and hydrophobic hydration.

A model composed of van der Waals-like and hydrogen bonding contributions that simulates the low-temperature anomalous thermodynamics of pure water while exhibiting a second, liquid-liquid critical point [P. H. Poole et al., Phys. Rev. Lett. 73, 1632 (1994)] is extended to dilute solutions of nonionic species. Critical lines emanating from such second critical point are calculated. While one infers that the smallness of the water molecule may be a relevant factor for those critical lines to move towards experimentally accessible regions, attention is mainly focused on the picture our model draws for the hydration thermodynamics of purely hydrophobic and amphiphilic non-electrolyte solutes. We first focus on differentiating solvation at constant volume from the corresponding isobaric process. Both processes provide the same viewpoint for the low solubility of hydrophobic solutes: it originates from the combination of weak solute-solvent attractive interactions and the specific excluded-volume effects associated with the small molecular size of water. However, a sharp distinction is found when exploring the temperature dependence of hydration phenomena since, in contrast to the situation for the constant-V process, the properties of pure water play a crucial role at isobaric conditions. Specifically, the solubility minimum as well as enthalpy and entropy convergence phenomena, exclusively ascribed to isobaric solvation, are closely related to water's density maximum. Furthermore, the behavior of the partial molecular volume and the partial molecular isobaric heat capacity highlights the interplay between water anomalies, attraction, and repulsion. The overall picture presented here is supported by experimental observations, simulations, and previous theoretical results.

Translational friction and momentum dissipation of a solute in simple liquid studied by generalized Langevin theory for liquid under external field

The translational friction coefficients on spherical solutes in the Lennard–Jones liquid are calculated as the function of solute size and the strength of the solute–solvent attractive interaction using the generalized Langevin theory for simple liquids under external fields recently proposed by us [T. Yamaguchi, T. Matsuoka, and S. Koda, J. Chem. Phys., 123 (2005) 034504.]. When the solute is as small as the solvent, the friction coefficient is a strong function of the solute–solvent attractive interaction, which means that it is the solute–solvent interaction rather than the solvent viscosity that determines the translational friction on the solute. On the other hand, when the solute is eight times larger than the solvent, the dependence on the solute–solvent attractive interaction becomes weak, indicating the dominant role of the solvent viscosity. The transient momentum transfer from the solute to the solvent is analyzed in connection with the crossover from microscopic to continuum views of the solvent.

Size dependence of the solubility of nonpolar compounds in different solvents

At 25°C, plots of the standard Gibbs energy change associated with the solvation of noble gases and aliphatic hydrocarbons vs. the size of the solutes prove to be approximately linear with a negative slope for common organic solvents but not for water. In the latter case, the plot has a characteristic V-shape. The slope is negative for noble gases, methane, and ethane, but is positive for larger alkanes. This means that the solubility of nonpolar solutes increases with solute size in every solvent except water. The solvation thermodynamics of noble gases and aliphatic hydrocarbons in five solvents (water, ethanol, benzene, c-hexane, and n-hexane) are analyzed in detail by a general theory, which is rederived to avoid risky misunderstandings. The calculations are performed in the same manner for all solvents, using simple formulas where the physical reliability is well established and the results are consistent. The work of cavity creation increases with solute size in every solvent, but to a far greater extent in water. Additionally, the work to turn on the solute–solvent attractive interactions increases in magnitude with solute size in every solvent, but to a lesser extent in water. By combining these two factors a satisfactory explanation for experimental data obtained emerges. The microscopic origins of the difference between water and common organic solvents are discussed.Key words: solvation, excluded-volume effect, solute–solvent interactions, enthalpy–entropy compensation, molecular size.

A mode-coupling approach to the attractive interaction effect on the solute diffusion in liquids

The effect of the solute–solvent and the solvent–solvent attractive interactions on the diffusion coefficient of the solute is studied by the mode-coupling theory. The mode-coupling part of the friction coefficient in our calculation is compared with that of molecular dynamics simulation. The theory qualitatively reproduces the increase in the friction coefficient with the solute–solvent attractive interaction, although the absolute value is larger than the simulation. A theoretical scheme is further provided to divide the memory function into attractive and repulsive components, and a mode-coupling approximation is introduced to both components separately. It is found that the solute–solvent interaction affects the attractive friction mainly, in agreement with the simulation. Further analyses reveal the presence of the large cross term of the attractive and repulsive random forces. When the solvent–solvent attractive interaction is weakened while keeping the solute–solvent interaction fixed, the mode-coupling friction is increased, which is opposite to the simulation results.

Molecular dynamics study on hydrophobic effects in aqueous urea solutions.

Hydrophobic effects in aqueous urea were analyzed by molecular dynamics simulations. The contribution of solvents to the potential of mean force between two methane molecules was calculated by using molecular dynamics simulations and was compared with the solubility data of hydrocarbons in aqueous urea. Both the simulation results and the solubility data indicated that urea stabilizes methane-methane association. The stabilization was due to increasing the solvation free energies of small hydrocarbons such as methane by addition of urea. The solvation free energies of larger hydrocarbons, on the other hand, are decreased by addition of urea. This effect of the solute size on hydrophobic free energies in aqueous urea was also analyzed by using molecular dynamics simulations by means of division of the solvation process into two parts: the cavity formation and the introduction of the solute-solvent attractive interactions. In the cavity formation, urea increased hydrophobic free energies, and in the introduction of the solute-solvent attractive interactions, urea decreased hydrophobic free energies. The influence of urea on hydrophobic free energies was determined by the balance of effects of the two parts of the solvation process.

Effects of solute-solvent and solvent-solvent attractive interactions on solute diffusion

A study is reported on the diffusion process of a solute molecule in a Lennard-Jones-like liquid near the triple point by a molecular dynamics simulation. Systematic changes were made to the strength of the solute-solvent or solvent-solvent attractive interaction in order to elucidate its effects on the diffusion coefficient. When the solute-solvent attractive interaction is enhanced, the diffusion coefficient of the solute becomes much smaller than that predicted by the Stokes-Einstein relationship with a stick boundary condition. The generalized friction coefficient on the solute molecule was investigated, and the attractive force between solute and solvent is found to be the main cause for the enhancement of the friction. When the attractive interaction between solvent molecules is weakened, the diffusion coefficient of a solute does not change, whereas that of a solvent does. Compared with the shear viscosity of the solvent, the diffusion coefficient of the solute breaks the Stokes-Einstein relationship, whereas the Stokes-Einstein relationship appears to hold in the case of the solvent molecule.

Interaction between solute molecules in medium density solvents

Solvation properties of solutes in supercritical, medium density solvents have been analysed using hypernetted-chain theory with the emphasis on the solvent-mediated interaction between solute molecules. The solvent and solute molecules are Lennard-Jones particles, and the solute is present at infinite dilution. Also a pair of solute molecules separated by different distances has been considered using reference interaction site model theory. Mainly, solvents at two typical densities (1.09pc and 2.91pc; pc is the critical density) that are in medium and high density regions, respectively, are treated. The temperature is set at 1.04TC (Tc is the critical temperature). When the solute size is larger than the solvent size and the strength of the solute-solvent attractive interaction is greater than that of the solvent-solvent in the medium density region, the solvent structure confined between a pair of solute molecules is largely different from that near a single solute molecule. The confined solvent becomes denser and more stabilized as the distance between the solute molecules decreases, and an attractive interaction is induced between them. The interaction becomes even more attractive as the strength of the solute-solvent attractive interaction increases. The observations are qualitatively different from those in the high density region. Another high density region, which is well below the critical temperature, has been considered, but the behaviour observed is similar to that in the high density region above the critical temperature.

Vibrational Lifetimes and Spectral Shifts in Supercritical Fluids as a Function of Density: Experiments and Theory

Vibrational lifetime and spectral shift data for the asymmetric CO stretching mode of $W(CO)_6$ in supercritical ethane, carbon dioxide, and fluoroform as a function of density at two temperatures are presented, and the lifetime $(T_1)$ measurements are compared to theory. The data and theory are refinements of previous work. Measurements at zero density allow the contribution from solute-solvent interactions to be separated from strictly intramolecular contributions to $T_1$. The results in the polyatomic SCFs are compared to data in Ar. The density functional/thermodynamic $theory^1$ has been extended to include contributions at large wavevector (k). Very good, quantitative agreement between theory and data taken in ethane and fluoroform is achieved, but the agreement for data taken in carbon dioxide, while reasonable, is not as good. The theory uses a variety of input information on the SCF properties obtained from the fluids' equations of state and other tabulated thermodynamic data. The solute-solvent spatial distribution is described in terms of hard spheres. The theory is able to reproduce the data without solute-solvent attractive interactions or clustering in the calculation of the spatial distribution.

Molecular dynamics simulation of solute diffusion in Lennard-Jones fluids

We performed a molecular dynamics (MD) simulation for a system of 5 solute molecules in 495 solvent molecules interacting through the Lennard-Jones (LJ) 12-6 potential, in order to study solvent density effects on the diffusion coefficients in supercritical fluids. The effects of the size of the solute and the strength of the solute-solvent attractive interaction on the diffusion coefficient of the solute were examined. The diffusion coefficients of the solute molecules were calculated at T = 1.5 (in the LJ reduced unit), slightly above the critical temperature, from ρ = 0.1 to ρ = 0.95, where ρ is the number density in the LJ reduced unit. The memory function in the generalized Langevin equation was calculated, in order to know the molecular origin of the friction on a solute. The memory function is separated into fast and slow components. The former arises from the solute-solvent repulsive interaction, and is interpreted as collisional Enskog-like friction. The interaction strength dependence of the collisional friction is larger in the low- and medium-density regions, which is consistent with the ‘clustering’ picture, i.e., the local density enhancement due to the solute-solvent attractive interaction. However, the slow component of the memory function suppresses the effect of the local density on the diffusion coefficients, and as a result the effect of the attractive interaction is smaller on the diffusion coefficients than on the local density. Nonetheless, the solvent density dependence of the effect of the attraction on the diffusion coefficient varies with the local density, and it is concluded that the local density is the principal factor that determines the interaction strength dependence of the diffusion coefficient in the low- and medium-density regions (ρ < 0.6).

Effect of Solute-Solvent Interaction on the Molecular Association in a Hard Sphere Fluid

Effect of solute-solvent interaction on the solute-solute distribution function is studied for a system in which the solute molecules are dissolved in a hard-sphere fluid at infinite dilution. The solute-solvent interaction is a square-well potential and the solvent molecule in the well is assumed to have a bond with the solute molecule. An ideal-gas approximation reveals that a solute-solvent attractive interaction, as well as a repulsive interaction, can stabilize the solute pair through the fluctuation of the bond number. The Monte Carlo calculations showed that the ideal-gas approximation becomes invalid with increasing density. Solvent-solvent correlation at high densities brings about a complicated fluid structure around a solute pair and reduces the fluctuation of the bond-number.

Solvent Density Dependence of Translational Diffusion of Transient Radicals in the Medium-Density Region of Trifluoromethane and Carbon Dioxide

Translational diffusion constants of transient radicals produced by photochemical reactions of pyrazine, α-naphthoquinone, quinoxaline, and benzophenone with triethylamine (TEA) have been measured in trifluoromethane from ρr = 1.4 to 2.0, and α-naphthoquinone with TEA in carbon dioxide from ρr = 1.2 to 1.93, where ρr is the reduced density by the critical density of the solvent. The diffusion constants of the transient radicals almost linearly depend on the reciprocal of the solvent viscosity in the measured density region. The diffusion constants of the radicals are relatively smaller than those of stable molecules of similar molecular sizes, and the difference becomes smaller with increasing the molecular size, as is observed in liquid solution. However, the difference is smaller than that observed in the liquid solvent, which suggests that the effect of the solute-solvent attractive interaction on the diffusion constant is smaller in fluids at the medium-density region than in liquids.

Solute-solute potential of mean force in simple fluids at infinite dilution

The effects of solvent density on the solute-solute potential of mean force in Lennard-Jones fluids are studied by means of the integral equation method. The solvent density plays an important role in solute-solute correlation in solution: the effect of the solute-solvent attractive interaction is quite different in the medium-density and high-density regions. In the latter the strong solute-solvent attractive interaction destabilizes the contact solute pair, and the strong solvent-solvent attractive interaction stabilizes the contact solute pair, which is expected from both ‘desolvation’ and the ‘solvophobic’ effect. In the medium-density region, however, curious phenomena are observed that cannot be interpreted by a simple picture: for example, the solute-solvent attractive interaction stabilizes both the solvent-separated pair and the contact pair of solute molecules under some conditions. The characteristics of the medium-density region are discussed in terms of the density fluctuation around the solu...