Theory for the solvation of nonpolar solutes in water

We recently developed an angle-dependent Wertheim integral equation theory (IET) of the Mercedes-Benz (MB) model of pure water [Silverstein et al., J. Am. Chem. Soc. 120, 3166 (1998)]. Our approach treats explicitly the coupled orientational constraints within water molecules. The analytical theory offers the advantage of being less computationally expensive than Monte Carlo simulations by two orders of magnitude. Here we apply the angle-dependent IET to studying the hydrophobic effect, the transfer of a nonpolar solute into MB water. We find that the theory reproduces the Monte Carlo results qualitatively for cold water and quantitatively for hot water.     

Effect of sugars on the solubility of hydrophobic solutes in water

It was found that the cosolvent effect of sugars on the solubilities of n-octanol, n-heptanol, and sodium dodecyl sulfate monomer in water depended on a set of factors that included molecular weight and concentration ofthe sugars, the kind of monosaccharides, the type of glycosidic linkages involved, and the temperature. All hexoses examined, D-glucose, D-galactose, and D-mannose, caused solubility depression of the hydrophobic solutes at low concentrations but to widely different extents. As the molecular weight of the sugar increased, the solubility depression was considerably lessened and further, as the concentration of the sugars increased, the solubility-increasing effect predominated leading to increased solubilities of the hydrophobic solutes relative of their solubility in pure water. The solubility-increasing effect was markedly enhanced at high temperatures. The free energy of the spontaneous transfer of octanol from water to the sugar solutions is entropic in nature and is attributed primarily to hydrophobic bond formation between the solute molecule and the hydrophobic surfaces of the sugar molecules.     

Excluded volume effect for large and small solutes in water

The cavitation effect, i.e., the process of the creation of a void of excluded volume in bulk solvent (a cavity), is considered. The cavitation free energy is treated in terms of the information theory (IT) approach [Hummer, G.; Garde, S.; Garcia, A. E.; Paulaitis, M. E.; Pratt, L. R. J. Phys. Chem. B 1998, 102, 10469]. The binomial cell model suggested earlier is applied as the IT default distribution p(m) for the number m of solute (water) particles occupying a cavity of given size and shape. In the present work, this model is extended to cover the entire range of cavity size between small ordinary molecular solutes and bulky biomolecular structures. The resulting distribution consists of two binomial peaks responsible for producing the free energy contributions, which are proportional respectively to the volume and to the surface area of a cavity. The surface peak dominates in the large cavity limit, when the two peaks are well separated. The volume effects become decisive in the opposite limit of small cavities, when the two peaks reduce to a single-peak distribution as considered in our earlier work. With a proper interpolation procedure connecting these two regimes, the MC simulation results for model spherical solutes with radii increasing up to R = 10 A [Huang, D. H.; Geissler, P. L.; Chandler, D. J. Phys. Chem. B 2001, 105, 6704] are well reproduced. The large cavity limit conforms to macroscopic properties of bulk water solvent, such as surface tension, isothermal compressibility and Tolman length. The computations are extended to include nonspherical solutes (hydrocarbons C1-C6).     

Thermodynamics of associated solutions: Henry's constants for nonpolar solutes in water

Hu, Y., Azevedo, D., Lüdecke, D. and Prausnitz, J., 1984. Thermodynamics of associated solutions: Henry's constants for nonpolar solutes in water. Fluid Phase Equilibria,17: 303–321.A systematic derivation is presented for the Helmholtz energy of a van der Waals fluid mixture whose nonideality is ascribed to both chemical and physical interactions; this derivation, applicable to all fluid densities, leads to an equation of state which contains chemical equilibrium constants in addition to the customary physical van der Waals constants a and b. Attention is given to the need for simplifying assumptions and to the variety of symplifying assumptions that can lead to useful results. A particular equation of state is used to correlate Henry's constants for nonpolar solutes in water over a wide temperature range. The correlation, however, is only partly successful, because a one-fluid van der Waals theory of mixtures is not satisfactory for mixtures containing molecules that differ appreciably in size, especially in the dilute region.     

Confusing cause and effect: energy-entropy compensation in the preferential solvation of a nonpolar solute in dimethyl sulfoxide/water mixtures

We performed molecular simulations to analyze the thermodynamics of methane solvation in dimethyl sulfoxide (DMSO)/water mixtures (298 K, 1 atm). Two contributions to the interaction thermodynamics are studied separately: (i) the introduction of solute-solvent interactions (primary contribution) and (ii) the solute-induced disruption of cohesive solvent-solvent interactions (secondary contribution). The energy and entropy changes of the secondary contribution always exactly cancel in the free energy (energy-entropy compensation), hence only the primary contribution is important for understanding changes of the free energy. We analyze the physical significance of the solute-solvent energy and solute-solvent entropy associated with the primary contribution and discuss how to obtain these quantities from experiments combining solvation thermodynamic and solvent equation of state data. We show that the secondary contribution dominates changes in the methane solvation entropy and enthalpy: below 30 mol % DMSO in the mixture, methane, because of more favorable dispersion interactions with DMSO molecules, preferentially attracts DMSO molecules, which, in response, release water molecules into the bulk, causing an increase in the entropy. This large energy-entropy compensating process easily causes a confusion in the cause for and the effect of preferred methane-DMSO interactions. Methane-DMSO dispersion interactions are the cause, and the entropy change is the effect. Procedures that infer thermodynamic driving forces from analyses of the solvation entropies and enthalpies should therefore be used with caution.     

Effect of small solutes on the water populations in inverted micelles

Different types of water are present in the core of inside-up vesicles; their relative populations are modified when small molecules like aminoacids or oligopeptides are dissolved in the aqueous compartment. I.R. spectroscopy allows the monitoring of the water organization in terms of greater or lesser molecular freedom.     

On the influence of solute polarizability on the hydrophobic interaction

The authors have performed molecular dynamics simulations of polarizable solutes in water to investigate how solute polarizability affects solute-solute hydrophophic interactions. A degree of polarization similar to the one expected in biomolecules, corresponding to a dielectric response of epsilon=2-20, results in dramatic changes in the hydrophobic forces. They find that this degree of polarizability is enough to inhibit drying between hydrophobic solutes and to stabilize a reduced water density phase whose density is smaller than the bulk water density. The hydrophobic forces associated with such reduced density states are still very significant with values of the order of several tens of piconewtons. Their results suggest that polarizability plays an important role in determining the hydrophobic force acting between weakly polar surfaces.     

Effects of nonpolar solutes on the thermodynamic response functions of aqueous mixtures

We investigate the effect of adding nonpolar solutes at atmospheric pressure on water's temperature of maximum density, isothermal compressibility, and isobaric heat capacity, using a statistical mechanical model of water solutions [H. S. Ashbaugh, T. M. Truskett, and P. G. Debenedetti, J. Chem. Phys. 116, 2907 (2002)]. We find that the temperature of maximum density increases with solute hydrophobicity, as characterized by its size, and decreases with its van der Waals attractive parameter a, in agreement with experiment. We predict similar trends for the addition of solutes on the isothermal compressibility and isobaric heat capacity: solute hydrophobicity causes an upward shift in water's anomalies, whereas dispersive interactions as measured by the solute's van der Waals attractive parameter shift the anomalies to lower temperatures. The locus along which the competing contributions of solute size sigma and interaction strength a to the shift in water's response functions balance each other obeys the scaling relationship sigma6 approximately a.     

The effect of temperature on the solubility of benzoic acid derivatives in water

Using a laser monitoring observation technique, solubilities of o-nitro-benzoic acid, p-hydro-benzoic acid, p-methyl-benzoic acid and m-methyl-benzoic acid in water have been measured in the temperature range 290.15–323.15 K. The experimental data are regressed with the Wilson equation and the λH equation. The experimental results show that solubilities of these compounds in the range of 10⁻⁴–10⁻⁵ mole fraction in water, increase significantly with temperature. Except for o-nitro-benzoic acid, the solubility data are described adequately with the Wilson equation. The λH equation gives good agreement with all experimental data. The results indicate that the molecular structure and interactions affect the solubilities significantly.     

Isotope effect on the solubility of amino acids in water

The solubilities of glycine, alanine, phenylalanine, and proline in H₂O and D₂O from 283 K to 335 K were determined. It was found that glycine and alanine are less soluble in heavy water than in light water but proline is more soluble in heavy water over the whole temperature range studied. Phenylalanine is more soluble in H₂O than D₂O below 310 K but above that temperature heavy water becomes a better solvent. An influence of H/D isotope substitution on the enthalpies of solution is also observed. In the case of glycine and alanine enthalpies of solution in heavy water increase by a small amount and in the same time the solution enthalpy for phenylalanine in D₂O increases markedly. No change in the solution enthalpy for proline was observed. The isotope effects on solubility and the solution enthalpy are qualitatively discussed.     

Effect of solute size and solute-water attractive interactions on hydration water structure around hydrophobic solutes.

Using Monte Carlo simulations, we investigated the influence of solute size and solute-water attractive interactions on hydration water structure around spherical clusters of 1, 13, 57, 135, and 305 hexagonally close-packed methanes and the single hard-sphere (HS) solute analogues of these clusters. We obtain quantitative results on the density of water molecules in contact with the HS solutes as a function of solute size for HS radii between 3.25 and 16.45 A. Analysis of these results based on scaled-particle theory yields a hydration free energy/surface area coefficient equal to 139 cal/(mol A2), independent of solute size, when this coefficient is defined with respect to the van der Waals surface of the solute. The same coefficient defined with respect to the solvent-accessible surface decreases with decreasing solute size for HS radii less than approximately 10 A. We also find that solute-water attractive interactions play an important role in the hydration of the methane clusters. Water densities in the first hydration shell of the three largest clusters are greater than bulk water density and are insensitive to the cluster size. In contrast, contact water densities for the HS analogues of these clusters decrease with solute size, falling below the bulk density of water for the two largest solutes. Thus, the large HS solutes dewet, while methane clusters of the same size do not.     

Model calculations of changes of thermodynamic variables for the transfer of nonpolar solutes from water to water-d2

A recently introduced modified hydration shell hydrogen bond model for rationalizing the thermodynamic consequences of hydrophobic hydration is adapted for use with heavy water. The required adjustment of parameters employs the assumption that breaking hydrogen bonds in water-d₂ involves a greater enthalpy change and a larger entropy increase than bond breaking in ordinary water. It also makes some use of information derived from studies of gas solubilities in the two solvents, although a review of the data leads to serious questions about the reliability of results obtained in this way. The model permits calculations of hydrogen bonding contributions to the changes, G _t ^o , H _t ^o , S _t ^o , and C _p,t ^o , for transfer of nonpolar solutes from water to water-d₂ and implies that such data should show regular trends. Although some of the numerical results depend strongly on the values chosen for the parameters, the pattern defined by these trends is nearly independent of parameters. Predicted values of C _p,t ^o are large and positive for all nonpolar solutes, while S _t ^o is expected to be negative near 0°C, becoming progressively less negative on warming and eventually positive. Both of these quantities should be proportional to the molecular surface area of the solute. Analogous predictions regarding G _t ^o and H _t ^o can also be made, but only if it is permissible to neglect possible contributions to these quantities from van der Waals interactions.     

The entropy effects in binary mixtures of polar mesogenic solvent/nonpolar solute

The paper concerns two aspects of the entropy in mesogenic systems: (i) the entropy jump (Delta S (0) NI) at the phase transition from the isotropic liquid (I) to the nematic liquid crystalline state (N), and (ii) the entropy increment (Delta S) caused by the ordering action of the probing electric field applied to the dipolar system. The system studied are the mixtures of strongly polar mesogenic solvent n-hexylcyanobiphenyl (C 6H 13PhPhCN, 6CB) and the nonpolar nonmesogenic admixture 4-ethylcyclohexyl-4'- n-nonylphenyl (C 2H 5CyHxPhC 9H 19, 2CyPh9). The entropy jump at the I-N phase transition in pure 6CB [Delta S (0) NI= 1.52 J/(mol K)] was evaluated from the analysis of the phase diagram of the mixture 6CB + 2CyPh9 with use of the Landau-Lifshitz theory; the resulting value of the transition enthalpy (Delta H (0) NI = T NIDelta S (0) NI = 0.50 kJ/mol) agrees well to that obtained with the calorimetric methods. The field-induced entropy increment (Delta S) was calculated, at the given temperature, from the static dielectric permittivity derivative value (depsilon s/d T), with use of the Fr?hlich theory. The singularities in dependence of the entropy increment on the temperature and on the mixtures composition are discussed in terms of the prenematic molecular self-organization extent in mesogenic liquids of different density of dipoles.     

Structure in aqueous solutions of nonpolar solutes from the standpoint of scaled-particle theory

Underlying assumptions have been examined in scaled-particle theory for the case of a rigid-sphere solute in liquid water. As a result, it has been possible to improve upon Pierotti's corresponding analysis in a way that explicitly incorporates measured surface tensions and radial-distribution functions for pure water. It is pointed out along the way that potential energy nonadditivity should create an orientational bias for molecules in the liquid-vapor interface that is peculiar to water. Some specific conclusions have been drawn about the solvation mode for the nonpolar rigid-sphere solute.This paper is substituted for the talk given at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.Editor's note