Excess properties of aqueous solutions: hard spheres versus pseudo-hard bodies

The Journal of chemical physics, 2014

We propose an efficient method for studying the solvent-induced interaction of two solvophobic particles immersed in a liquid solvent. The method is based on the combination of the probabilistic hydrogen bond model with the density functional theory. An analytic expression for the number of hydrogen bonds per water molecule near two spherical hydrophobes is derived as a function of the molecule distance to both hydrophobes, distance between hydrophobes, and their radii. Using this expression, one can construct an approximation for the distribution of fluid (liquid water) molecules in the system which provides a reasonably good (much faster and accurate enough) alternative to a standard iteration procedure. Such an approximate density distribution constitutes an efficient foundation for studying the length-scale and temperature dependence of hydrophobic interactions. The model is applied to the interaction of solvophobic solutes in both associated and non-associated liquids. Of these...

Proceedings of the National Academy of Sciences of the United States of America, 2013

The osmotic second virial coefficients, B2, for atomic-sized hard spheres in water are attractive (B2 < 0) and become more attractive with increasing temperature (ΔB2/ΔT < 0) in the temperature range 300 K ≤ T ≤ 360 K. Thus, these hydrophobic interactions are attractive and endothermic at moderate temperatures. Hydrophobic interactions between atomic-sized hard spheres in water are more attractive than predicted by the available statistical mechanical theory. These results constitute an initial step toward detailed molecular theory of additional intermolecular interaction features, specifically, attractive interactions associated with hydrophobic solutes.

The equivalence of the asymptotic expression for the interaction energy between a pair of large hard spheres in a fluid of small hard spheres that has been obtained by Roth et al.

Journal of Chemical Physics, 2014

The mutual entropic depletion force felt by two solute "big" hard spheres immersed in a binary mixture solvent of nonadditive "small" hard spheres is calculated as a function of the surface-to-surface distance by means of canonical Monte Carlo simulations and through a recently proposed rational-function approximation [Phys. Rev. E 84, 041201 (2011)]. Four representative scenarios are investigated: symmetric solute particles and the limit where one of the two solute spheres becomes a planar hard wall, in both cases with symmetric and asymmetric solvents. In all cases, the influence on the depletion force due to the nonadditivity in the solvent is determined in the mixed state. Comparison between results from the theoretical approximation and from the simulation shows a good agreement for surface-to-surface distances greater than the smallest solvent diameter.

Physica A: Statistical Mechanics and its Applications, 1997

The effective interaction between colloidal particles in a solvent of hard spheres, which can polymerize with the formation of chains and rings, is studied. Polymerization results in changes of the effective interaction between the colloid particles, compared with the effective interaction for unpolymerized solvents that has been extensively studied in previous publications. To describe the mixture, we use the associative Percus-Yevick approximation for Wertheim's Ornstein-Zernike integral equation. Both the infinite dilution and the nonvanishing concentration limits for the colloid species are considered. It is shown that, in the model of Wertheim for polymerization, the intercolloidal potential of mean force (PMF) depends primarily on the solvent density and to a lesser extent on the degree of polymerization. A depletion of the solvent density around the colloid particles is observed. If the solvent consists of longer chains, compared with hard spheres, the correlations between the colloid particles are of longer range. The oscillations of the PMF depend upon the average chain length but not so strongly as in the model with chains having a fixed number of beads; this is probably due to the flexibility of the chain conformations in Wertheim's model. We also study the effect of the size ratio of the colloid spheres and the solvent monomers. We observe that the polymer hypernetted chain closure is difficult to apply to the systems under study. This closure shares difficulties, especially near the critical point, that are well known in the study of the phase diagram of fluids with both repulsive and attractive interactions. For large colloidal particles, the mixed closure of Henderson is useful.

The Journal of Chemical Physics, 2007

We consider binary mixtures of soft repulsive spherical particles and calculate the depletion interaction between two big spheres mediated by the fluid of small spheres, using different theoretical and simulation methods. The validity of the theoretical approach, a virial expansion in terms of the density of the small spheres, is checked against simulation results. Attention is given to the approach toward the hard-sphere limit, and to the effect of density and temperature on the strength of the depletion potential. Our results indicate, surprisingly, that even a modest degree of softness in the pair potential governing the direct interactions between the particles may lead to a significantly more attractive total effective potential for the big spheres than in the hard-sphere case. This might lead to significant differences in phase behavior, structure and dynamics of a binary mixture of soft repulsive spheres. In particular, a perturbative scheme is applied to predict the phase diagram of an effective system of big spheres interacting via depletion forces for a size ratio of small and big spheres of 0.2; this diagram includes the usual fluid-solid transition but, in the soft-sphere case, the metastable fluid-fluid transition, which is probably absent in hard-sphere mixtures, is close to being stable with respect to direct fluid-solid coexistence. From these results the interesting possibility arises that, for sufficiently soft repulsive particles, this phase transition could become stable. Possible implications for the phase behavior of real colloidal dispersions are discussed.

The Journal of chemical physics, 2015

A general class of nonadditive sticky-hard-sphere binary mixtures, where small and large spheres represent the solvent and the solute, respectively, is introduced. The solute-solute and solvent-solvent interactions are of hard-sphere type, while the solute-solvent interactions are of sticky-hard-sphere type with tunable degrees of size nonadditivity and stickiness. Two particular and complementary limits are studied using analytical and semi-analytical tools. The first case is characterized by zero nonadditivity, lending itself to a Percus-Yevick approximate solution from which the impact of stickiness on the spinodal curves and on the effective solute-solute potential is analyzed. In the opposite nonadditive case, the solvent-solvent diameter is zero and the model can then be reckoned as an extension of the well-known Asakura-Oosawa model with additional sticky solute-solvent interaction. This latter model has the property that its exact effective one-component problem involves onl...

The Journal of Chemical Physics, 2000

The structure of mixtures of dipolar hard sphere fluids with components of equal size but different dipole moments around a single ion is studied. The solvation energy and the polarization around the ion is obtained in the framework of the mean spherical approximation ͑MSA͒. Our theoretical results and the results of other workers are compared with simulation data obtained from Monte Carlo simulations. An interpretation of the meaning of preferential solvation is given in terms of the contrasting behaviors of partial polarization in the bulk and near the ion.

Condensed Matter Physics, 2013

Very recently the effect of equisized charged hard sphere solutes in a mixture with core-softened fluid model on the structural and thermodynamic anomalies of the system has been explored in detail by using Monte Carlo simulations and integral equations theory [J. Chem. Phys., 2012, 137, 244502]. Our objective of the present short work is to complement this study by considering univalent ions of unequal diameters in a mixture with the same soft-core fluid model. Specifically, we are interested in the analysis of changes of the temperature of maximum density (TMD) lines with ion concentration for three model salt solutes, namely sodium chloride, potassium chloride and rubidium chloride models. We resort to Monte Carlo simulations for this purpose. Our discussion also involves the dependences of the pair contribution to excess entropy and of constant volume heat capacity on the temperature of maximum density line. Some examples of the microscopic structure of mixtures in question in terms of pair distribution functions are given in addition.

Macromolecules, 2000

We explore the depletion attractions that arise between hard colloidal spheres immersed in a nonadsorbing polymer solution of DNA. We spatially confine two 1.25 µm silica spheres in a scanning optical tweezer and quantitatively examine their interaction potential in double-stranded DNA solutions of different concentrations. The potentials obtained display variations in depth and range that are consistent with scaling behavior expected for semiflexible polymers near the θ-point. In particular, we clearly observe the crossover from a dilute solution of Gaussian coils to the weakly fluctuating semidilute regime dominated by two-point collisions Pincus, P. Macromolecules 1980, 13, 1280-1289. We also quantitatively test the Asakura-Oosawa model for these systems and show that it may be used in both the semidilute as well as the dilute regime. At fixed DNA concentration, we find that the range and depth of the interparticle potentials do not change significantly for ionic concentrations between 1 and 50 mM.