Hydrophobic interactions and osmotic second virial coefficients for methanol in water

A recent theory of the hydrophobic effect together with a simple model for an alcohol molecule is used to calculate the osmotic (McMillan-Mayer) second virial coefficientB ₂ for methanol dissolved in water. We use this calculation to study the validity of common arguments that try to draw microscopic structural information from experimental virial coefficient data. In disagreement with many workers, we find that the hydrophobic interaction between hard spheres in water is attractive and that its strength diminishes as temperature is raised. Models that have come to the opposite conclusions have neglected complications inherent to real solutes such as the role of the hydroxy groups in affecting the correlations between the apolar portions of neighboring alcohols. The calculations reported here indicate that this neglect is a poor approximation for methanol. Our calculations also show that osmotic virial coefficients are sensitive to subtle details in the potentials of mean force. Therefore, slowly varying (e.g., dispersion) interactions may also contribute significantly to the values of these coefficients without significantly changing the solvent structure near the solute molecules.     

The osmotic and activity coefficients of some thioureas in water and in ethylene carbonate

The osmotic and activity coefficients of thiourea and 1,3-dimethylthiourea have been determined from isopiestic measurements in aqueous solutions. Osmotic coefficients of these substances in ethylene carbonate solutions have been calculated from freezing-temperature measurements. The results of this and previously reported work enable the following conclusions. The substitution of alkyl groups for hydrogen atoms or sulfur for oxygen in the urea molecule diminishes the osmotic coefficients in aqueous solutions. Micelle formation is suspected as the cause. Urea tends to dimerize in ethylene carbonate while thiourea does not.     

Estimation of activity and osmotic coefficients of strong electrolytes in water at elevated temperatures

It was found over a wide concentration and temperature range that differences between osmotic coefficients of a given electrolyte and a standard electrolyte, - ^st , can be approximated by a linear relation. Sodium chloride, calcium chloride and sodium sulfate were chosen as standard electrolytes for 1:1, 2:1 and 1:2 electrolytes, respectively. Special numerical procedures were developed for the prediction and estimation of activity and osmotic coefficients at elevated temperatures based on data at two temperatures or on data at room temperature only. Reasonable agreement was found between activity coefficients estimated by the present procedure and those correlated by Pitzer's and Meissner's equations.     

From transition paths to transition states and rate coefficients

Transition states are defined as points in configuration space with the highest probability that trajectories passing through them are reactive (i.e., form transition paths between reactants and products). In the high-friction (diffusive) limit of Langevin dynamics, the resulting ensemble of transition states is shown to coincide with the separatrix formed by points of equal commitment (or splitting) probabilities for reaching the product and reactant regions. Transition states according to the new criterion can be identified directly from equilibrium trajectories, or indirectly by calculating probability densities in the equilibrium and transition-path ensembles using umbrella and transition-path sampling, respectively. An algorithm is proposed to calculate rate coefficients from the transition-path and equilibrium ensembles by estimating the frequency of transitions between reactants and products.     

A new isopiestic apparatus for the determination of osmotic coefficients

A new isopiestic apparatus has been designed and constructed following several criteria. It consists mainly of several small sample cups for holding small quantities of reference standard solutions, and a big sample cup for a bigger quantity of a test solution. Using this apparatus, experiments on NaOH solutions have been performed. The experimental procedure, the consistency among the samples in equilibrium, the equilibration process, and the determined osmotic coefficients of NaOH solutions are discussed. The apparatus is found to ensure a consistent temperature among the samples in equilibrium, meeting the experimental requirements for samples of molalities less than 0.05 mol · kg⁻¹. Inside the apparatus, the temperature can reach the desired uniform temperature within less than 0.5 d. In the experiments, the equilibration process is essentially determined by changes in the reference standard solutions in the small cups. Thus the apparatus is not only reliable and stable, but is also suitable for experiments on solutions of viscous, complex and unstable solutes. The equilibration time of the experiments is fast, which is practical for samples of molalities less than 0.05 mol · kg⁻¹. Moreover, with the new apparatus it is easy to determine the end point of the equilibration.     

Comparison of similarity coefficients for clustering and compound selection

Recent studies into the use of a selection of similarity coefficients, when applied to searches of chemical databases represented by binary fingerprints, have shown considerable variation in their retrieval performance and in the sets of compounds being retrieved. The main factor influencing performance is the density distribution of the bitstrings for the active class, a feature which is closely related to molecular size. If this is the case when these coefficients are applied to similarity searches, then we would expect considerable variation in performance when applied to dissimilarity methods, namely clustering and compound selection. Here we report on several studies which have been undertaken to investigate the relative performance of 13 association and correlation coefficients, which have been shown to exhibit complementary performance in similarity searches, when applied to hierarchical and nonhierarchical clustering methods and to a compound selection methodology. Results suggest that the correlation coefficients perform consistently well for clustering and compound selection, as does the Baroni-Urbani/Buser association coefficient. Surprisingly, these often outperform the Tanimoto coefficient, while the Simple Match (effectively the complement of the Squared Euclidean Distance) performs very poorly.     

Virial coefficients and osmotic pressure in polymer solutions in good-solvent conditions

We determine the second, third, and fourth virial coefficients appearing in the density expansion of the osmotic pressure Pi of a monodisperse polymer solution in good-solvent conditions. Using the expected large-concentration behavior, we extrapolate the low-density expansion outside the dilute regime, obtaining the osmotic pressure for any concentration in the semidilute region. Comparison with field-theoretical predictions and experimental data shows that the obtained expression is quite accurate. The error is approximately 1%-2% below the overlap concentration and rises at most to 5%-10% in the limit of very large polymer concentrations.     

Partial molar volumes and activity coefficients of the water in aqueous polyol solutions and the osmotic pressures of these solutions

The freezing-point depression and density of aqueous polyol (alditol) solutions were measured, and the osmotic pressure and the partial molar volume of the water of these solutions were calculated. The osmotic pressures calculated from the freezing-point depression data were compared with those calculated with van't Hoff's equation and fairly good agreement was found. The partial molar volumes of the water in the solutions were equal or almost equal to the molar volume of pure water up to the highest concentrations examined. Also, the activity coefficient of the water was unity or almost unity up to the highest concentrations examined.     

Fugacity coefficients for water vapor

The fugacity coefficient of water vapor were calculated along the saturation line at temperatures from 273.16 to 647.14 K and at supercritical temperatures from 673.15 to 1073.15 K and pressures from 5 to 1000 MPa.     

From ar clustering dynamics to Ar solvation for Na+-benzene

The gradual evolution from cluster rearrangement to solvation dynamics is discussed by considering the rearrangement of n (n = 1, ..., 19) Ar atoms around Na+-benzene clusters and using an atom-bond potential energy surface. The nature of the bonding is discussed on the basis of the decomposition of the interaction energy and of the formation of the possible conformers. The benzene molecule is found to remain strongly bound to Na+ independently of the number of solvating rare-gas atoms, although due to the anisotropy of the interaction potential, the Ar atoms solvate the Na+-benzene cluster preferentially on the side of the cation. Other specific features of the solvation process are discussed.     

The osmotic and activity coefficients of some guanidinium salts at 298.15 K

Osmotic and activity coefficients are reported for aqueous solutions of six guanidinium salts at (298.15 ± 0.01) K, and these coefficients are compared with those of other uni-univalent electrolytes. The results are consistent with previously reported spectral results that indicated hydrogen bonding of guanidinium ion with chloride ion in aqueous solutions.     

Higher-order virial coefficients of water models

We use the Mayer sampling method, with both direct and overlap sampling, to calculate and compare classical virial coefficients up to B6 for various water models (SPC, SPC/E, MSPC/E, TIP3P, and TIP4P). The precision of the computed values ranges from 0.1% for B2 to an average of 25% for B6. When expressed in a form scaled by the critical properties, the values of the coefficients for SPC water are observed to greatly exceed the magnitude of corresponding coefficients for the simple Lennard-Jones model. We examine the coefficients in the context of the equation of state and the Joule-Thomson coefficient. Comparisons of these properties are made both to established molecular simulation data for each respective model and to real water. For all models, the virial series up to B5 describes the equation of state along the saturated vapor line better than the series that includes B6. At supercritical temperatures, however, the sixth-order series often describes pressure-volume-temperature behavior better than the fifth-order series. For example, the sixth-order virial equation of state for SPC/E water predicts the 673 K isotherm within 8% of published molecular simulation values up to a density of 9 mol/L (roughly half the critical density of SPC/E water).     

Determination of osmotic coefficients in polyelectrolyte solutions by potentionmetric measurements during phase separation

A method of evaluating osmotic coefficients of polyelectrolyte solutions is suggested for systems in which a two-phase equilibrium is maintained between the polymer solution and a polymer precipitate so that the “polymer” terms C_pdμ_p in the Gibbs-Duhem relationship may be neglected. The method is applied to a polybase system, and the osmotic coefficients calculated from the experimental data are discussed in terms of the theory of Alexandrowicz. The results are found to be in line with the above theory but the characteristic parameter ?_p, the fraction of free counterions, is about twice as large as values previously observed in polyacids.     

Calculation of the osmotic and activity coefficients of seawater at 25°C

The equation of Reilly, Wood, and Robinson was used to predict the osmotic coefficient of seawater and of its concentrates at molal ionic strengths of 0.5 to 6.0 at 25°C. The results agree closely with experimental data at ionic strengths below 5. The average difference in osmotic coefficients over the entire concentration range is 0.0014. The only serious discrepancy is at an ionic strength of 6, where a difference of 0.0068 is found. The accuracy of the predictions of osmotic coefficients prompted the calculation of the activity coefficients of NaCl, Na₂SO₄, MgCl₂, MgSO₄, KCl, and K₂SO₄ in the mixture. The calculated activity coefficients of NaCl and Na₂SO₄ agree within experimental error with previous measurements. This agreement demonstrates the prediction of activity and osmotic coefficients for complex mixtures.     

Viscosity B coefficients of polyethyleneglycols in water

The viscosity B coefficients of polyethylene glycols (M=62–1000) are determined at 25 °C. The B coefficient increases non-linearly with the number of ethyleneoxide (EO) units. The increase of the B coefficient per EO(0.111 dm³/mole) is less than the B value for two methylene groups (0.160 dm³/mole). This is discussed in terms of changes in the configurations of polyethylene glycols with long EO chains.Molecular size is the major factor that contributes to B at shorter chains, but solvation (hydration) becomes dominant as the number of ethyleneoxide groups increases. The hydration parameter,(gH₂O/g ethyleneglycol), shows a linear dependence on B at low mass followed by a non-linear increase at high molecular mass and the viscosity C coefficient accounts for the solute-solute interactions.Symbols absolute viscosity - _d absolute viscosity of dispersion medium - _r relative viscosity - _sp specific viscosity - ¦ _o ¦ intrinsic viscosity at infinite dilution - ¦ _c ¦ intrinsic viscosity as a function of solute concentration - partial specific volume - volume fraction - hydration (weight of H₂O hydrating 1 g of polyethylene glycol) - _c hydration as a function of solute concentration - K shape function - K _c shape function as a function of solute concentration     

Lanthanide salts solutions: representation of osmotic coefficients within the binding mean spherical approximation

Osmotic coefficients of aqueous solutions of lanthanide salts are described using the binding mean spherical approximation (BIMSA) model based on the Wertheim formalism for association. The lanthanide(III) cation and the co-ion are allowed to form a 1-1 ion pair. Hydration is taken into account by introducing concentration-dependent cation size and solution permittivity. An expression for the osmotic coefficient, derived within the BIMSA, is used to fit data for a wide variety of lanthanide pure salt aqueous solutions at 25 degrees C. A total of 38 lanthanide salts have been treated, including perchlorates, nitrates, and chlorides. For most solutions, good fits could be obtained up to high ionic strengths. The relevance of the fitted parameters has been discussed, and a comparison with literature values has been made (especially the association constants) when available.     

Mechanism of water transport in controlled porosity osmotic devices

Controlled drug delivery devices that release the drug from an osmotic core by a pumping mechanism are promising for the oral administration of water-soluble drugs. This study considers various possible paths for the transport of water through a porous cellulose acetate membrane which constitutes the outer wall of a particular osmotic device. It is shown that the mechanism of water transport is not solely by diffusion through the semipermeable cellulose acetate or water-filled voids: solute-excluding water-filled regions in the coat as well as the polymeric portion of the coat are important in water ingress into the osmotic pump.