Water anomalous thermodynamics, attraction, repulsion, and hydrophobic hydration

Abstract

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.

Osmotic Second Virial Coefficients of Aqueous Solutions from Two-Component Equations of State

“…Finally, for the solvent water, as in ref , we add the hydrogen-bonding term of Poole et al to the original free energy in each equation of state . Such a term (see Appendix) is characterized by four parameters, two for volumetric effects, ρ HB and σ HB , and another two for energetic and entropic effects, ε HB and Ω, respectively …”

Glass polymorphism and liquid–liquid phase transition in aqueous solutions: experiments and computer simulations

“…The behavior of the LLCP upon the addition of hydrophobic solutes was also studied on theoretical grounds. 188,315,316 In ref. 315 and 316 the canonical partition function for a family of binary solutions is considered and the corresponding phase behaviors are explored using numerical calculations.…”

“…188,315,316 In ref. 315 and 316 the canonical partition function for a family of binary solutions is considered and the corresponding phase behaviors are explored using numerical calculations. The theoretical models for the solutions are based on the van der Waals model and complemented by terms to account for the presence of hydrogen bonds.…”

“…For the third set of parameters the critical line emanating from the LLCP merges with yet another of the five critical lines identified in the binary solution. 316 Cerdeiriña and Debenedetti 315 vary both a and b and find that the ratio a/b determines the magnitude and the direction of the LLCP shift as x is increased. Sufficiently large solutes shift the LLCP to higher T and lower p with increasing solute concentration.…”

“…The authors suggest that such a behavior might be found in solutions containing moderately large amphiphilic molecules. 315…”