Water’s Thermal Pressure Drives the Temperature Dependence of Hydrophobic Hydration

Abstract

With the aid of literature experimental data and reported results from molecular simulation, two thermodynamic relations are found to provide a theoretical basis for the understanding of a variety of characteristic features associated with the solvation of small nonpolar molecules in water. Thus, the large and positive solvation heat capacity, enthalpy-entropy compensation, the solubility minimum and solvation free energy maximum with respect to temperature, enthalpy convergence, and entropy convergence are rationalized in a unified way. Our key finding is that all of these phenomena are driven by the thermal pressure coefficient of pure water, which, via the isobaric thermal expansivity and the isothermal compressibility, reflects its unusual thermodynamics. Remarkably, the solubility minimum is found to be a direct consequence of water's density maximum.

Temperature, Pressure, and Length-Scale Dependence of Solvation in Water-like Solvents. I. Small Solvophobic Solutes

“…The thermodynamics of solvation of nonpolar, solvophobic solutes in liquid water has been described in a number of authoritative reports. − One aspect of the problem that we are currently addressing concerns the role played by solvent water itself. , Specifically, we are trying to accurately establish to which extent is the thermodynamics of aqueous solvation of nonpolar solutes driven by the thermodynamics of liquid water. In the first part of this series, we approach this issue for small hard-sphere solutes interacting with solvent molecules via harsh repulsive forces.…”

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

“…21 This is due to a large and positive temperature derivative of the Tα p v ̅ p term relying on solvent's water-like unusually large and positive (∂α p /∂T) p . 13 Larger solutes characterized by larger v ̅ p values lead to steeper Tα p v ̅ p versus T curves. However, Tα p v ̅ p = 0 at T = T MD for any solute because α p (T MD ) = 0.…”

“…This implies that the results of this series regarding the role of water-like unusual thermodynamics on solvation might remain essentially the same when such solute–solvent attractive interactions are present. Accordingly, our previous analysis of experimental data for the solvation of short hydrocarbons and noble gases in water evidences that solvent’s water-like unusual thermodynamics manifests basically in the same way for small hard spheres and for real hydrophobic molecules such as methane or argon interacting with water via weak dispersion forces. Conversely, a recent report on polypeptide hydration claims that peptide–solvent attractive interactions dominate the temperature dependence of the solvation free energy, albeit it also points out that the solvent’s isobaric thermal expansivity must be taken into account.…”

“…Certainly, the steep increase in the s̅ p * ( T ) curve in Figure implies a large and positive isobaric solvation heat capacity, C̅ pp * ≡ T (∂ s̅ p * /∂ T ) p , historically regarded the first manifestation of the unusual pattern of solvation of nonpolar solutes in water . This is due to a large and positive temperature derivative of the T α p v̅ p term relying on solvent’s water-like unusually large and positive (∂α p /∂ T ) p …”

“…All these are reflections of an overall pattern of behavior that also results in an intriguing pressure dependence and distinguishes water and water-like fluids from standard, "simple" fluids. 8−11 Consistent with recent reports, 12,13 as well as with standard theories of solvation, 14−20 we proved in ref 1 that solvent's water-like unusual thermodynamics is crucial to the T and p dependence of the solvation quantities of hard spheres as small as methane or argon. We shall henceforth explain this statement in a detailed manner with the aid of Figures 2 and 3, which illustrate the pattern of solvation of small hard-sphere solutes reported in ref 1.…”