Temperature and length scale dependence of solvophobic solvation in a single-site water-like liquid

Abstract

The temperature and length scale dependence of solvation properties of spherical hard solvophobic solutes is investigated in the Jagla liquid, a simple liquid that consists of particles interacting via a spherically symmetric potential combining a hard core repulsion and a longer ranged soft core interaction, yet exhibits water-like anomalies. The results are compared with equivalent calculations for a model of a typical atomic liquid, the Lennard-Jones potential, and with predictions for hydrophobic solvation in water using the cavity equation of state and the extended simple point charge model. We find that the Jagla liquid captures the qualitative thermodynamic behavior of hydrophobic hydration as a function of temperature for both small and large length scale solutes. In particular, for both the Jagla liquid and water, we observe temperature-dependent enthalpy and entropy of solvation for all solute sizes as well as a negative solvation entropy for sufficiently small solutes at low temperature. This feature of water-like solvation is distinct from the strictly positive and temperature independent enthalpy and entropy of cavity solvation observed in the Lennard-Jones fluid. The results suggest that, compared to a simple liquid, it is the presence of a second thermally accessible repulsive energy scale, acting to increasingly favor larger separations for decreasing temperature, that is the essential characteristic of a liquid that favors low-density, open structures, and models hydrophobic hydration, and that it is the presence of this second energy scale that leads to the similarity in the behavior of water and the Jagla liquid. In addition, the Jagla liquid dewets surfaces of large radii of curvature less readily than the Lennard-Jones liquid, reflecting a greater flexibility or elasticity in the Jagla liquid structure than that of a typical liquid, a behavior also similar to that of water's hydrogen bonding network. The implications of the temperature and length scale dependence of solvation free energies in water-like liquids are explored with a simple model for the aggregation of solvophobic solutes. We show how aggregate stability depends upon the size of the aggregate and the size of its constituent solutes, and we relate this dependence to cold-induced destabilization phenomena such as the cold-induced denaturation of proteins.

Scaled-particle theory analysis of cylindrical cavities in solution

“…The growth of the contact correlations appears to approach an asymptotic plateau with increasing radius, consistent with the expected contact density for a flat interface of f3P/ p = 7.75 (Table I) as dictated by the wall theorem [40]. Compared against the spherical solute, the cylinder's contact correlation (13) and (18)], the fit of spherical RSPT to the spherical cavity simulations [11,14], and predictions of the cylindrical contact correlation using the end cap approximation [Eq. (12)].…”

“…Following Stillinger, Ashbaugh and Pratt developed a revised SPT (RSPT) in which multibody solvent correlations determined from simulations are used to provide an improved description of molecular-scale solvation. Revised SPT has successfully been applied to quantitatively evaluate the solvation free energy of spherical solutes in water [12] and waterlike [13] solvents as well as Lennard-Jones [11,14] and organic solvents [15,16].…”

Properties of a soft-core model of methanol: An integral equation theory and computer simulation study

“…Recently, it has been shown that many structural, thermodynamic, and dynamic properties of water can be reproduced using soft-core spherically symmetrical potentials [7][8][9][10] although certain tradeoffs are necessary due to the transferability issue. 11,12 Angular dependence of hydrogen bonding is necessarily forgone and compensated for with two characteristic lengths that these models must have.…”

The hydrophobic effect in a simple isotropic water-like model: Monte Carlo study

“…The same behavior is encountered when using Jagla potential or cavity equation of state (C-EoS) for water. 60 A different solvation mechanism for small and large particles is proposed as an explanation for this behavior.…”