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

Abstract

We use molecular simulation to determine solvation free energies, isochoric solvation energies and entropies, isobaric solvation enthalpies and entropies, partial molecular volumes, and isothermal density derivatives of the solvation free energy as a function of temperature and pressure for hard-sphere solutes with diameters ranging from 4 to 36 Å in TIP4P/2005 and Jagla water-like solvents exhibiting unusual thermodynamics. An important piece of our discussion focuses on the nanometer-sized solutes, for which simulation results are found to be accounted for by the most basic classical thermodynamic treatment contemplating bulk and interfacial contributions to the solvation free energy. Thus, since water’s liquid–vapor surface tension is only special inasmuch as it takes unusually large values, solvent’s water-like unusual thermodynamics manifests through a term proportional to the pressure in the solvation free energy. As a result, such solvent’s unusual thermodynamics is found to be relevant to the temperature and pressure dependence of the isochoric solvation energy and entropy as well as to the isothermal density derivative of the solvation free energy. This sharply contrasts with the findings of the first part of this series indicating that the solvation free energy of small hard spheres responds to temperature and pressure changes as solvent’s density does, with such a contrasting picture embodying a “pressure–density dichotomy.” As for the length-scale dependence, we find the zero nominal pressure and the solvent’s temperature of the maximum density as singular conditions for cavity surface-area size scaling of large solutes to occur for all solvation quantities. We finally argue that the overall study undertaken in this series suggests that water’s unusual thermodynamics may be relevant to the thermodynamic stability of clusters of solvophobic units in the temperature–pressure plane. Some comments on the role of solute–solvent attractive interactions are also depicted.

Water’s Unusual Thermodynamics in the Realm of Physical Chemistry

“…When it comes to solutes with typical dimensions of a few nanometers, water’s unusual thermodynamics manifests in the pattern of solvation in a sharply distinct way . For such large solutes, Classical Thermodynamics dictates that μ* is made up of a term varying with T much like solvent’s liquid–vapor surface tension σ lv and a second one proportional to p .…”

“…When it comes to solutes with typical dimensions of a few nanometers, water’s unusual thermodynamics manifests in the pattern of solvation in a sharply distinct way. 5 For such large solutes, Classical Thermodynamics dictates that μ* is made up of a term varying with T much like solvent’s liquid–vapor surface tension σ lv and a second one proportional to p . Since water’s σ lv is only unusual inasmuch as its value is large relative to that of common liquids, there is no any significant distinction between water and other solvents as the μ*( T ) behavior along isobars is concerned.…”