Water’s unusual thermodynamics in the realm of physical Chemistry

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGWhile it is known since the early work by Edsall, Frank and Evans, Kauzmann, and others that the thermodynamics of solvation of nonpolar solutes in water is unusual and has implications for the thermodynamics of protein folding, only recently have its connections with the unusual temperature dependence of the density of solvent water been illuminated. Such density behavior is, in turn, one of the manifestations of a nonstandard thermodynamic pattern contemplating a second, liquid−liquid critical point at conditions of temperature and pressure at which water exists as a deeply supercooled liquid. Recent experimental and computational work unambiguously points toward the existence of such a critical point, thereby providing concrete answers to the questions posed by the 1976 pioneering experiments by Speedy and Angell and the associated “liquid−liquid transition hypothesis” posited in 1992 by Stanley and co-workers. Challenges of this phenomenology to the branch of Statistical Mechanics remain.Agencia Estatal de Investigación | Ref. PID2020-115722GB-C2

Liquid-liquid criticality in TIP4P/2005 and three-state models of water

Molecular dynamics simulations leading to the isothermal compressibility, the isobaric thermal expansivity, and the isobaric heat capacity of TIP4P/2005 water are found to be consistent with the coordinates of its second, liquid–liquid critical point reported recently by Debenedetti et al. [ Science 2020, 369, 289−292]. In accord with the theory of critical phenomena, we encounter that the rise in the magnitude of these response functions as temperature is lowered is especially marked along the critical isochore. Furthermore, response-function ratios provide a test for thermodynamic consistency at the critical point and manifest nonuniversal features sharply distinguishing liquid–liquid from standard gas–liquid criticality. The whole pattern of behavior revealed by simulations is qualitatively the same as the one of a three-state Ising model of water exhibiting a low-temperature liquid–liquid critical point. Exact solutions for the two-state components of such a three-state model are also provided.Agencia Estatal de Investigación | Ref. PID2020-115722GB-C22Universidade de Vigo/CISU