Solid−liquid critical behavior of water in nanopores

Abstract

Nanoconfined liquid water can transform into low-dimensional ices whose crystalline structures are dissimilar to any bulk ices and whose melting point may significantly rise with reducing the pore size, as revealed by computer simulation and confirmed by experiment. One of the intriguing, and as yet unresolved, questions concerns the observation that the liquid water may transform into a low-dimensional ice either via a first-order phase change or without any discontinuity in thermodynamic and dynamic properties, which suggests the existence of solid−liquid critical points in this class of nanoconfined systems. Here we explore the phase behavior of a model of water in carbon nanotubes in the temperature−pressure− diameter space by molecular dynamics simulation and provide unambiguous evidence to support solid−liquid critical phenomena of nanoconfined water. Solid−liquid first-order phase boundaries are determined by tracing spontaneous phase separation at various temperatures. All of the boundaries eventually cease to exist at the critical points and there appear loci of response function maxima, or the Widom lines, extending to the supercritical region. The finite-size scaling analysis of the density distribution supports the presence of both first-order and continuous phase changes between solid and liquid. At around the Widom line, there are microscopic domains of two phases, and continuous solid−liquid phase changes occur in such a way that the domains of one phase grow and those of the other evanesce as the thermodynamic state departs from the Widom line.T he possibility of the solid-liquid critical point has been reported by computer simulation studies of various systems in quasi-one, quasi-two, and three dimensions that exhibit both continuous and discontinuous changes in thermodynamic functions and other order parameters (1-7). However, the idea that a solid-liquid phase boundary never terminates at the critical point is still commonly accepted as a law of nature, largely because of the famous symmetry argument (8, 9) together with the lack of experimental observations. Furthermore, critical phenomena in quasi-1D systems are often considered impossible from a different point of view; that is, to begin with, there is no first-order phase transition in 1D systems as proved for solvable models (10) or shown by the phenomenological argument (9). Therefore, a thorough investigation is much needed to support or reject the possibility of the solid-liquid critical point. We examine the phase behavior of a model system of water confined in a quasi-1D nanopore (1,(11)(12)(13)(14)(15) and provide evidence to support the existence of first-order phase transitions and solid-liquid critical points. Results and DiscussionsFirst, we explore possible solid-liquid critical points of the confined water by calculating isotherms in the "pressure-volume" plane, where the pressure is actually P zz , a component of pressure tensor along the tube axis, or simply the axial pressure, and the volume is ℓ z , the length of simulatio...

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