期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：27
页码：8221-8226
DOI：10.1073/pnas.1422829112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceIt is commonly believed that the solid-liquid critical point does not exist, because of the famous symmetry argument and the lack of experimental observation so far. However, recently, the intriguing possibility of the critical point has been suggested for strongly confined substances. Here we perform molecular dynamics simulations of water confined in carbon nanotubes and provide unambiguous evidence of the solid-liquid critical point for water in the hydrophobic nanopore: macroscopic solid-liquid phase separation below a critical temperature Tc, diverging heat capacity and isothermal compressibility at around Tc, and the loci of response function maxima above Tc. We also give a molecular level explanation for how liquid water continuously freezes to ice along a thermodynamic path avoiding the first-order phase boundary. 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.
关键词：water ; solid−liquid critical point ; carbon nanotube ; ice ; Widom line
