WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan.
J Phys Chem B. 2011 Dec 8;115(48):14210-6. doi: 10.1021/jp205045k. Epub 2011 Sep 16.
Water confined in narrow nanopores that prevent ice crystallization is usually studied as a means to understand the anomalous behavior of bulk liquid water. Nevertheless, there is no agreement on the similarity of the thermodynamics of bulk and nanoconfined liquid water. In this work, we use molecular dynamics simulations with the mW water model to investigate the phase behavior of liquid water in bulk and confined in a 1.5 nm cylindrical pore with water-surface interactions identical to water-water interactions. Through analysis of the isochors of bulk liquid water V(T,p) we extrapolate the locus of a putative liquid-liquid critical point (LLCP) in bulk mW water at 190 K and 1215 atm. This is a "virtual LLCP", as it would lie in a region of the phase diagram where fast crystallization of water impedes equilibration of the liquid. We find that confinement has a weak effect on the loci of the thermodynamic anomalies: the maxima in density and heat capacity of confined water occur at T,p similar to that in bulk. The heat capacity peak of confined water is due to a transformation within the confined liquid; we verify that ice does not form in the pores. Confined mW water presents a heat capacity maximum C(p)(max)(p) up to the highest pressure we investigated, 4000 atm. The magnitude of the heat capacity peak C(p)(max)(p) has a nonmonotonous dependence with pressure, attaining a maximum at conditions close to those of the locus of the bulk water's virtual LLCP. We do not find, however, direct evidence of a first-order liquid-liquid transition in confined water for pressures above or below the locus of the maximum response function. The extreme value of the response functions of confined water could be a rounded manifestation of an equivalent feature in the free energy surface of bulk water.
水被限制在阻止冰晶形成的狭窄纳米孔中，通常被用作理解体相液体水异常行为的一种手段。然而，对于体相和纳米受限液体水的热力学相似性，尚无共识。在这项工作中，我们使用具有 mW 水模型的分子动力学模拟来研究体相和受限在具有与水-水相互作用相同的水-表面相互作用的 1.5nm 圆柱形孔中的液体水的相行为。通过分析体相液体水 V(T,p)的等压线，我们推断出在 190K 和 1215atm 下，体相 mW 水中假定的液-液临界点 (LLCP) 的位置。这是一个“虚拟 LLCP”，因为它位于相图的一个区域，在该区域中水的快速结晶会阻碍液体的平衡。我们发现，受限对热力学异常的位置有微弱的影响：受限水的密度和热容最大值发生在 T,p 类似于体相的位置。受限水的热容峰值是由于受限液体内部的转变；我们验证了冰不会在孔中形成。受限 mW 水在我们研究的最高压力 4000atm 下，表现出最大热容 C(p)(max)(p)。热容峰值 C(p)(max)(p)的大小与压力呈非单调依赖性，在接近体相水虚拟 LLCP 位置的条件下达到最大值。然而，我们没有在压力高于或低于最大响应函数位置的受限水中发现直接的一阶液-液转变的证据。受限水的响应函数的极值可能是体相水自由能表面等效特征的圆滑表现。
