限域单分子层冰-水两相平衡体系的分子动力学模拟

Molecular Dynamics Simulation of Monolayer Confined Ice-Water Phase Equilibrium

限域水因有极其丰富的结构物相变化而成为近年来水科学研究的一个热点.然而,不同相限域水之间的相平衡结构与性质却鲜有报道.论文提出一套分子动力学模拟技术,可实现纳米尺度限域条件下冰和水的不同结构相间形成的低维固-液界面(线)的平衡态模拟.应用此模拟技术,我们探索了0.65 nm限域尺寸、5000 bar限域压强条件下,单分子层厚度的冰-水(固-液)两相平衡,计算了该平衡体系一系列热力学量在界线附近的分布.平衡态的分子模拟结果直观地展示了粗糙型固-液界线的热毛细涨落、界线固-液结构转变的微观机制、以及缺陷在固-液相变区附近的形成与输运.各种热力学量分布函数呈现了二维限域冰-水共存界面(线)的特殊性质,如:相平衡区域的尺寸异于块体材料固-液界面,固-液界线处于切向压缩状态等.

Confined water became a recent hot topic in water science due to its extremely abundant structural phase behavior. However, there exist few studies focused on the coexistence of two or more confined water phases and their related properties. We present a methodology for studying the coexistences of two confined phases of water, based on a series equilibrium molecular-dynamics（MD） simulations using isobaric-isoenthalpic ensembles to iteratively predict the melting temperatures of the low dimensional confined crystal phase of water. The methodology is applied to the coexistence of the monolayer ice and water（described with a simple water model, i.e. SPC/E model） confined in the 0.65 nm size pore, yielding a direct determination of the melting point and extensive atomic-scale characterization for the mono-molecular layer containing the confined ice-water coexistence line. A finite value of lateral pressure（5000 bar） is adopted in the simulation, to mimic the high-pressure environment of the water molecules confined in the bi-graphene pocket in a recent experiment by Algara-Siller et al. [Nature, 519, 443（2015）]. The rough structural type and the capillary fluctuation of the line, the microscopic mechanism of the solid-liquid structural transition along the line, as well as the transport of the point defect in the solid side of the coexistence line are identified directly from the MD trajectories. Various profiles of different thermodynamic properties across the coexistence line illustrate the unique features for the in-plane coexistence of the monolayer confined ice-water system, e.g., the unexpected large width of the crystal-melt transition region, and the compression state along the solid-liquid phase coexistence line. The methodology presented in the current study can be easily applied to the coexistence of multilayer confined ice and water phases, as well as the many other types of water models beyond the SPC/E used in current work. The achievement of the low dimensional confined ice-water phase coexistence could potentially facilitate the fundamental advancements in thermodynamics and kinetic theories of the low dimensional water science.