摘要/Abstract
受限条件下水的介电性质因测量极具挑战，其在诸多电化学过程与反应输运过程中如何扮演关键角色从未被定量地澄清.本工作利用平衡态分子动力学模拟和受限体系介电性质计算方法，系统性地探索了0.65 nm限域尺寸、5×108 Pa限域压强、不同温度条件下单分子受限冰和受限水的介电性质.详细比较了恒定偶极矩SPC/E水分子模型和可极化的SWM4-NDP水分子模型在描述受限冰、水结构与介电性质上的优劣势，包括统计分析SWM4-NDP模型模拟的单分子层受限水和受限冰的瞬时分子偶极矩概率密度分布，计算每个模拟体系的静态结构因子、静态偶极空间关联函数、静态介电常数、体系偶极时间关联函数和德拜弛豫时间.首次发现了极化水分子模型描述的低维度受限水和受限冰的奇异分子极性变化，并观察到两种模型描述静态结构性质的效果相当，SWM4-NDP模型对于静态介电常数描述的优势会因受限条件的增强而被大幅削减.但在受限水介电极化弛豫动力学性质描述上SWM4-NDP模型明显优于SPC/E模型.我们推断SWM4-NDP模型在探索受限水结构相变动力学以及受限体系离子输运和溶剂化动力学等过程的模拟研究中是比SPC/E模型更好的选择.本工作将在进一步开展基于受限水系统储能、传感、输运的设计工作中提供一定的理论指导意义.
关键词: 受限水, 方冰, 极化水模型, 分子动力学, 介电常数
The direct measurement of the dielectric properties of the confined water is exceedingly challenging, result in the lack of a quantitative understanding of its critical roles in electrochemistry, interfacial reactivity and transport thermodynamics. In this paper, we employ the equilibrium molecular dynamics simulation and the linear response theory-based analytical expressions for the local permittivity tensor, to calculate the static and dynamic dielectric response properties of the monolayer ice and water confined in the 0.65 nm size hydrophobic slab pore under 5×108 Pa lateral pressure and different temperatures. We carry out a detailed comparative study on the performance of predicting the confined structure and dielectric response properties between two well known water molecule models, i.e., constant dipole moment SPC/E model and polarizable SWM4-NDP water model. We have analyzed the probability distributions of the instantaneous SWM4-NDP water molecular dipole moments and calculated the static structure factor, radial dipole-dipole correlation function, static dielectric tensor, total dipole autocorrelation function and Debye relaxation time of each simulation system. For the first time, we found the novel variation of the water molecular polarities, in the monolayer confined liquid and solid phase of water, due to the extreme confinement condition. The performance in describing the structural properties are found comparable between the two water models, and the enhancement of the confinement weakens the advantage of the SWM4-NDP model in predicting the static dielectric property. However, in the prediction of the dynamic properties such as dielectric relaxation time, SWM4-NDP water model is superior to the SPC/E model. Therefore, we suggest that using SWM4-NDP model in the future investigation of the structural phase transition kinetics, ionic transportation and solvation kinetics would be the better choice. The current achievement of the fundamental insight and computational data could potentially facilitate the theoretical advancements in designing new devices of energy storage, sensor, and medicine delivery based on confined water systems.
Key words: confined water, square ice, polarizable water model, molecular dynamics, dielectric property
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