In this work, we explored, via molecular dynamics simulations, layer-wise structural and spatio-temporal heterogeneity features of confined water inside rigid spherical reverse micelles of 55 Å inner diameter. These confined aqueous pools were divided into four fictitious concentric layers of 5 Å thickness and a central core layer. Reverse micellar confinements were constructed using model potentials mimicking AOT (charged) and IGEPAL (neutral) surfactant molecules for encapsulating SPC/E water. Density profiles for confined water were obtained and compared to validate the present simulations. The simulated layer-wise structural features were: dipole orientation distributions, tetrahedral angle distributions, tetrahedral order parameter, and the average number of H-bonds per water molecule and the relevant population distributions. Simulated dynamical features included mean-square displacements, velocity autocorrelation functions, non-Gaussian parameters, single-particle displacement distributions, dynamic susceptibilities, and the collective single-particle reorientational relaxations of first and second ranks. Analyses of simulation results revealed a strong impact of the confinement on bulk water structure and dynamics. The chemical nature of the confinement was found to influence both structure and dynamics. Interfacial water molecules were found to be the most severely affected ones, and the successive progression toward the center revealed a tendency for restoration of the bulk limit, although the bulk values were never fully recovered. A close inspection of the simulated results revealed an overlap among the layer-wise structural and dynamical features. These observations suggest a breakdown of the two-state core–shell model even for large reverse micelles (RMs) where an ample amount of “free” water is available. The simulated collective reorientational relaxations of reverse micellar water agree well with the existing time-resolved two-dimensional infrared (2D-IR) measurements.

中文翻译：

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大型反胶束水的动态磁化率和结构异质性：通过探测层状特征检验核-壳模型

在这项工作中，我们通过分子动力学模拟探索了内径55Å的刚性球形反胶束内承压水的分层结构和时空异质性。这些封闭的水库被分为四个厚度为5Å的虚拟同心层和一个中央核心层。使用模拟AOT（带电）和IGEPAL（中性）表面活性剂分子的模型电势来构造胶束反向约束，以封装SPC / E水。获得了承压水的密度分布图，并将其进行比较以验证当前的模拟。模拟的逐层结构特征是：偶极子取向分布，四面角分布，四面体阶数参数，每个水分子的H键平均数和相关的种群分布。模拟的动力学特征包括均方位移，速度自相关函数，非高斯参数，单粒子位移分布，动态磁化率以及第一和第二等级的集体单粒子重新定向松弛。对模拟结果的分析表明，该限制对大量水的结构和动力学有很大的影响。发现该禁闭的化学性质影响结构和动力学。界面水分子被发现是受影响最严重的分子，尽管其体积值从未完全恢复，但朝中心的连续进展表明恢复了体积极限的趋势。对模拟结果的仔细检查发现，分层结构和动力学特征之间存在重叠。这些观察结果表明，即使有大量的“游离”水可用的大型反胶束（RM），也无法对两态核-壳模型进行分解。反向胶束水的模拟集体方向性弛豫与现有的时间分辨二维红外（2D-IR）测量非常吻合。