Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, JD Block, Sector-III, Salt Lake, Kolkata 700106, India.
Indian Institute of Science Education and Research, Mohanpur, Nadia, Kolkata 741246, India.
J Phys Chem B. 2020 Apr 9;124(14):2848-2863. doi: 10.1021/acs.jpcb.9b11895. Epub 2020 Mar 31.
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.
在这项工作中，我们通过分子动力学模拟探索了刚性球形反胶束（内直径为 55Å）内部受限水的层状结构和时空异质性特征。这些受限水腔被分为四个虚构的同心层，每层厚 5Å，还有一个中心核层。反胶束限制使用模拟 AOT（带电）和 IGEPAL（中性）表面活性剂分子包封 SPC/E 水的模型势来构建。获得了受限水的密度分布，并进行了比较，以验证本模拟的结果。模拟的层状结构特征包括偶极取向分布、四面体角分布、四面体有序参数、每个水分子的平均氢键数以及相关的分布。模拟的动力学特征包括均方根位移、速度自相关函数、非高斯参数、单个粒子位移分布、动态灵敏度和一阶和二阶的集体单粒子重取向松弛。对模拟结果的分析表明，限制对体相水的结构和动力学有很大的影响。限制的化学性质被发现同时影响结构和动力学。界面水分子是受影响最严重的，并且向中心的连续推进显示出恢复体相极限的趋势，尽管体相值从未完全恢复。对模拟结果的仔细检查揭示了层状结构和动力学特征之间的重叠。这些观察结果表明，即使对于具有大量“自由”水的大反胶束，两态核壳模型也会失效。模拟的反胶束水的集体重取向松弛与现有的时间分辨二维红外（2D-IR）测量结果吻合良好。
