Harpham Michael R, Ladanyi Branka M, Levinger Nancy E
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Phys Chem B. 2005 Sep 8;109(35):16891-900. doi: 10.1021/jp052773l.
In this study, mobility and structure of water molecules in Aerosol OT (bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles with water content w0 = 5 and Na+, K+, Cs+ counterions have been explored with molecular dynamics (MD) simulations. Using the Faeder/Ladanyi model (J. Phys. Chem. B, 2000, 104, 1033) of the reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function, FS(Q,t), for water hydrogen atoms that could be measured in a quasielastic neutron scattering experiment. Separate intermediate scattering functions FRS(Q,t) and FCMS(Q,t) were determined for rotational and translational motion. We find that the decay of FCMS(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior arises from decreased water mobility for molecules close to the interface and from confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay, which is consistent with relatively rapid restricted rotation and slower rotational relaxation over the full angular range. Rotational relaxation is anisotropic, with the O-H bond short-time rotational mobility considerably higher than that of the molecular dipole. This behavior is related to the decreased density of water-water hydrogen bonds in the vicinity of the interface compared to core or bulk water. We find that the interfacial mobility of water molecules is quite different for the three counterion types, but that the core mobility exhibits weak counterion dependence. Differences in interfacial mobility are strongly correlated with structural features, especially ion-water coordination, and the extent of disruption by the counterions of the water hydrogen bond network.
在本研究中，利用分子动力学（MD）模拟探索了水含量w0 = 5且含有Na⁺、K⁺、Cs⁺抗衡离子的气溶胶OT（双（2-乙基己基）磺基琥珀酸酯，AOT）反胶束中水分子的流动性和结构。使用反胶束内部的Faeder/Ladanyi模型（《物理化学杂志B》，2000年，104卷，1033页），进行MD模拟以计算可在准弹性中子散射实验中测量的水氢原子的自中间散射函数FS(Q,t)。分别确定了旋转和平移运动的中间散射函数FRS(Q,t)和FCMS(Q,t)。我们发现FCMS(Q,t)的衰减是非指数形式的，并且我们对MD数据的分析表明，这种行为源于靠近界面的分子的水流动性降低以及平移位移范围的限制诱导限制。旋转弛豫也表现出非指数衰减，这与在整个角度范围内相对快速的受限旋转和较慢的旋转弛豫一致。旋转弛豫是各向异性的，O-H键的短时间旋转流动性远高于分子偶极的流动性。这种行为与界面附近与核心或本体水相比水-水氢键密度降低有关。我们发现，对于三种抗衡离子类型，水分子的界面流动性有很大不同，但核心流动性表现出较弱的抗衡离子依赖性。界面流动性的差异与结构特征密切相关，特别是离子-水配位以及抗衡离子对水氢键网络的破坏程度。
