Tan Howe-Siang, Piletic Ivan R, Fayer M D
Department of Chemistry, Stanford University, California 94305, USA.
J Chem Phys. 2005 May 1;122(17):174501. doi: 10.1063/1.1883605.
The time-resolved orientational anisotropies of the OD hydroxyl stretch of dilute HOD in H(2)O confined on a nanometer length scale in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles are studied using ultrafast infrared polarization and spectrally resolved pump-probe spectroscopy, and the results are compared to the same experiments on bulk water. The orientational anisotropy data for three water nanopool sizes (4.0, 2.4, and 1.7 nm) can be fitted well with biexponential decays. The biexponential decays are analyzed using a wobbling-in-a-cone model that involves fast orientational diffusion within a cone followed by slower, full orientational relaxation. The data provide the cone angles, the diffusion constants for motion within the cones, and the final diffusion constants as a function of the nanopool size. The two processes can be interpreted as a local angular fluctuation of the OD and a global hydrogen bond network rearrangement process. The trend in the relative amplitudes of the long and short exponential decays suggest an increasing rigidity as the nanopool size decreases. The trend in the long decay constants indicates a longer hydrogen bond network rearrangement time with decreasing reverse micelle size. The anisotropy measurements for the reverse micelles studied extrapolate to approximately 0.33 rather than the ideal value of 0.4, suggesting the presence of an initial inertial component in the anisotropy decay that is too fast to resolve. The very fast decay component is consistent with initial inertial orientational motion that is seen in published molecular-dynamics simulations of water in AOT reverse micelles. The angle over which the inertial orientational motion occurs is determined. The results are in semiquantitative agreement with the molecular-dynamics simulations.
利用超快红外偏振和光谱分辨泵浦 - 探测光谱技术，研究了在双（2 - 乙基己基）磺基琥珀酸钠（AOT）反胶束中受限在纳米长度尺度上的H₂O中稀HOD的OD羟基拉伸的时间分辨取向各向异性，并将结果与对本体水进行的相同实验进行比较。三种水纳米池尺寸（4.0、2.4和1.7纳米）的取向各向异性数据可以用双指数衰减很好地拟合。使用圆锥内摆动模型分析双指数衰减，该模型涉及圆锥内的快速取向扩散，随后是较慢的完全取向弛豫。数据给出了圆锥角、圆锥内运动的扩散常数以及作为纳米池尺寸函数的最终扩散常数。这两个过程可以解释为OD的局部角波动和全局氢键网络重排过程。长、短指数衰减相对幅度的趋势表明，随着纳米池尺寸减小，刚性增加。长衰减常数的趋势表明，随着反胶束尺寸减小，氢键网络重排时间变长。所研究的反胶束的各向异性测量外推到约0.33，而不是理想值0.4，这表明在各向异性衰减中存在一个初始惯性分量，其衰减太快而无法分辨。非常快的衰减分量与在已发表的AOT反胶束中水的分子动力学模拟中看到的初始惯性取向运动一致。确定了发生惯性取向运动的角度。结果与分子动力学模拟半定量一致。
