Piletic Ivan R, Moilanen David E, Spry D B, Levinger Nancy E, Fayer M D
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Phys Chem A. 2006 Apr 20;110(15):4985-99. doi: 10.1021/jp061065c.
A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.
核/壳模型常被用于描述限制在反胶束内部的水。使用四个实验观测值，即羟基伸缩吸收光谱、振动布居弛豫时间、取向弛豫速率和光谱扩散动力学，研究了该模型对于直径在1.7至28纳米（w0 = 2 - 60）范围内的AOT/异辛烷反胶束中封装的水以及本体水的有效性。随时间变化的观测值通过超快红外光谱分辨泵浦 - 探测和振动回波光谱进行测量。当系统从本体水转变为最小的水纳米池（w0 = 2）时，所有观测值都会出现主要的渐进变化。与本体水的响应相比，直径小于7纳米（w0 = 20）的反胶束的动力学很容易区分。结果还表明，使用核 - 壳模型可以定量预测尺寸依赖性吸收光谱和布居弛豫时间，其中核（纳米池内部）的性质被视为本体水的性质，而壳（与头基相关的水）的性质被视为w0 = 2时的性质。核和壳成分的加权和再现了尺寸依赖性光谱和非指数布居弛豫动力学。然而，相同的模型无法再现光谱扩散和取向弛豫实验。有人提出，当涉及氢键结构重排（取向弛豫和光谱扩散）时，壳与核之间的动力学耦合会使水纳米池表现出更均匀的动力学。因此，吸收光谱和振动寿命衰减可以辨别不同的氢键环境，而取向和光谱扩散相关函数预测动力学是尺寸依赖性的，但在反胶束内空间依赖性不强。
