Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy
Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy
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DOI：
10.1021/jp061065c
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发表时间：
2006-04-20
影响因子：
2.9
通讯作者：
Fayer, MD
中科院分区：
文献类型：
--
作者：
Piletic, IR;Moilanen, DE;Fayer, MD
核/壳模型经常被用来描述限制在反胶束内部的水。该模型适用于直径为1.7 ~ 28 nm （w(0) = 2 ~ 60）的AOT/异辛烷反胶束中包裹的水和散装水，通过四个实验观测值：羟基拉伸吸收光谱、振动种群弛豫时间、取向弛豫速率和光谱扩散动力学来研究。利用超快红外光谱分辨泵浦探针和振动回波光谱测量了随时间变化的观测值。当系统从散装水移动到最小的水纳米池w(0) = 2时，所有可观测值都出现了重大的渐进式变化。与散装水的反应相比，粒径小于7纳米的反胶束（w(0) = 20）的动力学很容易区分。结果还表明，可以使用核-壳模型定量预测大小相关的吸收光谱和种群松弛时间，其中核（纳米池内部）的性质被认为是散装水的性质，壳（与头基团相关的水）的性质被认为是w(0) = 2的性质。核和壳分量的加权和再现了尺寸依赖谱和非指数种群松弛动力学。然而，相同的模型不能再现光谱扩散和取向弛豫实验。提出当氢键结构重排（取向弛豫和光谱扩散）发生时，壳核之间的动力学耦合使纳米水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识别不同的氢键环境，而取向和光谱扩散相关函数预测，在反向胶束中，动力学是尺寸依赖的，而不是强烈的空间依赖。
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 (w(0) = 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, w(0) = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w(0) = 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 w(0) = 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.