Balakrishnan Sangeetha, Javid Nadeem, Weingärtner Hermann, Winter Roland
Physical Chemistry II, Ruhr-University Bochum,D-44780 Bochum, Germany.
Chemphyschem. 2008 Dec 22;9(18):2794-801. doi: 10.1002/cphc.200800506.
The state of water confined in Aerosol-OT-hydrocarbon-water reverse micelles with cyclohexane, n-pentane, n-octane, and n-dodecane as apolar solvents is investigated by small-angle X-ray scattering and near-infrared vibrational spectroscopy of the first overtone of the OH stretching mode of water. The experiments focus on water/AOT molecular ratios W(0)=2-20, where water is strongly affected by the confinement and surface-water interactions. The pair-distance distribution functions derived from the small-angle scattering patterns allows a detailed characterization of the topology of these systems, and they indicate deviations from monodisperse, spherical water pools for some of these hydrocarbon systems. In contrast to a common assumption, the pool size does not scale linearly with W(0) in going from dry reverse micelles (W(0)-->0) to essentially bulk-like water (W(0)>20). The first overtone of the OH-stretching vibration exhibits highly structured spectra, which reveal significant changes in the hydrogen bonding environment upon confinement. The spectra are rationalized by a core/shell model developed by Fayer and co-workers. This model subdivides water into core water in the interior of the micelle and shell water close to the interface. Core water is modelled by the properties of bulk water, while the properties of shell water are taken to be those of water at W(0)=2. The model allows the representation of the spectra at any hydration level as a linear combination of the spectra of core and shell water. Different approaches are critically reviewed and discussed as well.
以环己烷、正戊烷、正辛烷和正十二烷作为非极性溶剂，通过小角X射线散射和水的OH伸缩模式第一泛音的近红外振动光谱，研究了限制在气溶胶-OT-烃-水反胶束中的水的状态。实验聚焦于水/AOT分子比W(0)=2-20，在此范围内水受到限制和表面水相互作用的强烈影响。从小角散射图案导出的对距分布函数能够详细表征这些体系的拓扑结构，并且表明其中一些烃体系偏离了单分散的球形水池。与通常的假设相反，从干燥的反胶束（W(0)→0）到基本呈本体状的水（W(0)>20），水池尺寸并不随W(0)线性变化。OH伸缩振动的第一泛音呈现出高度结构化的光谱，这揭示了限制时氢键环境的显著变化。光谱通过法耶尔及其同事提出的核/壳模型得到合理解释。该模型将水细分为胶束内部的核心水和靠近界面的壳层水。核心水用水体水的性质来模拟，而壳层水的性质取为W(0)=2时水的性质。该模型能够将任何水合水平下的光谱表示为核心水和壳层水光谱的线性组合。还对不同方法进行了批判性的回顾和讨论。
