Physik Department, Technische Universität München, 85478 Garching, Germany.
J Chem Phys. 2012 Sep 14;137(10):104704. doi: 10.1063/1.4749380.
Proteins, molecules, and macromolecular assemblies in water are surrounded by a nanometer-sized hydration layer with properties very different from bulk water. Here, we use classical molecular dynamics simulations to study the dielectric response of hydration water next to hydrophobic and hydrophilic planar surfaces. We find the interfacial dielectric absorption of water to be strongly anisotropic: compared to bulk water, which shows a broad dielectric absorption maximum around 15 GHz in the imaginary part of the dielectric function, the absorption for electric fields parallel to the surface is of similar strength and shows a slight redshift, while for perpendicular electric fields it is strongly attenuated and blueshifted. This anisotropy is generic for hydrophobic and hydrophilic surfaces. From our spatially resolved dielectric functions and a modified Maxwell-Garnett theory that accounts for anisotropic hydration layers around spherical particles, the dielectric absorption of solutions of organic molecules and micelles is derived to exhibit the experimentally known attenuation in combination with a redshift. These two features are traced back to the subtle interplay of interfacial depolarization effects and the dielectric anisotropy in the hydration layer. By a detailed analysis of the individual water molecule dynamics the perpendicular blueshift is shown not to be linked to accelerated water reorientation, but rather to dielectric boundary effects. Carefully conducted angularly resolved experiments at planar aqueous interfaces will be able to resolve this dielectric anisotropy and thus to confirm the subtle connection between spectral absorption features and the molecular water dynamics in hydration layers.
蛋白质、分子和水的大分子组装体被一个纳米大小的水化层包围，其性质与体相水有很大的不同。在这里，我们使用经典分子动力学模拟来研究疏水和亲水平面表面附近水化水的介电响应。我们发现水的界面介电吸收具有很强的各向异性：与体相水相比，体相水在介电函数的虚部中在 15GHz 左右表现出宽的介电吸收最大值，平行于表面的电场的吸收强度相似，并略有红移，而垂直于表面的电场则强烈衰减和蓝移。这种各向异性对于疏水和亲水表面都是通用的。从我们的空间分辨介电函数和一个修正的 Maxwell-Garnett 理论，该理论考虑了球形粒子周围各向异性的水化层，我们推导出了有机分子和胶束溶液的介电吸收，表现出实验已知的衰减和红移。这两个特征可以追溯到界面去极化效应和水化层介电各向异性之间的微妙相互作用。通过对单个水分子动力学的详细分析，我们表明垂直蓝移与加速水分子取向无关，而是与介电边界效应有关。在平面水界面上进行精心设计的角度分辨实验将能够分辨这种介电各向异性，从而证实光谱吸收特征与水化层中分子水动力学之间的微妙联系。
