Frunza Ligia, Schönhals Andreas, Frunza Stefan, Parvulescu Vasile I, Cojocaru Bogdan, Carriazo Daniel, Martín Cristina, Rives Vicente
National Institute of Materials Physics, R-077125 Magurele, Romania.
J Phys Chem A. 2007 Jun 21;111(24):5166-75. doi: 10.1021/jp0717140. Epub 2007 May 31.
The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma0 in detail gives some hint that sigma0(T) has also an unusual saddlelike form.
通过介电谱在10⁻²至10⁷Hz的频率范围以及宽广的温度区间内，研究了局限于具有水镁石结构的层状氧化物材料中的水的旋转分子动力学。层状双氢氧化物样品呈现出一个弛豫过程，该过程归因于强烈吸附在氧化物表面形成一层的水分子的波动。弛豫速率的温度依赖性具有以最大值为特征的不寻常鞍状形状。最近用于描述多孔玻璃中水分子动力学的Ryabov等人（《物理化学杂志B》2001年，105卷，1845页）的模型也被用于这些层状材料。该模型假定两种相互竞争的效应：水分子的旋转波动与缺陷形成同时发生，从而允许产生重新取向所需的自由体积。从该模型对数据的拟合中获得旋转波动的活化能、缺陷形成能、预指数因子和缺陷浓度作为主要参数。将这些参数的值与在局限于纳米多孔分子筛、多孔玻璃或块状冰中的水所发现的值进行了比较。详细讨论了几种相关性，例如缺陷形成能的值越低，缺陷数量越高。预指数因子随着活化能的增加而增加，作为补偿定律的一种表现，并表明运动过程的协同性质。进一步讨论了表面OH基团和层间阴离子的氧原子在氢键形成中的作用。对于水钠锰矿样品，弛豫过程可能被占主导的电导率贡献所叠加，对其频率和温度依赖性进行了分析。发现水钠锰矿的电导率符合半导体无序材料的特征。特别是满足了Barton/Nakajima/Namikawa关系。详细分析直流（dc）电导率σ₀的温度依赖性给出了一些线索，即σ₀(T)也具有不寻常的鞍状形式。
