高岭石-水体系中水分子结构的分子动力学模拟

以Hendricks模型为初始结构, 利用CLAYFF力场对高岭石-水体系进行无晶体学限制的分子动力学模拟. 结果表明, 层间水有三种类型: I型类似于Costanzo提出的“洞水”分子, 其HH矢量(水分子中从一个氢原子位置指向另一个氢原子位置的方向矢量)平行于(001)平面, 而C2轴稍微倾斜于(001)面法线; II型类似于“连接水”, 一个氢氧键指向临近的层间四面体氧形成氢键, 另一个氢氧键与(001)面近似平行; III型水分子在层间近似保持为竖直状, 一个氢与层间四面体氧形成氢键, 而另一个氢与对面层的羟基氧形成氢键. 高岭石羟基氢沿(001)晶面法线的浓度曲线显示一部分羟基指向变为近似平行于(001)面, 羟基氧因此能够暴露出来与层间水分子氢形成氢键. 此外, 模拟中还观察到部分II型水分子氧偏离于层间的平均位置而更靠近四面体层, 这和Costanzo的实验结果一致, 可能是X射线谱图中(002)弱衍射峰出现的原因.

Molecular Dynamics Simulation on Structure of Water Molecules in a Kaolinite-Water System

Using Hendricks's model as initial structure, a molecular dynamics simulation without crystallographic restrictions was used to investigate the kaolinite-water systemwith the CLAYFF force field. Results showthat interlayer water molecules have three types: type I is similar to Costanzo's“hole water”molecule and its HH vector (a vector from one hydrogen atompointing to another hydrogen atomin water molecule) is parallel to the (001) plane while its C2-axis is slightly sloped and is nearly normal to the (001) plane; type II is similar to“associated water”where one O—H bond points to its neighboring layer tetrahedral oxygen and its hydrogen forms hydrogen bonds with oxygen while the other O—Hbond is approximately parallel to the (001) plane; type III watermolecule approximates a vertical form where one hydrogen forms a bond with tetrahedral oxygen and the other forms a hydrogen bond with a hydroxyl oxygen in the opposite clay layer. The concentration curve of the hydroxyl hydrogen of kaolinite along the normal line of the (001) plane shows that a portion of the hydroxyls change their orientation to be nearly parallel to the (001) plane. The corresponding oxygen atoms in the hydroxyl can therefore be exposed and forma hydrogen bond with interlayer water. Furthermore, some oxygen atoms of Type II water molecule deviate from their average position in the interlayer space and are closer to the tetrahedral layer and this result is consistent with Costanzo's result and may be the reason for the weak (002) peak observed in X-ray diffraction.