有机粘土疏水性吸附过程的分子模拟:烷基碳原子数和有机物logK_(ow)的影响

基于从分子尺度上解析改性剂烷基碳原子数和有机物疏水性对有机粘土吸附能力的影响机制,以从头算的量子化学计算方法(MP2/6-31g(d))模拟了有机粘土在水溶液中的吸附过程.通过计算吸附前后几何构型、能量、化学键和电荷分布等参数的变化,分析了影响有机粘土疏水性吸附的关键因素,并对疏水性吸附作用力的化学本质进行了初步探讨.计算结果表明,烷基碳原子数增加对提高有机物吸附能的作用并不显著,长链烷基提高有机粘土吸附容量的真正原因在于其可提供更多的有机物吸附位.有机物的疏水性(以正辛醇-水分配系数logKow表征)是影响疏水性吸附过程的关键因子,水分子的引力作用对疏水性吸附具有决定性影响,理论上解释了普遍存在的有机物吸附量与logKow呈正相关的实验现象.疏水性吸附力的作用前提是有机物为疏水性,即当水分子的引力作用较弱时疏水性作用力才可以表现出来;而对于亲水性有机物,疏水性吸附力会被水分子的引力所抵消,吸附强度减弱.模拟红外光谱和电荷分布结果表明,吸附前后无化学键断裂和电子得失的发生,只有原子电荷密度的微小变化,据此推测疏水性吸附力以分子间弱相互作用力——London色散力为主.

Molecular simulation of the hydrophobic adsorption of organoclay: Effects of the alkyl chain length and the hydrophobicity of organic compounds

Based on the effects of the alkyl chain length of modifier and the hydrophobicity of organic compounds, the adsorption behavior and capability of organoclay in aqueous system were simulated by the quantum chemical calculation (ab initio MP2/6-31g(d)). By calculating the changes of geometric configuration, energy, chemical bond and charge distribution before and after the adsorption, the key factors and the chemical nature of hydrophobic force were investigated and discussed. The calculation results showed that the increase of the alkyl chain length (from 2 to 6) had an insignificant effect on the adsorption energy. The reason responsible for improvement of adsorption capacity was that long chain-alkyl can provide more sites for organic molecules. The hydrophobicity of the organic compounds, characterized as octanol-water partition coefficient (logK_ow), was the crucial factor for the hydrophobic adsorption and the attraction strength by water molecules played a critical role in the adsorption. Based on these results, it can be theoretically explained that the uptake of organic compounds had a good linear correlation with their logK_ow. The simulation results of the infrared spectroscopy and the atom charge distribution indicated that the fracture of chemical bond and the electron transfer did not occur, and that there was only small change of atom charge distribution. It was inferred that London dispersion force, which is a kind of van der waals force, contributed to the hydrophobic force.