Institute of Radioecology and Radiation Protection, Leibniz Universität Hannover, Herrenhäuser Strasse 2, D-30419 Hannover, Germany.
Langmuir. 2012 Sep 18;28(37):13274-83. doi: 10.1021/la302658c. Epub 2012 Aug 31.
Organically modified clays exhibit adsorption capacities for cations, anions, and nonpolar organic compounds, which make them valuable for various environmental technical applications. To improve the understanding of the adsorption processes, the molecular-scale characterization of the structures of organic aggregates assembled on the external basal surfaces of clay particles is essential. The focus of this Monte Carlo simulation study was on the effects of the surface coverage and the alkyl chain length n on the structures of alkyltrimethylammonium chloride ((C(n)TMA)Cl) aggregates assembled on the montmorillonite-water interface. We found that the amount of adsorbed C(n)TMA(+) ions is independent of the alkyl chain length and increases with the C(n)TMA(+) surface coverage. The C(n)TMA(+) ions predominantly adsorb as inner-sphere complexes; the fraction of outer-sphere adsorbed ions equals only about 10%. The conformational order of the C(n)TMA(+) alkyl chains substantially decreases with decreasing alkyl chain length. In agreement with previous experiments, the amount of C(n)TMA(+) ions that are aggregated at the mineral surface increases with increasing chain length. The maximum value of 0.66 C(n)TMA(+) adsorption complex per unit cell area of the clay surface considerably exceeds the amount of cations required to compensate the negative charge of the montmorillonite surface. Furthermore, in most of the studied systems, fractions of Na(+) surface cations remain adsorbed on montmorillonite. The resulting interfacial positive charge excess is counterbalanced by coadsorbed chloride ions forming ion pairs with both C(n)TMA(+) and Na(+).
有机改性粘土对阳离子、阴离子和非极性有机化合物表现出吸附能力，这使它们在各种环境技术应用中具有价值。为了提高对吸附过程的理解，必须对组装在粘土颗粒外基面的有机聚集体的结构进行分子尺度的特征描述。这项蒙特卡罗模拟研究的重点是表面覆盖率和烷基链长 n 对组装在蒙脱石-水界面上的烷基三甲基氯化铵（(C(n)TMA)Cl）聚集体结构的影响。我们发现，吸附的 C(n)TMA(+)离子的量与烷基链长无关，而是随 C(n)TMA(+)的表面覆盖率增加而增加。C(n)TMA(+)离子主要以内球配合物形式吸附；外球吸附离子的分数仅约为 10%。C(n)TMA(+)烷基链的构象有序性随烷基链长度的减小而显著降低。与之前的实验一致，在矿物表面聚集的 C(n)TMA(+)离子的量随链长的增加而增加。粘土表面单位面积上最大的 0.66 C(n)TMA(+)吸附复合物数量大大超过了补偿蒙脱土表面负电荷所需的阳离子数量。此外，在所研究的大多数系统中，表面钠离子的分数仍吸附在蒙脱石上。由此产生的界面正电荷过剩由共吸附的氯离子平衡，氯离子与 C(n)TMA(+)和 Na(+)形成离子对。
