Monte Carlo simulations of parallel charged platelets as an approach to tactoid formation in clay.
Monte Carlo simulations of parallel charged platelets as an approach to tactoid formation in clay.
复制标题
平行带电片晶的蒙特卡罗模拟作为粘土中类晶团簇形成的方法。
作者：
A. Thuresson;M. Ullner;T. Åkesson;C. Labbez;B. Jönsson
用Monte Carlo模拟方法计算了平行带电荷片晶与二价抗衡离子相互作用的自由能，研究了静电对聚集的影响。薄片主要用于代表粘土颗粒。对于二价抗衡离子，由于离子-离子相关性引起的吸引力，两个片晶或两个类晶（平行片晶簇）的自由能在短距离处显示出最小值。在无盐体系中，相互作用的自由能在最小值之外有一个长程排斥尾。的排斥力增加与较大的聚集数的tactoids，而自由能最小值的深度逐渐减少。对于足够大的聚集数，排斥是占主导地位的，最小值不再是全局自由能最小值。这是由于溶液中游离的抗衡离子（在类触体外部）耗尽而产生的效应，因为抗衡离子和血小板聚集成类触体，并且系统中抗衡离子的重新分布改变了血小板和类触体之间的有效相互作用。类触体-类触体相互作用的差异作为聚集数的函数，可以通过添加足够的盐来掩盖消耗来去除。添加盐也减少了相互作用自由能的排斥尾，并提高了最小值。不依赖于聚集数表明，等键模型与单调衰减分布的聚集数可以用来描述粘土系统。这可能有助于解释tactoids中血小板平均数量较低的实验观察结果，尽管模拟模型中未包括的因素也可能起重要作用。
The free energy of interaction between parallel charged platelets with divalent counterions has been calculated using Monte Carlo simulations to investigate the electrostatic effects on aggregation. The platelets are primarily intended to represent clay particles. With divalent counterions, the free energy for two platelets or two tactoids (clusters of parallel platelets) shows a minimum at a short separation due to the attraction caused by ion-ion correlations. In a salt-free system, the free energy of interaction has a long-range repulsive tail beyond the minimum. The repulsion increases for tactoids with larger aggregation numbers, whereas the depth of the free-energy minimum is gradually reduced. For large enough aggregation numbers, the repulsion is dominating and the minimum is no longer a global free-energy minimum. This is an effect of the depletion of counterions free in solution (outside tactoids) as counterions and platelets aggregate into tactoids and the resulting redistribution of counterions in the system changes the effective interactions between platelets and tactoids. The difference in tactoid-tactoid interactions as a function of aggregation number can be removed by adding enough salt to mask the depletion. Adding salt also reduces the repulsive tail of the free energy of interaction and enhances the minimum. No dependence on the aggregation number suggests that an isodesmic model with a monotonically decaying distribution of aggregation numbers can be used to describe a clay system. This may help to explain the experimental observations of low average numbers of platelets in tactoids, although factors not included in the simulation model may also play an important role.