Monte Carlo Simulations of Parallel Charged Platelets as an Approach to Tactoid Formation in Clay

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• 作者：Axel Thuresson ; Magnus Ullner ; Torbj枚rn ; 脜kesson ; Christophe Labbez ; Bo J枚nsson
• 刊名：Langmuir
• 出版年：2013
• 出版时间：July 23, 2013
• 年：2013
• 卷：29
• 期：29
• 页码：9216-9223
• 全文大小：351K
• 年卷期：v.29,no.29(July 23, 2013)
• ISSN：1520-5827

文摘

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鈥搃on 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鈥搕actoid 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.