Department of Physical Chemistry, University of Pannonia, P.O. Box 158, H-8201 Veszprém, Hungary.
J Chem Phys. 2010 Oct 21;133(15):154507. doi: 10.1063/1.3489418.
We propose a simple model to explain the nonmonotonic concentration dependence of the mean activity coefficient of simple electrolytes without using any adjustable parameters. The primitive model of electrolytes is used to describe the interaction between ions computed by the adaptive grand canonical Monte Carlo method. For the dielectric constant of the electrolyte, we use experimental concentration dependent values. This is included through a solvation term in our treatment to describe the interaction between ions and water that changes as the dielectric constant changes with concentration. This term is computed by a Born-treatment fitted to experimental hydration energies. Our results for LiCl, NaCl, KCl, CsCl, NaBr, NaI, MgCl(2), CaCl(2), SrCl(2), and BaCl(2) demonstrate that the principal reason of the nonmonotonic behavior of the activity coefficient is a balance between the solvation and ion-ion correlation terms. This conclusion differs from previous studies that assumed that it is the balance of hard sphere repulsion and electrostatic attraction that produces the nonmonotonic behavior. Our results indicate that the earlier assumption that solvation can be taken into account by a larger, "solvated" ionic radius should be reconsidered. To explain second order effects (such as dependence on ionic size), we conclude that explicit water models are needed.
我们提出了一个简单的模型来解释简单电解质的平均活度系数的非单调浓度依赖性，而无需使用任何可调参数。电解质的原始模型用于描述通过自适应巨正则蒙特卡罗方法计算的离子之间的相互作用。对于电解质的介电常数，我们使用实验浓度相关值。通过我们处理中的一个溶剂化项将其包含在内，以描述随着介电常数随浓度变化而变化的离子与水之间的相互作用。该术语通过拟合实验水合能的 Born 处理来计算。我们对 LiCl、NaCl、KCl、CsCl、NaBr、NaI、MgCl2、CaCl2、SrCl2 和 BaCl2 的结果表明，活度系数非单调行为的主要原因是溶剂化和离子-离子相关项之间的平衡。这一结论与之前的研究不同，之前的研究假设是硬球排斥和静电吸引之间的平衡产生了非单调行为。我们的结果表明，应该重新考虑先前的假设，即溶剂化可以通过更大的“溶剂化”离子半径来考虑。为了解释二阶效应（例如对离子尺寸的依赖性），我们得出结论，需要明确的水模型。
