Department of Organic Chemistry I, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018 San Sebastian, Spain.
J Chem Phys. 2012 May 14;136(18):184501. doi: 10.1063/1.4705692.
Ions with like-charges repel each other with a magnitude given by the Coulomb law. The repulsion is also known to persist in aqueous solutions albeit factored by the medium's dielectric constant. In this paper, we report results from molecular dynamics simulations of alkali halides salt solutions indicating an effective attraction between some of the like-charged monovalent ions. The attraction is observed between anions, as well as between cations, leading to the formation of dimers with lifetimes on the order of few picoseconds. Two mechanisms have been identified to drive this counterintuitive attraction. The first is exhibited by high-charge density ions, such as fluoride, at low salt concentrations, yielding effective attractions with magnitude up to the order of 1-2 kT. In this case, the stronger local electric field generated when the two ions are in contact augments the alignment of neighboring waters toward the ions. This results in a gain of substantial favorable ion-water interaction energy. For fluorides, this interaction constitutes the major change among the different energy components compensating for the anion-anion repulsion, and therefore, rendering like-charge association possible. The second mechanism involves mediation by counterions, the attractions increase with salt concentration and are characterized by small magnitudes. In particular, clusters of ion triplets, in which a counterion is either bridging the two like-charged ions or is paired to only one of them, are formed. Although these two mechanisms may not yield net attractions in many cases, they might still be operational and significant, explaining effective repulsions between like-charged ions with magnitudes much smaller than expected based on continuum electrostatics.
带相同电荷的离子会相互排斥，排斥力的大小由库仑定律给出。尽管在水溶液中会受到介质介电常数的影响，但这种排斥力仍会持续存在。在本文中，我们报告了碱金属卤化物盐溶液的分子动力学模拟结果，表明在某些带相同电荷的单价离子之间存在有效吸引力。这种吸引力不仅存在于阴离子之间，也存在于阳离子之间，导致形成寿命约为数皮秒的二聚体。我们已经确定了两种机制来驱动这种反直觉的吸引力。第一种机制表现为高电荷密度离子（如氟离子）在低盐浓度下的行为，产生的有效吸引力可达 1-2 kT 的量级。在这种情况下，当两个离子接触时产生的更强的局部电场增强了相邻水分子向离子的定向排列。这导致了大量有利的离子-水相互作用能的增加。对于氟化物，这种相互作用是不同能量成分中主要的变化，补偿了阴离子-阴离子的排斥力，从而使带相同电荷的离子能够结合。第二种机制涉及反离子的介导，随着盐浓度的增加吸引力增加，并且具有较小的大小。特别是，形成了离子三聚体的簇，其中一个反离子要么桥接两个带相同电荷的离子，要么与其中一个配对。尽管这两种机制在许多情况下可能不会产生净吸引力，但它们仍然可能是有效的，并且非常重要，可以解释带相同电荷的离子之间的有效排斥力比基于连续体静电学预期的要小得多。
