Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
J Chem Phys. 2011 Oct 28;135(16):164705. doi: 10.1063/1.3656763.
In a previous theoretical and simulation study [G. I. Guerrero-García, E. González-Tovar, and M. Olvera de la Cruz, Soft Matter 6, 2056 (2010)], it has been shown that an asymmetric charge neutralization and electrostatic screening depending on the charge polarity of a single nanoparticle occurs in the presence of a size-asymmetric monovalent electrolyte. This effect should also impact the effective potential between two macroions suspended in such a solution. Thus, in this work we study the mean force and the potential of mean force between two identical charged nanoparticles immersed in a size-asymmetric monovalent electrolyte, showing that these results go beyond the standard description provided by the well-known Derjaguin-Landau-Verwey-Overbeek theory. To include consistently the ion-size effects, molecular dynamics (MD) simulations and liquid theory calculations are performed at the McMillan-Mayer level of description in which the solvent is taken into account implicitly as a background continuum with the suitable dielectric constant. Long-range electrostatic interactions are handled properly in the simulations via the well established Ewald sums method and the pre-averaged Ewald sums approach, originally proposed for homogeneous ionic fluids. An asymmetric behavior with respect to the colloidal charge polarity is found for the effective interactions between two identical nanoparticles. In particular, short-range attractions are observed between two equally charged nanoparticles, even though our model does not include specific interactions; these attractions are greatly enhanced for anionic nanoparticles immersed in standard electrolytes where cations are smaller than anions. Practical implications of some of the presented results are also briefly discussed. A good accord between the standard Ewald method and the pre-averaged Ewald approach is attained, despite the fact that the ionic system studied here is certainly inhomogeneous. In general, good agreement between the liquid theory approach and MD simulations is also found.
在之前的理论和模拟研究中[G. I. Guerrero-García、E. González-Tovar 和 M. Olvera de la Cruz，Soft Matter 6, 2056 (2010)]，已经表明在存在大小不对称单价电解质的情况下，单个纳米粒子的电荷中和和静电屏蔽会根据其电荷极性而不对称。这种效应也应该影响悬浮在这种溶液中的两个大分子离子之间的有效势能。因此，在这项工作中，我们研究了两个相同带电纳米粒子在大小不对称单价电解质中的平均力和平均势能，结果表明这些结果超出了由著名的 Derjaguin-Landau-Verwey-Overbeek 理论提供的标准描述。为了一致地包含离子尺寸效应，在 McMillan-Mayer 描述水平上进行了分子动力学（MD）模拟和液体理论计算，其中溶剂被隐式地作为具有适当介电常数的背景连续体考虑在内。通过广泛使用的 Ewald 求和方法和最初为均匀离子液体提出的预平均 Ewald 求和方法，在模拟中正确处理长程静电相互作用。发现两个相同纳米粒子之间的有效相互作用表现出相对于胶体电荷极性的不对称行为。具体而言，即使我们的模型不包括特定相互作用，也观察到两个带相同电荷的纳米粒子之间存在短程吸引力；对于阴离子纳米粒子在阳离子小于阴离子的标准电解质中，这些吸引力大大增强。还简要讨论了一些呈现结果的实际意义。尽管研究中离子系统肯定是不均匀的，但标准 Ewald 方法和预平均 Ewald 方法之间达到了很好的一致性。通常，液体理论方法和 MD 模拟之间也存在很好的一致性。
