Chen Yng-Gwei, Weeks John D
Department of Physics, University of Maryland, College Park, MD 20742, USA.
Proc Natl Acad Sci U S A. 2006 May 16;103(20):7560-5. doi: 10.1073/pnas.0600282103. Epub 2006 May 2.
Strong, short-ranged positional correlations involving counterions can induce a net attractive force between negatively charged strands of DNA and lead to the formation of ion pairs in dilute ionic solutions. However, the long range of the Coulomb interactions impedes the development of a simple local picture. We address this general problem by mapping the properties of a nonuniform system with Coulomb interactions onto those of a simpler system with short-ranged intermolecular interactions in an effective external field that accounts for the averaged effects of appropriately chosen long-ranged and slowly varying components of the Coulomb interactions. The remaining short-ranged components combine with the other molecular core interactions and strongly affect pair correlations in dense or strongly coupled systems. We show that pair correlation functions in the effective short-ranged system closely resemble those in the uniform primitive model of ionic solutions and illustrate the formation of ion pairs and clusters at low densities. The theory accurately describes detailed features of the effective attraction between two equally charged walls at strong coupling and intermediate separations of the walls. Analytical results for the minimal coupling strength needed to get any attraction and for the separation at which the attractive force is a maximum are presented.
涉及抗衡离子的强短程位置相关性可在稀离子溶液中诱导带负电的DNA链之间产生净吸引力，并导致离子对的形成。然而，库仑相互作用的长程性阻碍了简单局部图像的发展。我们通过将具有库仑相互作用的非均匀系统的性质映射到具有短程分子间相互作用的更简单系统的性质上，来解决这个一般问题，该更简单系统处于一个有效外场中，该外场考虑了库仑相互作用中适当选择的长程和缓慢变化分量的平均效应。其余的短程分量与其他分子核心相互作用相结合，并强烈影响密集或强耦合系统中的对关联。我们表明，有效短程系统中的对关联函数与离子溶液的均匀原胞模型中的对关联函数非常相似，并说明了在低密度下离子对和簇的形成。该理论准确地描述了在强耦合和壁的中间间距下两个等电荷壁之间有效吸引力的详细特征。给出了产生任何吸引力所需的最小耦合强度以及吸引力最大时的间距的解析结果。
