The Influence of Polymer and Ion Solvation on Counter-ion Cloud Formation and Charge Fluctuations in Highly Charged Polyelectrolytes

We investigate the influence of solvent affinity to counter-ions and to the polyelectrolyte (PE) backbone on the charge distribution about highly charged flexible polymer chains based on coarse-grained molecular dynamics simulations that include both explicit counter-ions and solvent. Based on this framework, we find that the competitive solvation of ions and PE chains leads to an extended “cloud” of counterions about the polymers that cannot be understood without modeling the solvent explicitly. The counter-ion cloud is highly dynamic and fluctuations of the charge can be expected to great influence the polarizability of PEs in solution. After reviewing recent findings on the influence of molecular topology and interactions on the average size of the counter-ion cloud, we estimated the PE potential of the mean interaction to determine the influence of the couterion-cloud and charge fluctuation effects on inter-PE interactions. Consistent with the Kirkwood-Schumaker theory, we find evidence that charge fluctuations gives rise to a long range attractive interaction having a 1/ r² asymptotic decay at large interpolymer separation distances r in the case of strongly hydrating counterions and a PE backbone that is not strongly hydrating, a model of relevance to many synthetic PEs.