Counter-ion solution modeling in salt-free polyelectrolytes depicts clustering mystery

Although one naturally expects charged particles and polymers of the same sign to repel, clustering and voids have often been observed in systems such as plasmas, colloidal solutions and polyelectrolyte solutions. This suggests that certain conditions give rise to effective attractions between particles and polymers of like charge.

Previous theories of electrolyte solutions, colloidal suspensions and polyelectrolyte solutions generally disregarded the solvation effects on molecule-particle interactions, and did not explain the large-scale clustering observed in charged particle and polymer systems. The origin of this ubiquitous clustering has been one of the great mysteries of charged colloidal and polyelectrolyte solutions.

Researchers at the National Institute of Standards and Technology developed a coarse-grained model of polyelectrolyte solutions in which the solvation of the counter-ions can be tuned by varying their dispersion interaction strength with the solvent. Specifically, the model entails a coarse-grained bead-spring model of polymer chains, an explicit solvent and counter-ions. All particles exhibit van der Waals interactions, modeled by a Lennard-Jones interaction potential. The counter-ions and polymer segments also interact by long-range Coulomb interactions.

Long-range many-body attractive interactions emerged between polyelectrolytes, arising from the formation of diffuse domains, rich in counter-ions around the chains that render the polyelectrolyte chains highly polarizable. The polyelectrolyte chains self-assemble into dynamic clusters at low polymer concentrations and form domains (“voids”) depleted of the polyelectrolyte chains and counter-ions. This phenomenon has been observed before in highly charged colloidal solutions and the clustering is evident by the anomalous low-angle scattering of the polyelectrolyte solutions.

The work confirms the “like-attraction” ideas suggested by Feynman, Ise and Langmuir, and demonstrates that a better understanding of such phenomena could lead to significant practical applications with the control of chain clustering and void formation in various substances.

Source: “Communication: Counter-ion solvation and anomalous low-angle scattering in salt-free polyelectrolyte solutions,” by Alexandros Chremos and Jack F. Douglas, Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.5010784 .