Counterion size affects the behavior of the charged polymer chain

Russian physicists investigated the change in the shape of a highly charged polymer chain in the presence of oppositely charged ions. They showed that the size of charged ions, called counterions, plays a decisive role in this. The research results are important for building new models in polymer physics. The work was published in Soft Matter magazine.

Polyelectrolytes – polymer molecules carrying a large number of charged groups on their surface – are common in natural and synthetic systems. For example, the most important biological polyelectrolytes are proteins, RNA and DNA molecules. Their behavior in solution, associated with a change in conformation (shape) and physicochemical properties, is rather difficult to predict. This problem is especially acute in situations where environmental conditions vary.

“The essence of our work is to study the conformational behavior of a highly charged polymer chain in the presence of counterions of various sizes. It has been shown that in low-polarity solvents, the size of the counterion plays a significant role, ”says Elena Kramarenko, professor of the Department of Polymer and Crystal Physics, Department of Solid State Physics, Faculty of Physics, Moscow State University.

A flexible, highly charged polyelectrolyte molecule was modeled using the molecular dynamics method. It turned out that in the presence of small counterions, the polymer chain folds into a globule (a dense spherical structure), and in the presence of large ones, it stretches. There are two reasons for this behavior. First, with an increase in the size of a charged particle, the excluded volume around it increases, where its other counterparts cannot get. Secondly, electrostatic interactions between charged molecules of the system are also important. Large counterions rather weakly interact with the polyelectrolyte, and therefore drift along its surface, not allowing their “rails” to collapse into a ball. In addition, with a certain content of counter-ions of both sizes, the system appears to be a globule with a core and a shell. Inside are the monomers and small counterions that make up the chain, and the large counterions are on the surface. The receipt of each of the states of the chain is determined by the balance of volumetric and electrostatic interactions.

“The study of the features of electrostatic interactions in charged polymer systems is a topical scientific topic in polymer physics. The competition between Coulomb (due to charge) and non-Coulomb interactions determines the realized state of polyelectrolyte chains, matrices, gels, and zwitter-polyions, the charge of which is compensated by the presence of positive and negative groups. In the future, it is planned to model them and to analyze in detail the influence of a wide range of parameters on the structures of charged polymer systems, ”concludes Elena Kramare

nko. Source: Soft Matter