Effect of Counterion Size on the Structure of a Flexible Polyelectrolyte Chain in Low-Polar Solvents

Abstract

The influence of the counterion size on the conformational behavior of a single polyelectrolyte chain in a dilute solution in low-polar media has been studied by molecular dynamics. Modeling has been performed both for small counterions, which are smaller in size than the monomer unit, and for large counterions, which are several times larger in size than the monomer unit. It has been found that the chain gyration radius Rg depends nonmonotonically on the counterion size. The minimum value of Rg is observed when the size of counterions is comparable with the size of the monomer unit. The growth of Rg with increasing size of counterions results from the increasing contribution of excluded volume interactions, whereas its growth with decreasing size of counterions is due to the formation of elongated chains of dipoles. The dependence of the multiplet structure formed in media with strong electrostatic interactions on the fraction of charged chain units and the counterion size is analyzed.