Abstract
Effect of multivalent salt and effect of chain stiffness on conformation of a single polyelectrolyte were investigated using Langevin dynamics simulations. A polyelectrolyte is classified into three categories according to its chain stiffness, based upon the variation of chain size (radius of gyration Rg) against the concentration of multivalent salt Cs. The variation of Rg for a flexible chain shows a tilted-L-shaped Rg curve. That for a semiflexible chain is characterized by a U-shaped curve. A stiff chain exhibitsan almost straight Rg curve. A polyelectrolyte at different stiffness shows different local structure. A flexible chain exhibits a zigzag local structure; a semiflexible chain reveals large-angle turns at the place where the counterions condense; a stiff chain stretches out smoothly. Upon addition of multivalent salt, a polyelectrolyte collapses and is then followed by chain reexpansion. The condensed structure of a flexible chain is disordered and spheroid-like. On the other hand, a semiflexible chain collapses into an ordered structure, such as rod or toroid. An increase in salt concentration is found to enhance the transition between these two ordered structures. The chain stiffness determines the occurrence of a condensation of a polyelectrolyte and the morphology of a condensate. It also affects the nature of the ‘coil-globule transition’. A flexible chain undergoes a continuous transition but a semiflexible chain undergoes a discrete transition. A state diagram is proposed in this study according to the prescribed effects.