A Parallel Tempering Monte-Carlo Study of Conformation Transitions of a Single Polyelectrolyte Chain in Solutions with Added Salt

The conformational change of polyelectrolyte chains under external stimuli such as temperature, solvent quality, pulling force, or salt, is of critical importance to numerous biological and synthetic processes, as well as to their potential applications. The conformation transition and ion condensation of a strongly charged polyelectrolyte chain in solvent with added salt are studied using parallel tempering Monte-Carlo simulation on a lattice model, focusing on the effects of salt concentration and valence. The variation of the mean-square radius of gyration (< R-g(2)>) with Bjerrum length (l(B)) is obtained in different salt concentrations and for salts of different valence. It is shown that the chain expands first with increased l(B), and then it shrinks and finally the chain assumes a collapse globule conformation. The chain size is related to both the concentration and the valence of the salt. In intermediate range of l(B), upon addition of monovalent salt, < R-g(2)> gradually decreases. By contrast, the addition of a multivalent salt leads to a more pronounced decrease in < R-g(2)> and a relatively low l(B) value for the chain collapse transition. When l(B) is appropriate, the chain re-expansion is observed with increased concentration of the multivalent salt. In the high multivalent salt solution, condensation of both multivalent counterions and co- ions happens when the charge of total multivalent counterions is greater than that of the chain segments, whereas the univalent counterions hardly condense. For the monovalent salt, the effective charge of chain decreases with increased salt concentration at a given l(B). For the multivalent salt, overcompensation of multivalent counterions is observed in appropriate range of l(B) when salt concentration exceeds a specific value, leading to inversion of effective charge of the chain The overcompensation of multivalent counterions also leads to condensation of the co-ions. The charge reversal and condensation of co-ions lead to the re-expansion of the chain at a high salt concentration. By comparing the chain size with the Debye length at N-s approximate to 64 and l(B) = 1.2, it is concluded that the condensation of multivalent counterions is the major reason for the chain collapsing, rather than the Debye screening in case with multivalent salt.</rg2 ></rg2 ></rg2 >