Lattice Monte Carlo simulations of a charged polymer chain:Effect of valence and concentration of the added salt

摘要

The configurational properties of a single polyelectrolyte chain accompanied by counterions andadded salt are simulated using the cooperative motion algorithm on the face-centered cubic lattice.In particular,a greater emphasis is put on the effect of valence z_s and concentration of the addedpositive (negative) salt ions n_s on the polymer behavior.This is achieved by inspecting two familiesof systems with widely varying numbers n_s of monovalent (z_s= 1) or multivalent (z_s=4) salt ions attwo fixed reduced temperatures T~*=0.5,1.The calculations indicate that especially at the lowertemperature the addition of some amount of multivalent salt has a tremendous impact on chainconformations compared to the situation with monovalent salt.Even for relatively lowconcentrations of the former,the mean radius of gyration ~(1/2) and the mean end-to-end distance(R~2)~(1/2) decrease sharply,i.e.,the polymer exists in strongly collapsed forms.This reduction ofpolymer size is also accompanied by a drop in the system inner energy e~* and the effective meancharge per monomer q~*.The analysis of various pair-correlation functions g_(ab)(r) indicates that thelatter effect-caused by condensation of ions onto the chain-is dominated by the multivalent ones.Furthermore,it is found that for z_s=4,the uncondensed salt ions tend to group themselves into smallclusters.