Monte Carlo simulations of a polyelectrolyte chain with added salt:Effect of temperature and salt valence

摘要

Using the cooperative motion algorithm,the effect of salt valence z_s and of the reduced temperature T* on a single polyelectrolyte chain as well as on counterions and salt ions themselves is studied.The calculations show that both parameters strongly influence the polymer,causing it to undergo conformational changes.For a given number of the added salt cations (anions) n_s and temperature T*,the chain takes more and more compact forms as z_s increases (z_s>0).For fixed z_s,in turn,the polymer size reduces sharply as T* drops down from intermediate to low.For high T* configurational the entropy dominates the chain statistics and the mean-square radius of gyration (s~2>~(1/2)(T*,n_s,z_s) approaches its athermal value.The low-temperature polymer collapse is also accompanied by a drop in the effective mean charge per monomer q*(T~*,n_s,z_s) (condensation of ions onto the chain) and the total inner energy e*(T* ,n_s,z_s).Furthermore,the local structure of the system is analyzed by means of pair-correlation functions g_(ab)(r,T* ,n_s,z_s).At lower T* they possess sharp local maxima at small interparticle distances r that disappear as T* grows.The former observation indicates that at lower T* the ions tend to group themselves close to each other.In particular,it is concluded that the condensation is dominated by the multivalent salt ions carrying charges of opposite sign to that of monomers.