Coil to rod transitions in Monte Carlo simulations of a short polyelectrolyte. I. New thermal and screening effects

Abstract

A Monte Carlo simulation of a single short polyelectrolyte chain immersed in a dielectric continuum solvent with screened Coulombic interactions is described. Two main effects are involved in the coil→rod conformational transitions that have been observed when the temperature is decreased: a new thermal effect and a screening effect. The pure thermal effect is resolved by simulating a bare fully ionized polyion with Coulombic interactions at various T. With decreasing T, the radius of gyration exhibits a crossover from a self-avoiding walk configuration to a rod-like shape. This phenomenon is explained by fundamental thermodynamic considerations. The screening effect at various temperatures, has been investigated for several ‘‘salt’’ concentrations using a screened Coulomb potential instead of a pure Coulomb one. Configurational properties such as the mean square end-to-end distance 〈R2〉, the mean square radius of gyration 〈S2〉, the average of the ratio of the means 〈S2〉/〈R2〉, the mean reduced electrostatic energy 〈UN/NkT〉, and the reduced mean electrostatic energy fluctuations Cν/Nk at various concentrations and temperatures were studied. Our results show that, at low salt concentration where C<0.001 M, the chain behaves like a bare polyelectrolyte chain at all temperatures (no screening at all). At high salt concentration, C>1 M, the chain is fully screened and exhibits a self-avoiding walk configuration at all temperatures, so that the thermal effect cannot be observed. At intermediate salt concentrations, 0.001 M<C<1 M, the chain is partially screened, exhibiting the coil to rod-like transition but with less extension under decreasing temperature. No increase in the effective screening due to temperature decrease is observed under these conditions, despite the decrease in κ−1, the Debye–Hückel screening length.