Dynamics of a polyelectrolyte under a constant electric field

We perform a molecular dynamics simulation of a polyelectrolyte in a viscous fluid under an external electric field to study the dynamics of gel-free electrophoresis. To incorporate the hydrodynamic effects, we employ a coarse-grained description of water by using multiparticle collision dynamics. We use a screened Coulomb interaction among the monomers and explicit monovalent counterions to model the electrostatic interactions in an ionic solution. The mobility of the polyelectrolyte µ is obtained as a function of the molecular weight N, the electric field strength E,and the Debye screening length of the solvent λ. The mobility is found to be independent of N for large N and to exhibit a maximum at a certain N for a large λ, which are in agreement with experimental results. The dependence of µ on E is also examined and discussed by considering the effects of an electric field on counterion condensation. The dependence of µ on λ shows a discrepancy between our simulation and experiments, which implies that the added salts not only screen out the Coulomb interaction but also participate in the counterion condensation significantly.