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Polyelectrolytes in multivalent salt solutions

We study conformational and electrophoretic properties of polyelectrolytes (PEs) in tetravalent salt solutions under the action of electric fields by means of molecular dynamics simulations. Chain conformations are found to have a sensitive dependence on the salt concentration Cs. As Cs is increased, the chains first shrink to a globular structure and subsequently re‐expand above a critical concentration Cs*. An external electric field can further alter the chain conformation. If the field strength E is larger than a critical value E*, the chains are elongated. E* is shown to be a function of Cs by using two estimators EI* and EII* through the study of the polarization energy and the onset point of chain unfolding, respectively. The electrophoretic mobility of the chains depends strongly on Cs, and the magnitude increases significantly, accompanying the chain unfolding, when E>EII*. We study the condensed ion distributions modified by electric fields and discuss the connection of the modification with the change of chain morphology and mobility. Finally, E* is studied by varying the chain length N. The inflection point is used as a third estimator EIII*. EIII* scales as N−0.63(4) and N−0.76(2) at Cs=0.0 and Cs*, respectively. EII* follows a similar scaling law to EIII* but a crossover appears at Cs=Cs* when N is small. The EI* estimator fails to predict the critical field, which is due to oversimplifying the critical polarization energy to the thermal energy. Our results provide valuable information to understand the electrokinetics of PE solutions at the molecular level and could be helpful in micro/nanofluidic applications.