摘要
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 salt concentrationC_(s) . AsC_(s)is increased, the chains first shrink to a globular structure and subsequently reexpand above a critical concentrationC_(s)^(*) . An external electric field can further alter the chain conformation. If the field strengthEis larger than a critical valueE^(*) , the chains are elongated.E^(*)is shown to be a function ofC_(s)by using two estimatorsE_(I)^(*)andE_(II)^(*)through the study of the polarization energy and the onset point of chain unfolding, respectively. The electrophoretic mobility of the chains depends strongly onC_(s) , and the magnitude increases significantly, accompanying the chain unfolding, whenE> E_(II)^(*) . 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 lengthN . The inflection point is used as a third estimatorE_(III)^(*) .E_(III)^(*)scales asN^(-0.63(4))andN^(-0.76(2))atC_(s)=0.0andC_(s)^(*) , respectively.E_(II)^(*)follows a similar scaling law toE_(III)^(*)but a crossover appears atC_(s)=C_(s)^(*)whenNis small. TheE_(I)^(*)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/nano-fluidics applications.