Effective Interactions Between Polyelectrolytes In Ionic Solutions

Ions play essential roles in effective interactions not only between like-charged charged polyelectrolytes but also between oppositely charged polyelectrolytes,e.g.,ions can mediate the effective interaction between the proteins with positive charges and the nucleic acids with negative charges.In addition to ions,the structural change of polyelectrolytes is closely related to the effective interaction between them.In this thesis,the ion-mediated effective interactions between oppositely charged spherical polyelectrolytes have been investigated by Monte Carlo simulations and the Poisson-Boltzmann theory and those between rotatable like-charged rod polyelectrolytes:1.Potentials of mean force between oppositely charged spherical polyelectrolytes in ion solutions.Ions can mediate the effective interaction between polyelectrolytes in ion solution,and the previous studies have shown that the well-known Poisson-Boltzmann theory cannot correctly predict the potential of mean force between like-charged polyelectrolytes in multivalent ion solutions.However,inadequate attention has been paid to ion-mediated interaction between oppositely charged polyelectrolytes in ion solution,thus we employed Monte Carlo simulations and the Poisson-Boltzmann theory to calculate the potentials of mean force between oppositely charged spherical polyelectrolytes in 1:1 and 2:2 salt solutions.Our calculations show that the effective interaction between the oppositely charged spherical polyelectrolytes are attractive over the wide range of 1:1 and 2:2 salt concentrations,and such attraction becomes weaker at higher 1:1 and 2:2 salt concentrations.Furthermore,the Poisson-Boltzmann theory can accurately predict the potential of mean force between oppositely charged spherical polyelectrolyte in 1:1 salt solutions,except for a slight deviation at very higher 1:1 salt concentration.For 2:2 salt solutions,the Poisson-Boltzmann theory generally overestimates the potential of mean force between oppositely charged spherical polyelectrolytes,and this overestimation becomes more pronounced with the increase of salt concentration or the charge density of polyelectrolytes.Our microscopic analyses show that the overestimation of the Poisson-Boltzmann theory for the potential of mean force between oppositely charged polyelectrolytes is attributed to the underestimation of the number of binding ions around polyelectrolytes.2.Potentials of mean force between charged rodlike polyelectrolyte with rotation freedom.To address the influence of polyelectrolyte structure change on the effective interaction between like-charged rod polyelectrolytes in ion solutions,we calculated the potentials of mean force between rotatable like-charged rod polyelectrolytes in 1:1 and 2:2 salt solutions in a comparision with tha case of parallel charged like-charged rod polyelectrolytes.Our calculations show that the potentials of mean force between the rotatable and parallel like-charged rod polyelectrolytes in 1:1 salt solutions are repulsive.Compared with the case of parallel configuration,the rotation freedom weakens the effective repulsion between charged like-charged rod polyelectrolytes.In 2:2 salt solutions,the potentials of mean force between the rotatable and parallel charged like-charged rod polyelectrolytes are attractive.Compared with the case of parallel configuration,the rotation freedom weekens such effective attractive interactions.Our analyses show that:For 1:1 salt,monovalent ion can only partially weaken the electrostatic repulsions between like-charged rod polyelectrolytes and the rotation freedom can drive the polyelectrolytes to ratate to non-parallel configuration with lower energy;For 2:2 salt,divalent ion-mediated attractive interaction is strongest for the parallel configuraton of polyelectrolytes,while rotational configuration entropy can weaken such divalent ion-mediated attractions at strongly correlated separation.