Influence of weak groups on polyelectrolyte mobilities

Abstract

The ionization of dissociable groups in weak polyelectrolytes does not occur in a homogenous fashion. Monomer connectivity imposes constraints on the localization of the dissociated (charged) monomers that affect the local electric potential. As a result, the mean bare charge along a weak polyelectrolyte can vary depending on the proximity to topological features (e.g. presence of crosslinks or dangling ends). Using reaction‐ensemble Monte‐Carlo simulations we calculate the dissociation inhomogeneities for a few selected PE configurations, linear, rod‐like, flexible four‐arm star, and a star with stiff arms. An ensemble preaverage is used to obtain the annealed bare charge profile for these different polymer configurations. Using molecular dynamics simulations within a Lattice‐Boltzman fluid, we investigate how the electrophoretic mobility is affected by the bare charge inhomogeneities arising from the annealed weak polyelectrolytes. Surprisingly, the mobility obtained for the situations corresponding to the predicted charge profile for annealed weak polyelectrolytes are not significantly different than the mobility obtained when all the monomers have an identical charge (under the constraint that the total polyelectrolyte bare charge is the same). This is also true for the stiff rod‐like variants where conformational changes induced from the localization of the monomer charges are negligible. In salty solutions, we find that counterions are affected by the electric potential modulations induced by the topological features. Since the counterions crowd in regions where the electric potential caused by the dissociated monomers is highest, they wash‐out the bare charge inhomogeneities and contribute to a more uniform effective backbone charge.

Numerical modelling of non-ionic microgels: an overview

“…However, for highly charged systems, the electrostatic force usually dominates over the other contributions, and thus the microscopic details become somewhat less important for the swelling transition 86. In this context, important questions linked to the counter-ion distribution113,114 and to the chemical equilibria of ions in weak polyelectrolyte nanogels115 have been the subject of recent work. Interestingly, since PNIPAM microgels are also weakly charged,116–118 with the effect of such a charge showing up close to the VPTT,119 some of these results may be broadly relevant.…”

Grand-Reaction Method for Simulations of Ionization Equilibria Coupled to Ion Partitioning

“…To simulate chemical reactions in a closed system uncoupled to a reservoir, we can use the Reaction-ensemble Monte Carlo − (RxMC) or the Constant-pH (cpH) algorithm. Various weak polyelectrolyte systems have been simulated using the RxMC method ,− or cpH method. ,− In RxMC, we simulate chemical reactions by inserting and deleting particles or by changing their chemical identity, as prescribed by the stoichiometry of the simulated reaction. Chemical equilibrium is achieved by performing the reaction moves in the forward and reverse directions of the reaction and by accepting the moves with a Metropolis-like acceptance probability.…”