Chain stiffness, salt valency, and concentration influences on titration curves of polyelectrolytes: Monte Carlo simulations

Abstract

Monte Carlo simulations have been used to study two different models of a weak linear polyelectrolyte surrounded by explicit counterions and salt particles: (i) a rigid rod and (ii) a flexible chain. We focused on the influence of the pH, chain stiffness, salt concentration, and valency on the polyelectrolyte titration process and conformational properties. It is shown that chain acid-base properties and conformational properties are strongly modified when multivalent salt concentration variation ranges below the charge equivalence. Increasing chain stiffness allows to minimize intramolecular electrostatic monomer interactions hence improving the deprotonation process. The presence of di and trivalent salt cations clearly promotes the chain degree of ionization but has only a limited effect at very low salt concentration ranges. Moreover, folded structures of fully charged chains are only observed when multivalent salt at a concentration equal or above charge equivalence is considered. Long-range electrostatic potential is found to influence the distribution of charges along and around the polyelectrolyte backbones hence resulting in a higher degree of ionization and a lower attraction of counterions and salt particles at the chain extremities.

Simulation of weak polyelectrolytes: a comparison between the constant pH and the reaction ensemble method

“…A negative prefactor defines a deprotonation reaction (diss) and a positive prefactor, vice versa assigns a protonation reaction (ass). Moreover, it has to be noted that the proposal probability for a protonation or deprotonation reaction in the constant pH method is asymmetric [11,30,31,15,16,17,14,18]. This can be shown by comparing the proposal probability for a deprotonation reaction which reads g(diss|ass) = NHA/N0, with the proposal probability for a protonation reaction defined as g(ass|diss) = N A − /N0, which implies g(ass|diss) = g(diss|ass).…”

“…The method was originally developed to simulate linear polyelectrolytes in presence and absence of excess salt and counterions [11,12,13,14] and reveals a good agreement with analytical results [15]. More effort was additionally spent on the study of different solvent conditions and their influence on polyelectrolyte conformations [16,17] and the properties of polyampholytes [18,5]. Moreover, the constant pH method can also be employed in slightly modified versions [19] in order to analyze shifts in the apparent reaction constant for proteins.…”

Thermo‐responsive gels in the presence of monovalent salt at physiological concentrations: A Monte Carlo simulation study

“…It should be mentioned, however, that PDEAEMA is a polyelectrolyte with ionizable groups whose degree of ionization depends on pH but also on ionic strength. In general, the degree of ionization of polyelectrolytes with weak acid or basic groups increases with the ionic strength, which has been previously analyzed with a similar coarse‐grained model as well . In any case, this dependence implies some coupling between charge and salt effects, which makes the discrimination between them quite difficult.…”

“…In general, the degree of ionization of polyelectrolytes with weak acid or basic groups increases with the ionic strength, which has been previously analyzed with a similar coarse-grained model as well. 46 In any case, this dependence implies some coupling between charge and salt effects, which makes the discrimination between them quite difficult. In this sense, computer simulations are extremely helpful since such effects can be easily decoupled and differentiated, as discussed previously.…”

Coupling of Charge Regulation and Conformational Equilibria in Linear Weak Polyelectrolytes: Treatment of Long Range Interactions via Effective Short-Ranged and pH-Dependent Interaction Parameters

“…[5,15,[21][22][23][24][25]. For systems with a small number of sites (N ≤ 20), the necessary thermal averages can be performed by direct enumeration, while for a large number of sites Monte Carlo (MC) simulations become necessary [26][27][28][29][30][31][32][33][34][35][36][37].…”