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

• Main Authors: Landsgesell, Jonas, Smiatek, Jens, Holm, Christian
• Format: Journal Article
• Language: English
• Published: 16-02-2017
• Subjects: Physics - Chemical Physics; Physics - Soft Condensed Matter
• DOI: 10.48550/arxiv.1702.04911

The reaction ensemble and the constant pH method are well-known chemical
equilibrium approaches to simulate protonation and deprotonation reactions in
classical molecular dynamics and Monte Carlo simulations. In this article, we
show similarity between both methods {under certain conditions}. We perform
molecular dynamics simulations of a weak polyelectrolyte in order to compare
the titration curves obtained by both approaches. Our findings reveal a good
agreement between the methods when the reaction ensemble is used to sweep the
reaction constant. Pronounced differences between the reaction ensemble and the
constant pH method can be observed for stronger acids and bases in terms of
adaptive pH values. These deviations are due to the presence of explicit
protons in the reaction ensemble method which induce a screening of
electrostatic interactions between the charged titrable groups of the
polyelectrolyte. The outcomes of our simulation hint to a better applicability
of the reaction ensemble method for systems in confined geometries and titrable
groups in polyelectrolytes with different pK$_\text{a}$ values.