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

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.Comment: 3 figure

Separation of chiral particles in micro- or nanofluidic channels

We propose a method to separate enantiomers in microfluidic or nanofluidic channels. It requires flow profiles which break chiral symmetry and have regions with high local shear. Such profiles can be generated in channels confined by walls with different hydrodynamic boundary conditions (e.g. slip lengths). Due to a nonlinear hydrodynamic effect, particles with different chirality migrate at different speed and can be separated. The mechanism is demonstrated by computer simulations. We investigate the influence of thermal fluctuations (i.e. the P\'eclet number) and show that the effect disappears in the linear response regime. The details of the microscopic flow are important and determine which volume forces are necessary to achieve separation.Comment: Accepted for publication in Physical Review Letter