Grand-reaction method for simulations of ionization equilibria coupled to ion partitioning

We developed a new method for coarse-grained simulations of acid-base equilibria in a system coupled to a reservoir at a given pH and concentration of added salt, that we term the Grand-reaction method. More generally, it can be used for simulations of any reactive system coupled to a reservoir of a known composition. Conceptually, it can be regarded as an extension of the reaction ensemble, combining explicit simulations of reactions within the system and Grand-canonical exchange of particles with the reservoir. To demonstrate its strength, we applied our method to a solution of weak polyelectrolytes in equilibrium with a reservoir. Our results show that the ionization and swelling of a weak polyelectrolyte are affected by the Donnan effect due to the partitioning of ions and by the polyelectrolyte effect due to electrostatic repulsion along the chain. Both effects lead to a similar shift in ionization and swelling as a function of pH; albeit for different physical reasons. By comparison with published results, we showed that neglecting one or the other effect may lead to erroneous predictions or misinterpretations of results. In contrast, the Grand-reaction method accounts for both effects on the results and allows us to quantify them. Finally, we outline possible extensions and generalizations of the method and provide a set of guidelines for its safe application by a broad community of users.<br><div><br></div>

Grand-reaction method for simulations of ionization equilibria coupled to ion partitioning

We developed a new method for coarse-grained simulations of acid-base equilibria in a system coupled to a reservoir at a given pH and concentration of added salt, that we term the Grand-reaction method. More generally, it can be used for simulations of any reactive system coupled to a reservoir of a known composition. Conceptually, it can be regarded as an extension of the reaction ensemble, combining explicit simulations of reactions within the system and Grand-canonical exchange of particles with the reservoir. To demonstrate its strength, we applied our method to a solution of weak polyelectrolytes in equilibrium with a reservoir. Our results show that the ionization and swelling of a weak polyelectrolyte are affected by the Donnan effect due to the partitioning of ions and by the polyelectrolyte effect due to electrostatic repulsion along the chain. Both effects lead to a similar shift in ionization and swelling as a function of pH; albeit for different physical reasons. By comparison with published results, we showed that neglecting one or the other effect may lead to erroneous predictions or misinterpretations of results. In contrast, the Grand-reaction method accounts for both effects on the results and allows us to quantify them. Finally, we outline possible extensions and generalizations of the method and provide a set of guidelines for its safe application by a broad community of users.<br><div><br></div>

Grand-reaction method for simulations of ionization equilibria coupled to ion partitioning

We developed a new method for coarse-grained simulations of acid-base equilibria in a system coupled to a reservoir at a given pH and concentration of added salt, that we term the Grand-reaction method. More generally, it can be used for simulations of any reactive system coupled to a reservoir of a known composition. Conceptually, it can be regarded as an extension of the reaction ensemble, combining explicit simulations of reactions within the system and Grand-canonical exchange of particles with the reservoir. To demonstrate its strength, we applied our method to a solution of weak polyelectrolytes in equilibrium with a reservoir. Our results show that the ionization and swelling of a weak polyelectrolyte are affected by the Donnan effect due to the partitioning of ions and by the polyelectrolyte effect due to electrostatic repulsion along the chain. Both effects lead to a similar shift in ionization and swelling as a function of pH; albeit for different physical reasons. By comparison with published results, we showed that neglecting one or the other effect may lead to erroneous predictions or misinterpretations of results. In contrast, the Grand-reaction method accounts for both effects on the results and allows us to quantify them. Finally, we outline possible extensions and generalizations of the method and provide a set of guidelines for its safe application by a broad community of users.<br><div><br></div>

Grand-Reaction Method for Simulations of Ionization Equilibria and Ion Partitioning in a Broad Range of pH and Ionic Strength

On 16th September 2019 we found a major flaw in the original manuscript, that has been uploaded to ChemRXiv. To prevent readers from implementing and using a flawed algorithm, we decided to replace the manuscript with this notice. We anticipate to upload a corrected version within few weeks.<br><br>

Grand-Reaction Method for Simulations of Ionization Equilibria and Ion Partitioning in a Broad Range of pH and Ionic Strength

We introduce the grand-reaction method for coarse-grained simulations of acid-base equilibria in a system coupled to a reservoir at a given pH and concentration of added salt. It can be viewed as an extension of the constant-pH method and the reaction ensemble, combining explicit simulations of reactions within the system, and grand-canonical exchange of particles with the reservoir. Unlike the previously introduced methods, the grand-reaction method is applicable to acid-base equilibria in the whole pH range because it avoids known artifacts. However, the method is more general, and can be used for simulations of any reactive system coupled to a reservoir of a known composition. To demonstrate the advantages of the grand-reaction method, we simulated a model system: A solution of weak polyelectrolytes in equilibrium with a buffer solution. By carefully accounting for the exchange of all constituents, the method ensures that all chemical potentials are equal in the system and in the multi-component reservoir. Thus, the grand-reaction method is able to predict non-monotonic swelling of weak polyelectrolytes as a function of pH, that has been known from mean-field predictions and from experiments but has never been observed in coarse-grained simulations. Finally, we outline possible extensions and further generalizations of the method, and provide a set of guidelines to enable safe usage of the method by a broad community of users.<br><br>

Simulations of ionization equilibria in weak polyelectrolyte solutions and gels

This article recapitulates the state of the art regarding simulations of ionization equilibria of weak polyelectrolyte solutions and gels.

Non-LTE Abundances in OB stars: Preliminary Results for 5 Stars in the Outer Galactic Disk

The aim of this study is to analyse and determine elemental abundances for a large sample of distant B stars in the outer Galactic disk in order to constrain the chemical distribution of the Galactic disk and models of chemical evolution of the Galaxy. Here, we present preliminary results on a few stars along with the adopted methodology based on securing simultaneous O and Si ionization equilibria with consistent NLTE model atmospheres.