arXiv subjects

Amin Bakhshandeh

Publications and source records attributed to Amin Bakhshandeh.

At least 19 recordsLinked to original sources

arXiv2023

Titration in Canonical and Grand-Canonical Ensembles

We discuss problems associated with the notion of pH in heterogeneous systems. For homogeneous systems, standardization protocols lead to a well defined quantity, which although different from Sørensen's original idea of pH, is well reproducible and has become accepted as the measure of the ``hydrogen potential". On the other hand, for heterogeneous systems, pH defined in terms of the chemical part of the electrochemical activity is thermodynamically inconsistent and runs afoul of the Gibbs-Guggenheim principle that forbids splitting of the electrochemical potential into separate chemical and electrostatic parts -- since only the sum of two has any thermodynamic meaning. The problem is particularly relevant for modern simulation methods which involve charge regulation of proteins, polyelectrolytes, nanoparticles, colloidal suspensions etc. In this paper we show that titration isotherms calculated using semi-grand canonical simulations can be very different from the ones obtained using canonical reactive Monte Carlo simulations.

cond-mat.soft↗

arXiv2023

A new method for reactive constant pH simulations

We present a simulation method that allows us to calculate the titration curves for systems undergoing protonation/deprotonation reactions -- such as charged colloidal suspensions with acidic/basic surface groups, polyelectrolytes, polyampholytes, proteins, etc. The new approach allows us to simultaneously obtain titration curves both for systems in contact with salt and acid reservoir (semi-grand canonical ensemble) and for isolated suspensions (canonical ensemble). To treat the electrostatic interactions, we present a new method based on Ewald summation -- which accounts for the existence of both Bethe and Donnan potentials within the simulation cell. We show that that the Donnan potential affects dramatically the pH of suspension. Counter intuitively, we find that for concentrated suspensions of low ionic strength, the number of deprotonated groups can be 100\% larger in an isolated system, compared to a system connected to a reservoir by a semi-permeable membrane -- with both systems being at exactly the same pH.

cond-mat.stat-mech↗

arXiv2023

Interaction between two overall neutral charged microscopically patterned surfaces

We study the interaction between heterogeneously charged surfaces in an electrolyte solution by employing classical Density Functional Theory (cDFT) and Monte Carlo simulations. We observe a consistent behavior between cDFT and Monte Carlo simulations regarding force curves and two-dimensional density profiles. Armed with the validated cDFT, we explore the system's behavior under parameters challenging to simulate directly .Our findings include impacts of domain size, domain charge, domain charge configuration, and bulk electrolyte concentration on the osmotic pressure. Remarkably, the force curve is more sensitive to the domain size for asymmetric configuration than symmetry configuration; the bulk concentration weakly influences the force curve independent of the system configurations.

cond-mat.soft↗

arXiv2023

Charge fluctuations in charge regulated systems: dependence on statistical ensemble

We investigate charge regulation of nanoparticles in concentrated suspensions, focusing on the effect of different statistical ensembles. We find that the choice of ensemble does not affect the mean charge of nanoparticles, but significantly alters the magnitude of its fluctuation. Specifically, we compared the behaviors of colloidal charge fluctuations in the semi-grand canonical and canonical ensembles, and identified significant differences between the two. The choice of ensemble -- whether the system is isolated or is in contact with a reservoir of acid and salt -- will, therefore, affect the Kirkwood-Shumaker fluctuation-induced force inside concentrated suspensions. Our results emphasize the importance of selecting an appropriate ensemble that accurately reflects the experimental conditions when studying fluctuation-induced forces between polyelectrolytes, proteins, and colloidal particles in concentrated suspensions.

cond-mat.soft↗

arXiv2023

Comment on "Simulations of ionization equilibria in weak polyelectrolyte solutions and gels" by J. Landsgesell, L. Nová, O. Rud, F. Uhlík, D. Sean, P. Hebbeker, C. Holm and P. Ko\v sovan, Soft Matter, 2019,15, 1155-1185

In a recent review~Landsgesell et al., Soft Matter {\bf 15}, 1155 (2019) stated that $\text{pH} - \text{pK}_a$ is a ``universal parameter" for titrating systems. We show that this is not the case. This broken symmetry has important implications for constant pH (cpH) simulations. In particular, we show that for concentrated suspensions the error resulting from the use of cpH algorithm described by Landsgesell et al. is very significant, even for suspension containing 1:1 electrolyte. We show how to modify the cpH algorithm to account for the grand-canonical nature of the cpH simulations and for the charge neutrality requirement.

cond-mat.soft↗

arXiv2022

Theory of Charge Regulation of Colloidal Particles in Electrolyte Solutions

We present a theory that enables us to calculate the effective surface charge of colloidal particles and to efficiently obtain titration curves for different salt concentrations. The theory accounts for the shift of pH of solution due to the presence of 1:1 electrolyte. It also accounts self-consistently for the electrostatic potential produced by the deprotonated surface groups. To examine the accuracy of the theory we have performed extensive reactive Monte Carlo simulations, which show excellent agreement between theory and simulations without any adjustable parameters.

cond-mat.soft↗

arXiv2022

Widom insertion method in simulations with Ewald summation

We discuss the application of Widom insertion method for calculation of the chemical potential of individual ions in computer simulations with Ewald summation. Two approaches are considered. In the first approach, an individual ion is inserted into a periodically replicated overall charge neutral system representing an electrolyte solution. In the second approach an inserted ion is also periodically replicated, leading to the violation of the overall charge neutrality. This requires an introduction of an additional neutralizing background. We find that the second approach leads to a much better agreement with the results of grand canonical Monte Carlo simulation for the total chemical potential of a neutral ionic cluster.

cond-mat.soft↗

arXiv2021

Reactive Monte Carlo Simulations for Charge Regulation of Colloidal Particles

We use a reactive Monte Carlo simulation method and primitive model of electrolyte to study acid-base equilibrium that controls charge regulation in colloidal systems. The simulations are performed in a semi-grand canonical ensemble in which colloidal suspension is in contact with a reservoir of salt and strong acid. The interior of colloidal particles is modeled as a low dielectric medium, different from the surrounding water. The effective colloidal charge is calculated for different number of surface acidic groups, pH, salt concentrations, and types of electrolyte. In the case of potassium chloride the titration curves are compared with the the experimental measurements obtained using potentiometric titration. A good agreement is found between simulations and experiments. In the case of lithium chloride specific ionic adsorption is taken into account through partial dehydration of lithium ion.

cond-mat.soft↗

arXiv2021

Equilibrium conformations and surface charge regulation of spherical polymer brushes in stretched regimes

In the present work, we study the equilibrium conformations of linear polyelectrolytes tethered onto a spherical, oppositely charged core in equilibrium with an ionic reservoir of fixed concentration. Particular focus is placed on the situation of stretched chains, where the monomer concentration is known to display an inverse square-law decay far away from the spherical surface, which is then further extrapolated all the way down to the grafting core. While the equilibrium distributions of mobile ions are computed in the framework of a classical Density Functional Theory (cDFT) that incorporates both their size and electrostatic correlations within the grafted polyelectrolyte, the equilibrium configuration of the latter is described by its averaged radius of gyration, which is taken as a variational parameter that guarantees mechanical equilibrium across the polymer-solvent interface. The averaged particle size is then analyzed over a wide range of polymerization degrees, ionic concentrations and functionality of the polymer backbones. Two distinct regimes can be identified: at high ionic strengths, swelling of the grafted polymers is dominated by ionic entropic contribution as well as polymer size effects, whereas at low ionic concentrations a balance between electrostatic and entropic effects is the main driven mechanism for particle stretching. Using Monte Carlo simulations, we then proceed to investigate the effects of charge regulation when the brush core is further decorated with active functional sites randomly distributed over its surface, which act as receptors onto which dissolved acidic ions can be adsorbed.

cond-mat.soft↗

arXiv2020

Interaction between charge-regulated metal nanoparticles in an electrolyte solution

We present a theory which allows us to calculate the interaction potential between charge-regulated metal nanoparticles inside an acid-electrolyte solution. The approach is based on the recently introduced model of charge regulation which permits us to explicitly -- within a specific microscopic model -- relate the bulk association constant of a weak acid to the surface association constant for the same weak acid adsorption sites. When considering metal nanoparticles we explicitly account for the effect of the induced surface charge in the conducting core. To explore the accuracy of the approximations, we compare the ionic density profiles of an isolated charge-regulated metal nanoparticle with explicit Monte Carlo simulations of the same model. Once the accuracy of the theoretical approach is established, we proceed to calculate the interaction force between two charge-regulated metal nanoparticles by numerically solving the Poisson-Boltzmann equation with charge regulation boundary condition. The force is then calculated by integrating the electroosmotic stress tensor. We find that for metal nanoparticles the charge regulation boundary condition can be well approximated by the constant surface charge boundary condition, for which a very accurate Derjaguin-like approximation was recently introduced. On the other hand, a constant surface potential boundary condition often used in colloidal literature, shows a significant deviation from the charge regulation boundary condition for particles with large charge asymmetry.

cond-mat.soft↗

arXiv2020

Electrolytes in regimes of strong confinements: surface charge modulations, osmotic equilibrium and electroneutrality

In the present work, we study an electrolyte solution confined between planar surfaces with nonopatterned charged domains, which has been connected to a bulk ionic reservoir. The system is investigated through an improved Monte Carlo (MC) simulation method, suitable for simulation of electrolytes in the presence of modulated surface charge distributions. We also employ a linear approach in the spirit of the classical Debye-Hückel approximation, which allows one to obtain explicit expressions for the averaged potentials, ionic profiles, effective surface interactions and the net ionic charge confined between the walls. Emphasis is placed in the limit of strongly confined electrolytes, in which case local electroneutrality in the inter-surface space might not be fulfilled. In order to access the effects of such lack of local charge neutrality on the ionic-induced interactions between surfaces with modulated charge domains, we consider two distinct model systems for the confined electrolyte: one in which a salt reservoir is explicitly taken into account {\it via} the osmotic equilibrium with an electrolyte of fixed bulk concentration, and a second one in which the equilibrium with a charge neutral ionic reservoir is implicitly considered. While in the former case the osmotic ionic exchange might lead to non-vanishing net charges, in the latter model charge neutrality is enforced through the appearance of an implicit Donnan potential across the charged interfaces. A strong dependence of the ionic-induced surface interactions in the employed model system is observed at all particle separations. These findings strongly suggest that due care is to be taken while choosing among different scenarios to describe the ionic exchanging in electrolytes confined between charged surfaces, even in cases when the monopole (non zero net charge) surface contributions are absent.

cond-mat.soft↗

arXiv2020

Charge regulation of colloidal particles in aqueous solutions

We study charge regulation of colloidal particles inside aqueous electrolyte solutions. To stabilize colloidal suspension against precipitation, colloidal particles are synthesized with either acidic or basic groups on their surface. In contact with water these surface groups undergo proton transfer reaction, resulting in colloidal surface charge. The charge is determined by the condition of local chemical equilibrium between hydronium ions inside the solution and at the colloidal surface. We use a model of Baxter sticky spheres to explicitly calculate the equilibrium dissociation constants and to construct a theory which is able to quantitatively predict the effective charge of colloidal particles with either acidic or basic surface groups. The predictions of the theory for the model are found to be in excellent agreement with the results of Monte Carlo simulations. The theory is further extended to treat colloidal particles with a mixture of both acidic and basic surface groups.

cond-mat.soft↗

arXiv2020

Theoretical investigation of a polarizable colloid in the salt medium

In the present work, we have extended a weak coupling theory [A. Bakhshandeh, A. P. dos Santos and Y. Levin Phys. Rev. Lett 107, 107801 (2011)] for systems with added 1:1 electrolyte. To study the accuracy of the developed theory, we compare its numerical predictions with Monte Carlo simulation data and a recent theory which accounts for the surface polarization. A very good agreement is found for the case of monovalent electrolytes, up to very high salt concentrations and different colloidal charges

cond-mat.soft↗

arXiv2020

Osmotic stress and pore nucleation in charged biological nanoshells and capsids

A model system is proposed to investigate the chemical equilibrium and mechanical stability of biological spherical-like nanoshells in contact with an aqueous solution with added dissociated electrolyte of a given concentration. The ionic chemical equilibrium across the permeable shell is investigated in the framework of an accurate Density Functional Theory (DFT) that incorporates electrostatic and hardcore correlations beyond the traditional mean-field ({\it e. g.}, Poisson-Boltzmann) limit. The accuracy of the theory is tested by a direct comparison with Monte Carlo (MC) simulations. A simple analytical expression is then deduced which clearly highlights the entropic, electrostatic, and self-energy contributions to the osmotic stress over the shell in terms of the calculated ionic profiles. By invoking a continuum mean-field elastic approach to account for the shell surface stress upon osmotic stretching, the mechanical equilibrium properties of the shell under a wide variety of ionic strengths and surface charges are investigated. The model is further coupled to a continuum mechanical approach similar in structure to a Classical Nucleation Theory (CNT) to address the question of mechanical stability of the shells against a pore nucleation. This allows us to construct a phase diagram which delimits the mechanical stability of capsids for different ionic strengths and shell surface charges.

cond-mat.soft↗

arXiv2020

Adsorption of polyelectrolytes on charged microscopically patterned surfaces

In the present study, we have investigated, using Monte Carlo simulations (MC), the adsorption of polyelectrolytes on the charged nanopatterned surfaces. Different surface patterns were considered and we noticed that the amount of adsorption is directly dependent on the size of the domains. Also in the case of checkerboard configuration, it was observed that the polyelectrolytes are aligned along the diagonal of square domains.

cond-mat.soft↗

arXiv2019

Charge Regulation of Colloidal Particles: Theory and Simulations

To explore charge regulation (CR) in physicochemical and biophysical systems, we present a model of colloidal particles with sticky adsorption sites which account for the formation of covalent bonds between the hydronium ions and the surface functional groups. Using this model and Monte Carlo simulations, we find that the standard Ninham and Parsegian (NP) theory of CR leads to results which deviate significantly from computer simulations. The problem of NP approach is traced back to the use of bulk equilibrium constant to account for surface chemical reactions. To resolve this difficulty we present a new theory of CR. The fundamental ingredient of the new approach is the sticky length, which is non-trivially related with the bulk equilibrium constant. The theory is found to be in excellent agreement with computer simulations, without any adjustable parameters. As an application of the theory we calculate the effective charge of colloidal particles containing carboxyl groups, as a function of pH and salt concentration.

cond-mat.soft↗

arXiv2018

Adsorption isotherms of charged nanoparticles

We present theory and simulations which allow us to quantitatively calculate the amount of surface adsorption excess of charged nanoparticles onto a charged surface. The theory is very accurate for weakly charged nanoparticles and can be used at physiological concentrations of salt. We have also developed an efficient simulation algorithm which can be used for dilute suspensions of nanoparticles of any charge, even at very large salt concentrations. With the help of the new simulation method, we are able to efficiently calculate the adsorption isotherms of highly charged nanoparticles in suspensions containing multivalent ions, for which there are no accurate theoretical methods available.

cond-mat.soft↗

arXiv2015

Interaction between random heterogeneously charged surfaces in an electrolyte solution

We study, using Monte Carlo simulations, the interaction between infinite heterogeneously charged surfaces inside an electrolyte solution. The surfaces are overall neutral with quenched charged domains. An average over the quenched disorder is performed to obtain the net force. We find that the interaction between the surfaces is repulsive at short distances and is attractive for larger separations.

cond-mat.soft↗