arXiv subjects

Yan Levin

Publications and source records attributed to Yan Levin.

At least 19 recordsLinked to original sources

arXiv2026

Electrolytes confined between polarizable surfaces in slit pores with anisotropic permittivity tensor

We present a method that enables efficient simulations of coarse-grained electrolyte solutions inside a narrow slit pore with an anisotropic dielectric permittivity tensor. The electrostatic equations for polarizable surfaces are solved using a 2D periodic Green's function method combined with a slab-corrected anisotropic 3D Ewald summation. We apply this approach in Monte Carlo simulations to study 1:1 electrolytes confined between both polarizable dielectric and metallic surfaces. Our results show that dielectric anisotropy aggressively reshapes the double-layer structure. While a coordinate stretching transformation demonstrates that individual ion-image interactions depend strictly on the bulk-like parallel permittivity, the suppression of the perpendicular permittivity dramatically amplifies direct in-plane ion-ion correlations. Under strong anisotropy, these lateral correlations dominate the thermodynamics completely, rendering the structural profiles of mutually opposing dielectric and metallic boundaries practically identical by forcing the smaller cations directly into the contact plane of the larger anions.

cond-mat.soft↗

arXiv2026

Comment on: Discontinuous codimension-two bifurcation in a Vlasov equation (arXiv:2212.01250)

We comment on the recent work by Yamaguchi and Barr\'e [Phys. Rev. E 107, 054203 (2023)], which uses linear stability analysis of the Vlasov equation to characterize phase transitions in a generalized Hamiltonian Mean Field (gHMF) model. By performing extensive molecular dynamics simulations with $N=10^8$ particles, we demonstrate that the bifurcation analysis of the initial stationary distribution is insufficient to predict either the location or the nature of the phase transition to a quasi-stationary state (qSS). Specifically, we show that for bimodal momentum distributions, the instability threshold identified by the authors does not correspond to a ferromagnetic transition; instead, the system remains in a paramagnetic state characterized by magnetization oscillations with a zero time-average. We find that the true paramagnetic-ferromagnetic transition is discontinuous (first-order) and occurs at significantly larger coupling strengths, characterized by a clear coexistence of states. These results indicate that linear bifurcation and symmetry-breaking phase transitions are distinct phenomena in long-range interacting systems, and that the former lacks the predictive power to describe the long-time fate of the system.

cond-mat.stat-mech↗

arXiv2025

Brownian dynamics simulations of electric double-layer capacitors with tunable metallicity

We introduce an efficient description of electrodes, characterized by their Thomas-Fermi screening length lTF inside the metal, for Brownian dynamics (BD) simulations of capacitors. Within a Born-Oppenheimer approximation for the electron charge density inside the electrodes, we derive the effective many-body potential for ions in an implicit solvent between Thomas-Fermi electrodes, taking into account the constraints of applied voltage and of global electro-neutrality of the system, as well as the 2D periodic boundary conditions along the electrode surfaces. We derive the average charge and the fluctuation-dissipation relation for the differential capacitance, highlighting the contribution of the fluctuations of the net ionic dipole moment, as well as those from the solvent polarization and of the electron density, whose fluctuations are suppressed within the Born-Oppenheimer description. We demonstrate the relevance of this model by validating its predictions against known results for the force on ions as a function of the ion-surface distance in simple geometries. The equilibrium ionic density profiles from BD simulations are in excellent agreement with those from an explicit electrode model for perfect metals, and are obtained at a significantly lower computational cost. Finally, we discuss with the present model the effect of the Thomas-Fermi screening length on the equilibrium ionic density profiles and the capacitance. While limited to parallel plate capacitors, the present simulation method allows to consider larger systems, lower concentrations, and longer time scales concentrations than molecular simulations in order to predict the electrochemical properties of Thomas-Fermi capacitors and correlate them with the ion dynamics.

physics.chem-ph↗

arXiv2025

Mixing and Ergodicity in Systems with Long-Range Interactions

We present a theory of collisionless relaxation in systems with long-range interactions. Contrary to Lynden-Bell's theory of violent relaxation, which assumes global ergodicity and mixing, we show that quasi-stationary states (qSS) observed in these systems exhibit broken global ergodicity. We propose that relaxation towards equilibrium occurs through a process of local mixing, where particles spread over energy shells defined by the manifold to which their trajectories are confined. To demonstrate our theory, we study the Hamiltonian Mean Field (HMF) model, a paradigmatic system with long-range interactions. Our theory accurately predicts the particle distribution functions in qSS observed in molecular dynamics simulations without any adjustable parameters. Additionally, it precisely forecasts the phase transitions observed in the HMF model.

cond-mat.stat-mech↗

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 S\o rensen'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

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. Nov\'a, O. Rud, F. Uhl\'ik, 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↗

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

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

Thermodynamic collapse in a lattice-gas model for a two-component system of penetrable particles

We study a lattice-gas model of penetrable particles on a square-lattice substrate with same-site and nearest-neighbor interactions. Penetrability implies that the number of particles occupying a single lattice site is unlimited and the model itself is intended as a simple representation of penetrable particles encountered in realistic soft-matter systems. Our specific focus is on a binary mixture, where particles of the same species repel and those of the opposite species attract each other. As a consequence of penetrability and the unlimited occupation of each site, the system exhibits thermodynamic collapse, which in simulations is manifested by an emergence of extremely dense clusters scattered throughout the system with energy of a cluster $E\propto -n^2$ where $n$ is the number of particles in a cluster. After transforming a particle system into a spin system, in the large density limit the Hamiltonian recovers a simple harmonic form, resulting in the discrete Gaussian model used in the past to model the roughening transition of interfaces. For finite densities, due to the presence of a non-harmonic term, the system is approximated using a variational Gaussian model.

cond-mat.soft↗

arXiv2020

Entropy production and Vlasov equation for self-gravitating systems

The evolution of a self-gravitating system to a non-equilibrium steady state occurs through a process of violent relaxation. In the thermodynamic limit the dynamics of a many body system should be governed by the Vlasov equation. Recently, however, a question was raised regarding the validity of Vlasov equation during the process of violent relaxation. In this paper we will explore the entropy production during the relaxation process using N-body molecular dynamics simulations. We will show that the entropy production time grows as $N^\alpha$, with $\alpha > 0$ and in the limit $N \rightarrow \infty$, entropy will remain constant, consistent with the Vlasov equation. Furthermore, we will show that the mean field dynamics constructed on the basis of the Vlasov equation is in excellent agreement with the full molecular dynamics simulations, justifying the applicability of Vlasov equation during the violent relaxation phase of evolution.

cond-mat.stat-mech↗

arXiv2020

Linear and non-linear instabilities of Kirchhoff's Elliptical Vortices

We investigate the stability of a uniform elliptical vortex in a two-dimensional incompressible Euler fluid. It's demonstrated that for small eccentricities, the vortex relaxes to a core-halo structure that undergoes rigid rotation with the central core remaining elliptical. For large eccentricities, the vortex splits into two quasi-circular vortices that revolve around the center of mass. Independent of the aspect ratio, the steady-state displays a low-density halo. We present a theory that qualitatively explains the transition between the two states. All theoretical results are compared with extensive molecular dynamics simulations based on the vortex-in-cell algorithm.

physics.flu-dyn↗

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↗

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↗