arXiv subjects

O. Patsahan

Publications and source records attributed to O. Patsahan.

At least 19 recordsLinked to original sources

arXiv2026

On the electrical double layer capacitance of the restricted primitive model: a link between the mesoscopic theory and the associative mean spherical approximation

The results for the electrical double layer capacitance and the density of ``free ions'' obtained from the mesoscopic theory are compared with the corresponding results of the associative mean spherical approximation. While the first theory takes into account the fluctuations of the charge density, the second theory assumes that the free ions and ion pairs are in chemical equilibrium according to the mass action law. Our results demonstrate a fairly good agreement between the two theories at high densities and low temperatures.

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arXiv2025

Mutual effect of charge- and number-density correlations in ionic liquids and concentrated electrolytes

Correlation functions in concentrated ionic systems are studied within the mesoscopic theory at the level of the Gaussian approximation. The previously neglected fluctuation contribution to the inverse charge-charge correlation function is taken into account to verify the accuracy of the previous results. We calculate the correlation lengths and the amplitudes and show that the fluctuation contribution does not lead to significant changes of the results. We also derive necessary conditions for the presence of both, the oscillatory and the monotonic decays of the charge-charge correlations that must be satisfied by the noncoulombic contributions to the inverse charge-charge correlation function. At the level of the Gaussian approximation, these conditions are not satisfied. Extension of the theory beyond the Gaussian approximation is necessary to verify whether the asymptotic decay of the charge-charge correlations is monotonous or oscillatory, as suggested by the surface force apparatus or by the SAXS experiments, respectively.

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arXiv2024

Mesoscopic theory for a double layer capacitance in concentrated ionic systems

Effect of an oscillatory decay of the charge density in concentrated ionic solutions and ionic liquids on the double-layer capacitance is studied in a framework of a mesoscopic theory. Only Coulomb and steric forces between the ions that are present in all ionic systems are taken into account. We show that the charge oscillations lead to a rescaled distance between the electrode and the virtual monolayer of counterions in the Helmholtz capacitance, and the scaling factor depends on the period of the charge oscillations. Our very simple formula for large density of ions and small voltage can serve as a reference point for the double layer capacitance in concentrated ionic solutions and ionic liquids, and can help to disentangle the universal and specific contributions to the capacitance in particular systems.

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arXiv2024

Phase behaviour of primitive models of molecular ionic liquids in porous media: effects of cation shape, ion association and disordered confinement

The phase behaviour of room-temperature ionic liquids (ILs) confined in disordered porous media is studied using a theoretical approach that combines an extension of scaled particle theory, Wertheim's thermodynamic perturbation theory, and the associative mean spherical approximation. Two models, differing in the shape of the molecular cation, are considered: one with cations formed as charged flexible chains and the other with cations modelled as charged hard spherocylinders. Each model is described by a mixture of dimerized and free ions, while the porous medium is represented as a disordered matrix of hard spheres. We focus on the effects of the molecular cation shape, partial ion association, and disordered confinement on the liquid-vapour-like phase behaviour of the model ILs. In the approximation considered, we find that both the critical temperature and critical density in the model with spherocylinder cations are lower than those in the model with chain cations, and the phase coexistence region is narrower. This is the first theoretical attempt to describe an IL model with molecular ions shaped as spherocylinders, particularly in a disordered porous medium.

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arXiv2024

Vapour-liquid phase behaviour of primitive models of ionic liquids confined in disordered porous media

We develop a theory for the description of ionic liquids (ILs) confined in a porous medium formed by a matrix of immobile randomly placed uncharged particles. The IL is modelled as an electroneutral mixture of hardsphere anions and flexible linear chain cations, represented by tangentially bonded hard spheres with the charge located on one of the terminal beads. The theory combines a generalization of the scaled particle theory, Wertheim's thermodynamic perturbation theory, and the associative mean-spherical approximation and allows one to obtain analytical expressions for the pressure and chemical potentials of the matrix-IL system. Using the theory, we calculate the vapour-liquid phase diagrams for two versions of the IL model, i.e., when the cation is modelled as a dimer and as a chain, in a complete association limit. The effects of the matrix confinement and of the non-spherical shape of the cations on the vapour-liquid phase diagrams are studied.

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arXiv2023

Spontaneous pattern formation in monolayers of binary mixtures with competing interactions

A model for a monolayer of two types of particles spontaneously forming ordered patterns is studied by a mesoscopic theory and by MC simulations. We assume hard-cores of the same size for both components, short-range attraction long-range repulsion between particles of the same species, and the cross-interaction of opposite sign. The model is inspired by oppositely charged hydrophilic and hydrophobic particles or macromolecules with preferential solubility in different components of a solvent that is close to a miscibility critical point. We determine the phase diagram in the chemical potentials and in the concentration - density planes for a few fixed temperatures in the mean-field one-shell approximation. We find that the presence of the second component significantly enlarges the temperature range of stability of the ordered phases. We obtain three stable phases with periodic concentration: the lamellar L phase with alternating stripes of the two components for similar chemical potentials, and a hexagonal arrangement of the clusters of the minority component in the liquid of the majority component. The latter two phases, however, are stable only at relatively high temperatures. At lower temperatures, the L phase coexists with a disordered one-component fluid or with very dilute gas with mixed components. At still lower temperatures, the one-component phase coexisting with the L phase can be disordered or ordered, depending on the chemical potentials. The theoretical results are confirmed by MC simulations for selected thermodynamic states.

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arXiv2022

Structure of ionic liquids and concentrated electrolytes from a mesoscopic theory

Recently, underscreening in concentrated electrolytes was discovered in experiments and confirmed in simulations and theory. It was found that the correlation length of the charge-charge correlations, $λ_s$, satisfies the scaling relation $λ_s/λ_D\sim (a/λ_D)^n$, where $λ_D$ is the Debye screening length and $a$ is the ionic diameter. However, different values of n were found in different studies. In this work we solve this puzzle within the mesocopic theory that yielded n=3 in agreement with experiments, but only very high densities of ions were considered [A. Ciach A. and O. Patsahan, {\it J.Phys.: Condens. Matter} {\textbf 33}, 37LT01 (2021)]. Here we apply the theory to a broader range of density of ions and find that different values of n in the above scaling can yield a fair approximation for $λ_s/λ_D$ for different ranges of $a/λ_D$. The experimentally found scaling holds for $2 <a/λ_D<4$, and we find n=3 for the same range of the reduced Debye length. For smaller $a/λ_D$, we find n=2 obtained earlier in several simulation and theoretical studies, and still closer to the Kirkwood line we obtain n=1.5 that was also predicted in different works. It follows from our theory that n=3 (i.e. $λ_s$ is proportional to the density of ions) when the variance of the local charge density is large, and $λ_s$ is proportional to this variance times the Bjerrum length. Detailed derivation of the theory is presented.

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arXiv2022

Fluid-fluid phase behaviour in the explicit solvent ionic model: hard spherocylinder solvent molecules

We study a fluid-fluid phase transition of the explicit solvent model represented as a mixture of the restricted primitive model (RPM) of ionic fluid and neutral hard spherocylinders (HSC). To this end, we combine two theoretical approaches, i.e., the scale particle theory (SPT) and the associative mean spherical approximation (AMSA). Whereas the SPT is sufficient to provide a rather good description of a reference system taking into account hard-core interactions, the AMSA is known to be efficient in treating the Coulomb interactions between the ions. Alternatively, we also use the mean spherical approximation (MSA) for comparison. In general, both approximations lead to similar qualitative results for the phase diagrams: the region of coexisting envelope becomes broader and shifts towards larger densities and higher temperatures when the pressure increases. However, the AMSA and the MSA produce different concentration dependences, i.e., contrary to the MSA, the AMSA phase diagrams show that the high-density phase mostly consists of the ions for all pressures considered. To demonstrate the effect of asphericity of solvent molecules on the fluid-fluid phase transition, we consider an "equivalent" mixture in which the HSC particles are replaced by hard spheres (HS) of the same volume. It is observed that in the case of HSC solvent (RPM-HSC model), the region of phase coexistence is wider than for the case of the solvent molecules being of spherical shape (RPM-HS model). It is also found that the critical temperature is higher in the RPM-HSC model than in the RPM-HS model, though it becomes the same at higher pressures in the MSA, while in the AMSA this difference remains essential.

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arXiv2021

Correct scaling of the correlation length from a theory for concentrated electrolytes

Self-consistent theory for concentrated electrolytes is developed. Oscillatory decay of the charge-charge correlation function with the decay length that shows perfect agreement with the experimentally discovered and so far unexplained scaling is obtained. For the density-density correlations, monotonic asymptotic decay with the decay length comparable with the decay length of the charge correlations is found. We show that the correlation lengths in concentrated electrolytes depend crucially on the local variance of the charge density.

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arXiv2020

Self-assembly in mixtures with competing interactions

A binary mixture of particles interacting with spherically-symmetric potentials leading to microsegregation is studied by theory and molecular dynamics (MD) simulations. We consider spherical particles with equal diameters and volume fractions. Motivated by the mixture of oppositely charged particles with different adsorption preferences immersed in near-critical binary solvent, we assume short-range attraction long-range repulsion for the interaction between like particles, and short range repulsion long-range attraction for the interaction between different ones. In order to predict structural and thermodynamic properties of such complex mixtures, we develop a theory combining the density functional and field-theoretic methods. We show that concentration fluctuations in mesoscopic regions lead to a qualitative change of the phase diagram compared to mean-field predictions. Both theory and MD simulations show coexistence of a low-density disordered phase with a high-density phase with alternating layers rich in the first and the second component. The density and the degree of order of the ordered phase decrease with increasing temperature, up to a temperature where the theory predicts a narrow two-phase region with increasing density of both phases for increasing temperature. MD simulations show that monocrystals of the solid and liquid crystals have a prolate shape with the axis parallel to the direction of concentration oscillations, and the deviation from the spherical shape increases with increasing periodic order.

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arXiv2020

Correlation functions in mixtures with energetically favoured nearest-neighbours of different kind: a size-asymmetric case

Binary mixtures of hard-spheres with different diameters and square-well attraction between different particles are studied by theory and Monte Carlo simulations. In our mesoscopic theory, local fluctuations of the volume fraction of the two components are taken into account. Semi-quantitative agreement between the simulation and theoretical results is obtained, except from very small distances. The correlation functions exhibit exponentially damped oscillations, with the period determined by the interaction potential, and both the amplitude and the correlation length increasing significantly with increasing diameter ratio. Increasing size asymmetry leads also to decreasing fluctuations of the number of the smaller particles in the attractive shell of the bigger ones. For small size asymmetry, the strongest correlations occur for comparable volume fraction of the two components. When the size ratio increases, the maximum of the structure factor moves to a larger volume fraction of the bigger particles, and for the size ratio as large as 4, the maximum goes beyond the accessible range of volume fractions. Our results show that when the neighbourhood of different particles is energetically favoured, the particles are much more uniformly distributed than in the random distribution even at relatively high temperature, especially for large size asymmetry.

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arXiv2020

Effects of fluctuations on correlation functions in inhomogeneous mixtures

Approximate expressions for correlation functions in binary inhomogeneous mixtures are derived in a framework of the mesoscopic theory [Ciach A., Mol. Phys., 2011, {\textbf{109}}, 1101]. Fluctuation contribution is taken into account in a Brazovskii-type approximation. Explicit results are obtained for two model systems. In the two models, the diameters of the hard cores of particles are equal, and the interactions favour a periodic arrangement of alternating species A and B. However, the optimal distance between the species A and B is much different in the two models. Theoretical results for different temperature and volume fractions of the two components are compared with the results of Monte Carlo simulations, and the structure is illustrated by simulation snapshots. Despite different interaction potentials and different length scale of the local ordering, properties of the correlation functions in the two models are very similar.

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arXiv2016

Vapour-liquid phase diagram for an ionic fluid in a random porous medium

We study the vapour-liquid phase behaviour of an ionic fluid confined in a random porous matrix formed by uncharged hard sphere particles. The ionic fluid is modelled as an equimolar binary mixture of oppositely charged equisized hard spheres, the so-called restricted primitive model (RPM). Considering the matrix-fluid system as a partly-quenched model, we develop a theoretical approach which combines the method of collective variables with the extension of the scaled-particle theory (SPT) for a hard-sphere fluid confined in a disordered hard-sphere matrix. The approach allows us to formulate the perturbation theory using the SPT for the description of the thermodynamics of the reference system. The phase diagrams of the RPM in matrices of different porosities and for different size ratios of matrix and fluid particles are calculated in the random-phase approximation and also when the effects of higher-order correlations between ions are taken into account. Both approximations correctly reproduce the basic effects of porous media on the vapour-liquid phase diagram, i.e., with a decrease of porosity the critical point shifts toward lower fluid densities and lower temperatures and the coexistence region is getting narrower. For the fixed matrix porosity, both the critical temperature and the critical density increase with an increase of size of matrix particles and tend to the critical values of the bulk RPM.

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arXiv2015

Gas-liquid critical point of the ultrasoft restricted primitive model from analytic theory

Gas-liquid criticality in the ultrasoft restricted primitive model (URPM) of polyelectrolytes is studied using the collective variables-based theory. For the model, an effective Hamiltonian is derived and explicit expressions for all the coefficients are found in a one-loop approximation. Based on this Hamiltonian, the phase and critical behaviour is analysed. Our results provide evidence that the nature of the gas-liquid criticality in the URPM is the same as in the restricted primitive model that includes a hard core.

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arXiv2014

Gas-liquid phase equilibrium in ionic fluids: Coulomb versus non-Coulomb interactions

Using the collective variables theory, we study the effect of competition between Coulomb and dispersion forces on the gas-liquid phase behaviour of a model ionic fluid, i.e. a charge-asymmetric primitive model with additional short-range attractive interactions. Both the critical parameters and the coexistence envelope are calculated in a one-loop approximation as a function of the parameter $α$ measuring the relative strength of the Coulomb to short-range interactions. We found the very narrow region of $α$ bounded from the both sides by tricritical points which separates the models with "nonionic" and "Coulombic" phase behaviour. This is at variance with the result of available computer simulations where no tricritical point is found for the finely-discretized lattice version of the model.

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arXiv2014

Gas-liquid phase coexistence and crossover behavior of binary ionic fluids with screened Coulomb interactions: The effect of an interaction range

We study the gas-liquid phase diagram and the crossover behavior of a simple model of ionic fluid: an equimolar binary mixture of equisized hard spheres interacting through screened Coulomb potentials which are repulsive between particles of the same species and attractive between particles of different species. Using our previous results, we obtain explicit expressions for the relevant coefficients of the effective $φ^{4}$ Ginzburg-Landau Hamiltonian in a one-loop approximation. Within the framework of this approximation, we calculate the critical parameters and gas-liquid coexistence curves for different values of the dimensionless inverse screening length $z$. The critical temperature scaled by the Yukawa potential contact value as well as the critical packing fraction rapidly decrease with an increase of the interaction range and then for $z<0.05$ slowly approach the values found for a purely ionic model, i.e., a restricted primitive model (RPM). The both trends are qualitatively consistent with the results of Monte Carlo simulations. We find that gas-liquid coexistence region reduces with an increase of $z$ and completely vanishes at $z\simeq 2.781$. This qualitatively agrees with the results of Monte Carlo simulations indicating a stable gas-liquid coexistence for $z\leq 4$. It is also shown that an increase in the interaction range from the one typical of simple fluids to the one typical of ionic fluids leads to a decrease of the crossover temperature. For $z\simeq 0.01$, the crossover temperature is the same as for the RPM. For $z\simeq 2.781$, our results indicate a tricritical point.

cond-mat.stat-mech↗

arXiv2012

The method of collective variables: a link with the density functional theory

Recently, based on the method of collective variables the statistical field theory for multicomponent inhomogeneous systems was formulated [O. Patsahan, I. Mryglod, J.-M. Caillol, Journal of Physical Studies, 2007, 11, 133]. In this letter we establish a link between this approach and the classical density functional theory for inhomogeneous fluids.

cond-mat.stat-mech↗

arXiv2012

Spatial inhomogeneities in ionic liquids, charged proteins and charge stabilized colloids from collective variables theory

Effects of size and charge asymmetry between oppositely charged ions or particles on spatial inhomogeneities are studied for a large range of charge and size ratios. We perform a stability analysis of the primitive model (PM) of ionic systems with respect to periodic ordering using the collective variables based theory. We extend previous studies [A. Ciach et al., Phys. Rev.E \textbf{75}, 051505 (2007)] in several ways. First, we employ a non-local approximation for the reference hard-sphere fluid which leads to the Percus-Yevick pair direct correlation functions for the uniform case. Second, we use the Weeks-Chandler-Anderson regularization scheme for the Coulomb potential inside the hard core. We determine the relevant order parameter connected with the periodic ordering and analyze the character of the dominant fluctuations along the $λ$-lines. We show that the above-mentioned modifications produce large quantitative and partly qualitative changes in the phase diagrams obtained previously. We discuss possible scenarios of the periodic ordering for the whole range of size- and charge ratios of the two ionic species, covering electrolytes, ionic liquids, charged globular proteins or nanoparticles in aqueous solutions and charge-stabilized colloids.

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