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Marcelo Lozada-Cassou

Publications and source records attributed to Marcelo Lozada-Cassou.

16 recordsLinked to original sources
arXiv2026

Topological control of local electroneutrality in confined electrolytes

Topology governs finite-size violations of local electroneutrality in confined electrolytes. Within Poisson-Boltzmann theory, we show that this topological control gives rise to a universal hierarchy of deviations in spherical, cylindrical, and planar confinement. We quantify this effect through an electroneutrality deviation ratio that captures the global electrostatic constraints associated with compactness and boundary multiplicity in the three topological classes corresponding to slit, cylindrical, and spherical cavities. Although local electroneutrality is asymptotically restored in the limit of infinite cavity size, finite-size deviations follow a robust topological hierarchy, being strongest in spherical cavities, weaker in cylindrical confinement, and weakest in planar slits. These results prove that topology is the organizing principle underlying confinement-induced charge redistribution and that violations of local electroneutrality constitute a general electrostatic constraint governing overcharging, charge reversal, and long-range charge correlations. More fundamentally, they demonstrate that changing the topology modifies the global electrostatic constraints without altering the local Poisson-Boltzmann equations. As a counterintuitive manifestation of nonlinear confinement in a point-ion model, we report the existence of inside confinement charge reversal (ICCR) in charged hollow cylindrical and spherical nanoparticles. Since physically consistent, more general electrolyte theories must recover the Poisson-Boltzmann description in the appropriate limiting case, the present results establish a benchmark against which topological effects should be assessed beyond the Poisson-Boltzmann description.

cond-mat.soft↗
arXiv2025

Contact theorems for electrolyte-filled hollow charged nanoparticles: Non-linear osmotic pressure in confined electrolytes

Analytical expressions for the osmotic pressure of electrolytes confined within electrolyte-filled, hollow, charged nanoparticles are studied using contact theorems for cavity nanoshells immersed in a low-concentration electrolyte. Three shell geometries are considered: planar, cylindrical, and spherical. The nanoparticles are modeled as charged cavities of internal radius R and wall thickness d, and are assumed to be at infinite dilution in the surrounding bulk electrolyte. Numerical calculations of the osmotic pressure are presented as a function of several model parameters. For cylindrical and spherical shells, the osmotic pressure exhibits absolute maxima as a function of shell size. This behavior arises from the competition between the violation of the local electroneutrality condition (VLEC) within the shell cavity and the nonlinear profile of the effective electric field. In contrast, the osmotic pressure of the planar (slit-shell) geometry is a monotonic, nonlinear, decreasing function of cavity width. The results are analyzed in terms of the steric and electrostatic (Maxwell stress tensor) contributions appearing in the corresponding contact theorems. Because the analysis is restricted to low surface charge densities and low electrolyte concentrations, the electric double layers (EDLs) inside and outside the shells are obtained from analytical solutions of the linearized Poisson-Boltzmann equation. We report two novel phenomena: confinement charge reversal (CCR) and confinement overcharging (CO). These arise naturally from the topology of the system. By construction, the confined and bulk electrolytes are maintained at the same chemical potential. These findings have implications for the design of synthetic nanocapsules and ion-selective membranes.

cond-mat.soft↗
arXiv2018

Very long-range attractive and repulsive forces in Model Colloidal Dispersions

Experiments with polymer latex solutions show the coexistence of order-disorder structures of macroions. Because of the large macroions' sizes, this order-disorder phase coexistence imply the existence of very long-range attractive and repulsive forces, which can not be explained in terms of conventional direct interaction potentials, which are short-range. Here we apply an integral equations theory to a simple model for colloidal dispersions, at finite concentrations, calculate the particles distribution functions and the involved effective forces. We find very long-range attractive and repulsive forces among the like-charged macroions. The distribution functions are in qualitative agreement with experimental results. The origin of these forces are discussed in terms of an energy-entropy balance.

cond-mat.soft↗
arXiv2017

Long-range overcharging and long-range charge reversal in model colloidal dispersions

Most theoretical and simulation studies on charged particles suspensions are at infinite dilution conditions. Hence these studies have been focused on the electrolyte structure around an isolated central particle (or elctrode), where phenomena as charge reversal, charge inversion and overcharging have been shown to be relevant. However, experimental studiesat finite volume fraction exhibit interesting phenomenology which imply very long-range correlations. Inthis paper we apply an integral equation theory to a simple model for a charged macroions suspension, at finite volume fraction, and find two new effects of long-range overcharging and long-range charge reversal. These new effects are different from the classical overcharging and charge reversal in that they occur at finite macroion's volume fraction, far away from the central macroion, are much more intense, and increase, not decrease, as a function of the distance to the central particle, which is indicative of correlations at large separations. We find our results to be qualitative consistent with existing experimental results, and Monte Carlo simulations.

cond-mat.soft↗
arXiv2010

Entropy driven key-lock assembly

The effective interaction between a sphere with an open cavity (lock) and a spherical macroparticle (key), both immersed in a hard sphere fluid, is studied by means of Monte Carlo simulations. As a result, a 2d map of the key-lock effective interaction potential is constructed, which leads to the proposal of a self-assembling mechanism: there exists trajectories through which the key-lock pair could assemble avoiding trespassing potential barriers. Hence, solely the entropic contribution can induce their self-assembling even in the absence of attractive forces. This study points out the solvent contribution within the underlying mechanisms of substrate-protein assembly/disassembly processes, which are important steps of the enzyme catalysis and protein mediated transport.

cond-mat.mes-hall↗
arXiv2008

Population inversion of a NAHS mixture adsorbed into a cylindrical pore

A cylindrical nanopore immersed in a non-additive hard sphere binary fluid is studied by means of integral equation theories and Monte Carlo simulations. It is found that at low and intermediate values of the bulk total number density the more concentrated bulk species is preferentially absorbed by the pore, as expected. However, further increments of the bulk number density lead to an abrupt population inversion in the confined fluid and an entropy driven prewetting transition at the outside wall of the pore. These phenomena are a function of the pore size, the non-additivity parameter, the bulk number density, and particles relative number fraction. We discuss our results in relation to the phase separation in the bulk.

cond-mat.stat-mech↗
arXiv2008

On the regimes of charge reversal

Charge reversal of the planar electrical double layer is studied by means of a well known integral equations theory. By a numerical analysis, a diagram is constructed with the onset points of charge reversal in the space of the fundamental variables of the system. Within this diagram two regimes of charge reversal are identified, referred to as oscillatory and non oscillatory. We found that these two regimes can be distinguished through a simple formula. Furthermore, a symmetry between electrostatic and size correlations in charge reversal is exhibited. The agreement of our results with other theories and molecular simulations data is discussed.

cond-mat.soft↗
arXiv2008

Overcharging and charge reversal in the electrical double layer near the point of zero charge

The ionic adsorption around a weakly charged spherical colloid, immersed in size-asymmetric 1:1 and 2:2 salts, is studied. We use the primitive model of an electrolyte to perform Monte Carlo simulations as well as theoretical calculations by means of the hypernetted chain/mean spherical approximation (HNC/MSA) and the unequal-radius modified Gouy-Chapman (URMGC) integral equations. Structural quantities such as the radial distribution functions, the integrated charge, and the mean electrostatic potential are reported. Our Monte Carlo "experiments" evidence that near the point of zero charge the smallest ionic species is preferentially adsorbed onto the macroparticle, independently of the sign of the charge carried by this tiniest electrolytic component, giving rise to the appearance of the phenomena of charge reversal and overcharging. Accordingly, charge reversal is observed when the macroion is slightly charged and the coions are larger than the counterions. In the opposite situation, i.e. if the counterions are larger than the coions, overcharging occurs. In other words, in this paper we present the first simulational data on overcharging, showing that this novel effect surges close to the point of zero charge as a consequence of the ionic size asymmetry. Further, it is seen that the inclusion of hard-core correlations in HNC/MSA leads to spatial regions near the macroion's surface in which the integrated charge and/or the mean electrostatic potential can decrease when the colloidal charge is augmented and vice versa. These observations aware about the interpretation of electrophoretic mobility measurements using the standard Poisson-Boltzmann approximation beyond its validity region.

cond-mat.soft↗
arXiv2004

The electrical double layer for a fully asymmetric electrolyte around a spherical colloid: an integral equation study

The hypernetted chain/mean spherical approximation (HNC/MSA) integral equation is obtained and solved numerically for a totally asymmetric primitive model electrolyte around a spherical macroparticle. The ensuing radial distribution functions show a very good agreement when compared to our Monte Carlo and molecular dynamics simulations for spherical geometry and with respect to previous anisotropic reference HNC calculations in the planar limit. We report an analysis of the potential vs charge relationship, radial distribution functions, mean electrostatic potential and cumulative reduced charge for representative cases of 1:1 and 2:2 salts with a size asymmetry ratio of 2. Our results are collated with those of the Modified Gouy-Chapman (MGC) and unequal radius Modified Gouy-Chapman (URMGC) theories and with those of HNC/MSA in the restricted primitive model (RPM) to assess the importance of size asymmetry effects. One of the most striking characteristics found is that,\textit{contrary to the general belief}, away from the point of zero charge the properties of an asymmetric electrical double layer (EDL) are not those corresponding to a symmetric electrolyte with the size and charge of the counterion, i.e. \textit{counterions do not always dominate}. This behavior suggests the existence of a new phenomenology in the EDL that genuinely belongs to a more realistic size-asymmetric model where steric correlations are taken into account consistently. Such novel features can not be described by traditional mean field theories like MGC, URMGC or even by enhanced formalisms, like HNC/MSA, if they are based on the RPM.

cond-mat.soft↗
arXiv2003

A simple model for semipermeable membrane: Donnan equilibrium

We study a model for macroions in an electrolyte solution confined by a semipermeable membrane. The membrane finite thickness is considered and both membrane surfaces are uniformly charged. The model explicitly includes electrostatic and size particles correlations. Our study is focused on the adsorption of macroions on the membrane surface and on the osmotic pressure. The theoretical prediction for the osmotic pressure shows a good agreement with experimental results.

cond-mat.soft↗
arXiv2003

Ion pairing in model electrolytes: A study via three particle correlation functions

A novel integral equations approach is applied for studying ion pairing in the restricted primitive model (RPM) electrolyte, i. e., the three point extension (TPE) to the Ornstein-Zernike integral equations. In the TPE approach, the three-particle correlation functions $g^{[3]}({\bf r}_{1},{\bf r}_{2},{\bf r}_{3})$ are obtained. The TPE results are compared to molecular dynamics (MD) simulations and other theories. Good agreement between TPE and MD is observed for a wide range of parameters, particularly where standard integral equations theories fail, i. e., low salt concentration and high ionic valence. Our results support the formation of ion pairs and aligned ion complexes.

cond-mat.soft↗
arXiv2003

Overcharging of DNA in the presence of salt: Theory and Simulation

A study of a model rod-like polyelectrolyte molecule immersed into a monovalent or divalent electrolyte is presented. Results from the hypernetted-chain/mean spherical approximation (HNC/MSA) theory, for inhomogeneous charged fluids, {\ch are} compared with molecular dynamics (MD) simulations. As a particular case, the parameters of the polyelectrolyte molecule are mapped to those of a DNA molecule. An excellent qualitative, and in some cases quantitative, agreement between HNC/MSA and MD is found. Both, HNC/MSA and MD, predict the occurrence of overcharging, which is not present in the Poisson-Boltzmann theory. Mean electrostatic potential and local concentration profiles, $ζ$-potential and charge distribution functions are obtained and discussed in terms of the observed overcharging effect. Particularly interesting results are a very non-monotonic behavior of the $ζ$-potential, as a function of the rod charge density, and the overcharging by {\em monovalent} counterions.

cond-mat.soft↗
arXiv2003

The effect of entropy on macroions adsorption

We study macroion adsorption on planar surfaces, through a simple model. The importance of entropy in the interfacial phenomena is stressed. Our results are in qualitative agreement with available computer simulations and experimental results on charge reversal and self-assembling at interfaces.

cond-mat.soft↗
arXiv2002

Overcharging: The Crucial Role of Excluded Volume

In this Letter we investigate the mechanism for overcharging of a single spherical colloid in the presence of aqueous salts within the framework of the primitive model by molecular dynamics (MD) simulations as well as integral-equation theory. We find that the occurrence and strength of overcharging strongly depends on the salt-ion size, and the available volume in the fluid. To understand the role of the excluded volume of the microions, we first consider an uncharged system. For a fixed bulk concentration we find that upon increasing the fluid particle size one strongly increases the local concentration nearby the colloidal surface and that the particles become laterally ordered. For a charged system the first surface layer is built up predominantly by strongly correlated counterions. We argue that this a key mechanism to produce overcharging with a low electrostatic coupling, and as a more practical consequence, to account for charge inversion with monovalent aqueous salt ions.

cond-mat.soft↗
arXiv2001

Overcharging by macroions: above all, an entropy effect

Model macroion solutions next to a charged wall show interface \textit{true overcharging}, charge reversal and inversion, and layering. Macroion layering is present, even if the wall or the macroparticle are \textit{uncharged} or if the wall and macroions are like-charged. An effective long-range attractive force between the adsorbed macroions is implied. The results are obtained through an integral equation theory and a new extended Poisson-Boltzmann theory, and are in accordance with experiments on confined macroions and polymer layering.

physics.chem-ph↗