arXiv subjects

Alexandre P. dos Santos

Publications and source records attributed to Alexandre P. dos Santos.

31 records · Page 2Linked to original sources

arXiv2016

Charge neutrality breakdown in confined aqueous electrolytes: theory and simulation

We study, using Density Functional theory and Monte Carlo simulations, aqueous electrolyte solutions between charged infinite planar surfaces, in a contact with a bulk salt reservoir. In agreement with recent experimental observations [Z. Luo et al., Nat. Comm. 6, 6358 (2015)], we find that the confined electrolyte lacks local charge neutrality. We show that a Density Functional Theory (DFT) based on a bulk- HNC expansion properly accounts for strong electrostatic correlations and allows us to accurately calculate the ionic density profiles between the charged surfaces, even for electrolytes containing trivalent counterions. The DFT allows us to explore the degree of local charge neutrality violation, as a function of plate separation and bulk electrolyte concentration, and to accurately calculate the interaction force between the charged surfaces.

cond-mat.soft↗

arXiv2015

Electrolytes between dielectric charged surfaces: Simulations and theory

We present a simulation method to study electrolyte solutions in a dielectric slab geometry using a modified 3D Ewald summation. The method is fast and easy to implement, allowing us to rapidly resum an infinite series of image charges. In the weak coupling limit, we also develop a mean-field theory which allows us to predict the ionic distribution between the dielectric charged plates. The agreement between both approaches, theoretical and simulational, is very good, validating both methods. Examples of ionic density profiles in the strong electrostatic coupling limit are also presented. Finally, we explore the confinement of charge asymmetric electrolytes between neutral surfaces.

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↗

arXiv2014

Reply to 'Comment on "Vortex distribution in a confining potential"

We argue that contrary to recent suggestions, non-extensive statistical mechanics has no relevance for inhomogeneous systems of particles interacting by short-range potentials. We show that these systems are perfectly well described by the usual Boltzmann-Gibbs statistical mechanics.

cond-mat.stat-mech↗

arXiv2014

Yukawa particles in a confining potential

We study the density distribution of repulsive Yukawa particles confined by an external potential. In the weak coupling limit, we show that the mean-field theory is able to accurately account for the particle distribution. In the strong coupling limit, the correlations between the particles become important and the mean-field theory fails. For strongly correlated systems, we construct a density functional theory which provides an excellent description of the particle distribution, without any adjustable parameters.

cond-mat.stat-mech↗

arXiv2014

Ion Specificity and Micellization of Ionic Surfactants: A Monte Carlo Study

We develop a simulation method which allows us to calculate the critical micelle concentrations for ionic surfactants in the presence of different salts. The results are in good agreement with the experimental data. The simulations are performed on a simple cubic lattice. The anionic interactions with the alkyl chains are taken into account based on the previously developed theory of the interfacial tensions of hydrophobic interfaces: the kosmotropic anions do not interact with the hydrocarbon tails of ionic surfactants, while chaotropic anions interact with the alkyl chains through a dispersion potential proportional to the anionic polarizability.

physics.chem-ph↗

arXiv2014

Adsorption of cationic polyions to a hydrophobic surface in the presence of Hofmeister salts

We study, using extensive Monte Carlos simulations, the behavior of cationic polyelectrolytes near hydrophobic surfaces in solutions containing Hofmeister salts. The Hofmeister anions are divided into kosmotropes and chaotropes. Near a hydrophobic surface, the chaotropes lose their solvation sheath and become partially adsorbed to the interface, while the kosmotropes remain strongly hydrated and are repelled from the interface. If the polyelectrolyte solution contains chaotropic anions, a significant adsorption of polyions to the surface is also observed. On the other hand, the kosmotropic anions have only a small influence on the polyion adsorption. These findings can have important implications for exploring the antibacterial properties of cationic polyelectrolytes.

cond-mat.soft↗

arXiv2012

Polarizable Surfaces: Weak and Strong Coupling Regimes

We study the ionic distribution near a charged surface. A new method for performing Monte Carlo simulations in this geometry is discussed. A theory is then presented that allows us to accurately reproduce the density profiles obtained in the simulations. In the weak-coupling regime, a theory accounts for the ion-image interactions, leading to a modified Poisson-Boltzmann equation. When the correlations between the ions are significant, a strong-coupling theory is used to calculate the density profiles near the surface and a Poisson-Boltzmann equation with a renormalized boundary condition to account for the counterion distribution in the far-field.

physics.chem-ph↗

arXiv2012

Equation of state of charged colloidal suspensions and its dependence on the thermodynamic route

The thermodynamic properties of highly charged colloidal suspensions in contact with a salt reservoir are investigated in the framework of the Renormalized Jellium Model (RJM). It is found that the equation of state is very sensitive to the particular thermodynamic route used to obtain it. Specifically, the osmotic pressure calculated within the RJM using the contact value theorem can be very different from the pressure calculated using the Kirkwood-Buff fluctuation relations. On the other hand, Monte Carlo (MC) simulations show that both the effective pair potentials and the correlation functions are accurately predicted by the RJM. It is suggested that the lack of self-consistency in the thermodynamics of the RJM is a result of neglected electrostatic correlations between the counterions and coions.

cond-mat.soft↗

arXiv2011

Ion specificity and the theory of stability of colloidal suspensions

A theory is presented which allow us to accurately calculate the critical coagulation concentration (CCC) of hydrophobic colloidal suspensions. For positively charged particles the CCC's follow the Hofmeister (lyotropic) series. For negatively charged particles the series is reversed. We find that strongly polarizable chaotropic anions are driven towards the colloidal surface by electrostatic and hydrophobic forces. Within approximately one ionic radius from the surface, the chaotropic anions loose part of their hydration sheath and become strongly adsorbed. The kosmotropic anions, on the other hand, are repelled from the hydrophobic surface. The theory is quantitatively accurate without any adjustable parameters. We speculate that the same mechanism is responsible for the Hofmeister series that governs stability of protein solutions.

cond-mat.soft↗

arXiv2010

Surface tensions and surface potentials of acid solutions

A theory is presented which allows us to quantitatively calculate the excess surface tension of acid solutions. The H^+, in the form of hydronium ion, is found to be strongly adsorbed to the solution-air interface. To account for the electrostatic potential difference measured experimentally, it is necessary to assume that the hydronium ion is oriented with its hydrogens pointing into the bulk water. The theory is quantitatively accurate for surface tensions and is qualitative for electrostatic potential difference across the air-water interface.

cond-mat.soft↗

arXiv2010

Surface tensions, surface potentials and the Hofmeister series of electrolyte solutions

A theory is presented which allows us to accurately calculate the surface tensions and the surface potentials of electrolyte solutions. Both the ionic hydration and the polarizability are taken into account. We find a good correlation between the Jones-Dole viscosity $B$-coefficient and the ionic hydration near the air-water interface. The kosmotropic anions such as fluoride, iodate, sulfate and carbonate, are found to be strongly hydrated and are repelled from the interface. The chaotropic anions such as perchlorate, iodide, chlorate and bromide are found to be significantly adsorbed to the interface. Chloride and bromate anions become weakly hydrated in the interfacial region. The sequence of surface tensions and surface potentials is found to follow the Hofmeister ordering. The theory, with only one adjustable parameter, quantitatively accounts for the surface tensions of 10 sodium salts for which there is experimental data.

cond-mat.soft↗

arXiv2009

Ions at the air-water interface: An end to one hundred year old mystery?

Availability of highly reactive halogen ions at the surface of aerosols has tremendous implications for the atmospheric chemistry. Yet neither simulations, experiments, nor existing theories are able to provide a fully consistent description of the electrolyte-air interface. In this paper a new theory is proposed which allows us to explicitly calculate the ionic density profiles, the surface tension, and the electrostatic potential difference across the solution-air interface. Predictions of the theory are compared to experiments and are found to be in excellent agreement. The theory also sheds new light on one of the oldest puzzles of physical chemistry -- the Hofmeister effect.

cond-mat.soft↗