Enrique González-Tovar

Monte Carlo simulation for a symmetrical electrolyte next to a charged spherical colloid particle

Results from Monte Carlo (MC) simulations for a restricted primitive model symmetrical electrolyte next to an isolated spherical macroion are reported. Calculations were made for various 1:1 and 2:2 electrolyte concentrations and macroion’s radii and charges. The MC results are compared to those from the hypernetted chain/mean spherical approximation (HNC/MSA) integral equation and the fifth version of the modified Poisson–Boltzmann (MPB5) and Poisson–Boltzmann (PB) differential equations. An overall good agreement of the HNC/MSA and MPB5 results with the MC data is found. On the other hand, the widely used PB theory is found to have important quantitative and qualitative disagreements with the MC results.Results from Monte Carlo (MC) simulations for a restricted primitive model symmetrical electrolyte next to an isolated spherical macroion are reported. Calculations were made for various 1:1 and 2:2 electrolyte concentrations and macroion’s radii and charges. The MC results are compared to those from the hypernetted chain/mean spherical approximation (HNC/MSA) integral equation and the fifth version of the modified Poisson–Boltzmann (MPB5) and Poisson–Boltzmann (PB) differential equations. An overall good agreement of the HNC/MSA and MPB5 results with the MC data is found. On the other hand, the widely used PB theory is found to have important quantitative and qualitative disagreements with the MC results.

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 for a totally asymmetric primitive model electrolyte around a spherical macroparticle is obtained and solved numerically in the case of size-asymmetric systems. 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 versus charge relationship, radial distribution functions, mean electrostatic potential, and cumulative reduced charge for representative examples 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, 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., 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 cannot be described by traditional mean-field theories such as MGC, URMGC, or even by enhanced formalisms, such as HNC/MSA, if they are based on the RPM.

A new correlation effect in the Helmholtz and surface potentials of the electrical double layer

The restricted primitive model of an electrical double layer around a spherical macroparticle is studied by using integral equation theories and Monte Carlo simulations. The resulting theoretical curves for the Helmholtz and surface potentials versus the macroparticle charge show an unexpected positive curvature when the ionic size of uni- and divalent electrolyte species is increased. This is a novel effect that is confirmed here by computer experiments. An explanation of this phenomenon is advanced in terms of the adsorption and layering of the electrolytic species and of the compactness of the diffuse double layer. It is claimed that the interplay between electrostatic and ionic size correlation effects, absent in the classical Poisson-Boltzmann view, is responsible for this singularity.

Entropic effects in the electric double layer of model colloids with size-asymmetric monovalent ions

The structure of the electric double layer of charged nanoparticles and colloids in monovalent salts is crucial to determine their thermodynamics, solubility, and polyion adsorption. In this work, we explore the double layer structure and the possibility of charge reversal in relation to the size of both counterions and coions. We examine systems with various size-ratios between counterions and coions (ion size asymmetries) as well as different total ion volume fractions. Using Monte Carlo simulations and integral equations of a primitive-model electric double layer, we determine the highest charge neutralization and electrostatic screening near the electrified surface. Specifically, for two binary monovalent electrolytes with the same counterion properties but differing only in the coions size surrounding a charged nanoparticle, the one with largest coion size is found to have the largest charge neutralization and screening. That is, in size-asymmetric double layers with a given counterions size the excluded volume of the coions dictates the adsorption of the ionic charge close to the colloidal surface for monovalent salts. Furthermore, we demonstrate that charge reversal can occur at low surface charge densities, given a large enough total ion concentration, for systems of monovalent salts in a wide range of ion size asymmetries. In addition, we find a non-monotonic behavior for the corresponding maximum charge reversal, as a function of the colloidal bare charge. We also find that the reversal effect disappears for binary salts with large-size counterions and small-size coions at high surface charge densities. Lastly, we observe a good agreement between results from both Monte Carlo simulations and the integral equation theory across different colloidal charge densities and 1:1-electrolytes with different ion sizes.

Critical parameters of asymmetric primitive model electrolytes in the mean spherical approximation

The effect of size and charge asymmetries on the critical parameters of primitive model electrolytes is comprehensively surveyed by using the mean spherical approximation.

Electrokinetic flow in ultrafine slits: Ionic size effects

The equilibrium density profiles for the restricted primitive model (RPM) for electrolytes, which takes into account the ionic size, are used to calculate several electrokinetic quantities for a 1:1 ionic solution in a charged slit. The three‐point‐extension hypernetted‐chain–mean‐spherical equation is used to calculate the RPM density profiles. The results are compared to those obtained through the Poisson–Boltzmann equation for a point‐ion model. Important quantitative and qualitative differences are found for ultrafine slits. These differences, as well as the general behavior of the electrokinetic quantities, are explained in terms of the structure of the electrical double layer inside the slit.

The primitive model of ionic fluids near its critical point in the Poisson–Boltzmann and modified Poisson–Boltzmann theories

The Poisson–Boltzmann (PB) and modified Poisson–Boltzmann (MPB) theories are used to investigate the primitive model of ionic fluids in the low density–large coupling regime where the liquid–vapor transition is situated. The PB and MPB spinodal curves for the restricted primitive model are calculated from the virial route and compared with those from the mean spherical approximation (energy route) and the hybrid hypernetted‐chain/mean spherical approximation (virial route). The effect of unequal ion sizes on the critical point and spinodal curves is also considered.

Computer simulation of charged hard spherocylinders

In this work we present a computer simulation study of charged hard spherocylinders of aspect ratio L/sigma=5, using NVT and NPT Monte Carlo methods. Coulombic interactions are handled using the Wolf method [D. Wolf, P. Keblinski, S. R. Phillpot, and J. Eggebrecht, J. Chem. Phys. 110, 8254 (1999)]. Thermodynamic and structural properties are in excellent agreement with the results obtained with the standard Ewald summation method. A partial prediction of the corresponding phase diagram is obtained by studying two isotherms of this system. The stability of the liquid crystalline phases is examined and compared with the phase diagrams of neutral hard spherocylinders and dipolar hard spherocylinders.

Monte Carlo simulations of the electrical double layer forces in the presence of divalent electrolyte solutions: effect of the ion size

In this paper, the effect of ion size on the mean forces between two charged plates in the presence of divalent counterions is analyzed with the help of Monte Carlo simulations in the framework of the primitive model. Inspired by a preliminary work in which a particular and isolated case was presented, we propose a more systematic survey considering both like and oppositely charged plates, different surface charge densities (with magnitudes ranging from 0.01 to 0.2 C m−2) and two very different ionic strengths (0.5 and 500 mM). The effect of ion size is probed comparing systematically results for the most commonly used ionic diameter (0.425 nm) and greater values reported in the scientific literature for hydrated ions. These greater values were previously and successfully employed in other areas (e.g., electrokinetic behaviour in the presence of divalent and trivalent ions). Our simulations show that force–distance profiles strongly depend on the ion size. Consequently, some of the ‘classical’ findings obtained from simulations must be carefully reconsidered. In particular, the widely known and reported attraction between like-charged plates (or macroions) becomes repulsion with increasing ion size.

Computer simulation of charged hard spherocylinders at low temperatures

In this work we report the stability of liquid crystalline phases of charged hard spherocylinders (CHSC) of aspect ratio L/σ=5 at low temperatures using NPT Monte Carlo computer simulations. Following the methodology used in previous work [C. Avendaño, A. Gil-Villegas, E. González-Tovar, J. Chem. Phys. 128, 044506 (2008); Chem. Phys. Lett. 470, 67 (2009)], long-range coulombic interactions are handled using the Wolf method. The supramolecular organization of CHSC is obtained by compression of a low-density isotropic state. The system under consideration exhibits the expected isotropic, nematic, smectic-A, and crystal phases. However two important phenomena emerge at low temperatures, namely the existence of an isotropic–nematic–smectic triple point, with the ending of the nematic phase for lower temperatures, and the apparent hexatic arrangement of the layers in the smectic phases. Assuming that the smectic-layers behave as quasi-bidimensional systems, lowering the temperatures is possible to observe the formation of hexatic phases, which are detected analysing the structure factor, order parameters and distribution functions. This hexatic ordering indicates that the CHSC phase diagram presents a smectic-B phase at low temperatures.