Unraveling the behavior of the individual ionic activity coefficients on the basis of the balance of ion-ion and ion-water interactions

We investigate the individual activity coefficients of pure 1:1 and 2:1 electrolytes using our theory that is based on the competition of ion-ion (II) and ion-water (IW) interactions (Vincze et al., J. Chem. Phys. 133, 154507, 2010). The II term is computed from Grand Canonical Monte Carlo simulations on the basis of the implicit solvent model of electrolytes using hard sphere ions with Pauling radii. The IW term is computed on the basis of Born's treatment of solvation using experimental hydration free energies. The two terms are coupled through the concentration-dependent dielectric constant of the electrolyte. With this approach we are able to reproduce the nonmonotonic concentration dependence of the mean activity coefficient of pure electrolytes qualitatively without using adjustable parameters. In this paper, we show that the theory can provide valuable insight into the behavior of individual activity coefficients too. We compare our theoretical predictions against experimental data measured by electrochemical cells containing ion-specific electrodes. As in the case of the mean activity coefficients, we find good agreement for 2:1 electrolytes, while the accuracy of our model is worse for 1:1 systems. This deviation in accuracy is explained by the fact that the two competing terms (II and IW) are much larger in the 2:1 case so errors in the two separate terms have less effects. The difference of the excess chemical potentials of cations and anions (the ratio of activity coefficients) is determined by asymmetries in the properties of the two ions: charge, radius, and hydration free energies.Comment: 32 pages, 8 figures, 1 TOC figur

The origin of interparticle potential of electrorheological fluids

The particles of electrorheological fluids can be modelled as dielectric spheres (DS) immersed in a continuum dielectric. When an external field is applied, polarization charges are induced on the surfaces of the spheres and can be represented as point dipoles placed in the centres of the spheres. When the DSs are close to each other, the induced charge distributions are distorted by the electric field of the neighbouring DSs. This is the origin of the interaction potential between the DSs. The calculation of this energy is very time consuming, therefore, the DS model cannot be used in molecular simulations. In this paper, we show that the interaction between the point dipoles appropriately approximates the interaction of DSs. The polarizable point dipole model provides better results, but this model is not pair-wise additive, so it is not that practical in particle simulations.Comment: 10 pages, 5 figure

The effect of the charge pattern on the applicability of a nanopore as a sensor

We investigate a model nanopore sensor that is able to detect analyte ions that are present in the electrolyte solution in very small concentrations. The nanopore selectively binds the analyte ions with which the local concentrations of the ions of the background electrolyte (KCl), and, thus, the ionic current flowing through the pore is changed. Analyte concentration can be determined from calibration curves. In our previous study (M\'{a}dai et al. J. Chem. Phys., 147(24):244702, 2017.), we proposed a symmetric model (surface charge is negative all along the pore). The mechanism of sensing was a competition between K$^{+}$ and positive analyte ions, so increasing analyte concentration decreased K$^{+}$ current. Here we allow asymmetric charge patterns on the pore wall (positive/negative/neutral along the pore), thus, gaining an additional device function, rectification, resulting in a dual responsive device. We find that a bipolar nanopore is an efficient geometry with Cl$^{-}$ ions being the main charge carriers. The mechanism of sensing is that more positive analyte ions attract more Cl$^{-}$ ions into the pore thus increasing the current. Also they make the pore less asymmetric and, thus, decrease rectification. We use a hybrid computer simulation method, where a generalization of the grand canonical Monte Carlo method to non-equilibrium (Local Equilibrium Monte Carlo) is coupled to the Nernst-Planck equation with which the flux is computed

Monte Carlo simulation of the electrical properties of electrolytes adsorbed in charged slit-systems

We study the adsorption of primitive model electrolytes into a layered slit system using grand canonical Monte Carlo simulations. The slit system contains a series of charged membranes. The ions are forbidden from the membranes, while they are allowed to be adsorbed into the slits between the membranes. We focus on the electrical properties of the slit system. We show concentration, charge, electric field, and electrical potential profiles. We show that the potential difference between the slit system and the bulk phase is mainly due to the double layers formed at the boundaries of the slit system, but polarization of external slits also contributes to the potential drop. We demonstrate that the electrical work necessary to bring an ion into the slit system can be studied only if we simulate the slit together with the bulk phases in one single simulation cell.Comment: 11 pages, 8 figure

The origin of the interparticle potential of electrorheological fluids

The particles of electrorheological fluids can be modelled a s dielectric sphere (DS) immersed in a continuum dielectric. When an external field is applied, polarization charges are induced on the surfaces of the spheres that can be represented as point dipoles placed in the centres of the spheres. When the DSs are close to each other, the induced charge distributions are distorted by the electric field of the neighbouring DSs. This is the origin of the interaction potential between the DSs. The calculation of this energy is very time consuming, therefore, the DS model cannot be used in molecular simulations. In this paper, we show that the interaction between the point dipoles approximates the interaction of DSs appropriately. The polarizable point dipole model provides better results, but this model is not pair-wise additive, so it is not so practical in particle simulations

Comment on “The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye–Hückel Theory”

Comment on “The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye–Hückel Theory

The effect of concentration- and temperature-dependent dielectric constant ont the activity coefficient of NaCl electrolyte solutions

Our implicit-solvent model for the estimation of the excess chemical potential (or, equivalently, the activity coefficient) of electrolytes is based on using a dielectric constant that depends on the thermodynamic state, namely, the temperature and concentration of the electrolyte, ε(c, T). As a consequence, the excess chemical potential is split into two terms corresponding to ion-ion (II) and ion-water (IW) interactions. The II term is obtained from computer simulation using the Primitive Model of electrolytes, while the IW term is estimated from the Born treatment. In our previous work [J. Vincze, M. Valiskó, and D. Boda, "The nonmonotonic concentration dependence of the mean activity coefficient of electrolytes is a result of a balance between solvation and ion-ion correlations," J. Chem. Phys. 133, 154507 (2010)], we showed that the nonmonotonic concentration dependence of the activity coefficient can be reproduced qualitatively with this II+IW model without using any adjustable parameter. The Pauling radii were used in the calculation of the II term, while experimental solvation free energies were used in the calculation of the IW term. In this work, we analyze the effect of the parameters (dielectric constant, ionic radii, solvation free energy) on the concentration and temperature dependence of the mean activity coefficient of NaCl. We conclude that the II+IW model can explain the experimental behavior using a concentration-dependent dielectric constant and that we do not need the artificial concept of "solvated ionic radius" assumed by earlier studies

Controlling ion transport through nanopores: modeling transistor behavior

We present a modeling study of a nanopore-based transistor computed by a mean-field continuum theory (Poisson-Nernst-Planck, PNP) and a hybrid method including particle simulation (Local Equilibrium Monte Carlo, LEMC) that is able to take ionic correlations into account including finite size of ions. The model is composed of three regions along the pore axis with the left and right regions determining the ionic species that is the main charge carrier, and the central region tuning the concentration of that species and, thus, the current flowing through the nanopore. We consider a model of small dimensions with the pore radius comparable to the Debye-screening length ($R_{\mathrm{pore}}/\lambda_{\mathrm{D}}\approx 1$), which, together with large surface charges provides a mechanism for creating depletion zones and, thus, controlling ionic current through the device. We report scaling behavior of the device as a function the $R_{\mathrm{pore}}/\lambda_{\mathrm{D}}$ parameter. Qualitative agreement between PNP and LEMC results indicates that mean-field electrostatic effects determine device behavior to the first order

Activity coefficients of individual ions in LaCl3 from the II+IW theory

We investigate the individual activity coefficients of ions in LaCl3 using our theory that is based on the competition of ion–ion (II) and ion–water (IW) interactions. The II term is computed from Grand Canonical Monte Carlo simulations on the basis of the implicit solvent model of electrolytes using hard sphere ions with Pauling radii. The IW term is computed on the basis of Born's treatment of solvation using experimental hydration-free energies. The results show good agreement with experimental data for La3+. This agreement is remarkable considering the facts that (i) the result is the balance of two terms that are large in absolute value (up to 20 kT) but opposite in sign, and (ii) that our model does not contain any adjustable parameter. All the parameters used in the model are taken from experiments: concentration-dependent dielectric constant, hydration free energies and Pauling radii