Three component model of cylindrical electric double layers containing mixed electrolytes: A systematic study by Monte Carlo simulations and density functional theory
ABSTRACT The structure of electric double layer around a hard rigid impenetrable cylindrical polyion is studied using density functional theory as well as Monte Carlo simulations. The three component model, presented here, is an extension of solvent primitive model where the solvent molecules are treated as the neutral hard spheres, counterions and coions as the charged hard spheres, all of equal diameters, and in addition the mixture of mono- and multivalent counterions are also considered. The theory is partially perturbative where the hard sphere interactions are treated within the weighted density approach and the corresponding ionic interactions have been evaluated through second-order functional Taylor expansion with respect to the bulk electrolyte. The theoretical predictions in terms of the density profiles and the mean electrostatic potential profiles are found to be in good agreement with the simulation results. The presence of neutral hard spheres incorporate the effects of exclude volume interactions (ionic size correlations) while the mixture of mono- and multivalent counterions enhance the ionic charge correlation effects. Thus, this model study shows clear manipulations of ionic size and charge correlations in dictating the ionic density profiles as well as mean electrostatic potential profiles of the diffuse layer. The behavior of diffused double layer has been characterized at varying ionic concentrations, at different concentration ratios of mono- and multivalent counterions of mixed electrolytes, at different diameters of hard spheres, and at varying polyion surface charge density.
- SourceAvailable from: Guillermo Ivan Guerrero-Garcia[Show abstract] [Hide abstract]
ABSTRACT: The ionic atmosphere around a nucleic acid regulates its stability in aqueous salt solutions. One major source of complexity in biological activities involving nucleic acids arises from the strong influence of the surrounding ions and water molecules on their structural and thermodynamic properties. Here, we implement a classical density functional theory for cylindrical polyelectrolytes embedded in aqueous electrolytes containing explicit (neutral hard sphere) water molecules at experimental solvent concentrations. Our approach allows us to include ion correlations as well as solvent and ion excluded volume effects for studying the structural and thermodynamic properties of highly charged cylindrical polyelectrolytes. Several models of size and charge asymmetric mixtures of aqueous electrolytes at physiological concentrations are studied. Our results are in good agreement with Monte Carlo simulations. Our numerical calculations display significant differences in the ion density profiles for the different aqueous electrolyte models studied. However, similar results regarding the excess number of ions adsorbed to the B-DNA molecule are predicted by our theoretical approach for different aqueous electrolyte models. These findings suggest that ion counting experimental data should not be used alone to validate the performance of aqueous DNA-electrolyte models.The Journal of Chemical Physics 12/2014; 141(22):225103. DOI:10.1063/1.4902407 · 3.12 Impact Factor
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ABSTRACT: A systematic theoretical investigation is performed for electrostatic potential of mean force (EPMF) between two similarly charged rods (modeling DNA) immersed in a primitive model electrolyte solution. Two scientific anomalies are disclosed that (i) although a like-charge attraction (LCA) generally becomes stronger with bulk electrolyte concentration, opposite effect unexpectedly occurs if the two rod surfaces involved are sufficiently charged; (2) contrary to what is often asserted that presence of multivalent counter ion is necessary to induce the LCA, it is found that univalent counter ion induces the LCA solely only if bulk electrolyte concentration rises sufficiently and the rod surface charge quantities are high. Based on the system energetics calculated first by a classical density functional theory (DFT) in three-dimensional space, a hydrogen bonding style mechanism is advanced to reveal origin of the LCA, and by appealing to fairly common-sense concepts such as bond energy, bond length, number of hydrogen bonds formed, and counter ion single-layer saturation adsorption capacity, the present mechanism successfully explains the scientific anomalies and effects of counter ion and co-ion diameters in eliciting the LCA first investigated in this work. To add weight to the hydrogen bonding style mechanism, I further perform theoretical investigation about effects of rod surface charge density, co-ion valence, relative permittivity of medium, temperature, non-electrostatic inter-ion interaction, and rod diameter in modifying the EPMF, and several novel phenomena are first confirmed, which is self-consistently explained by the present hydrogen bonding style mechanism.Langmuir 09/2013; 29(40). DOI:10.1021/la402860r · 4.38 Impact Factor
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ABSTRACT: A universal theoretical way, proposed recently by Zhou, provides a general scheme which transforms any one of hard sphere density functional approximations (DFAs) into a DFA workable for non-hard sphere fluids. In this article, the universal theoretical way is combined with a hard sphere DFA as a Lagrangian theorem-based DFA to perform calculations for density profiles of a hard core attractive Yukawa model fluid influenced by diverse external fields, and the calculation results are compared with corresponding results due to a 3rd + 2nd-order perturbation density functional theory (DFT) and computer simulation previously reported. Several conclusions are drawn from a detailed analysis on the degree of accuracy of the two approximations relative to the computer simulation and described below. (i) Correspondence between the change of relative performance of the two DFAs and the change of the performance of the bulk 2nd-order direct correlation function (DCF) used as an input as a function of bulk state indicates that the bulk 2nd-order DCF is a key input information determining the accuracy of the approximations using them. (ii) For large inhomogeneity, non-perturbation approximation is evidently more successful than a 3rd-order truncation on the functional perturbation expansion series. (iii) The universal theoretical way is very promising given that a successful hard sphere DFA and accurate bulk 2nd-order DCF are incorporated. Under the present conditions, the universal theoretical way gives an excellent account of the density profiles even if the ‘dangerous’ coexistence bulk states are being under consideration. (iv) The present study helps in formulating new approximations for the inhomogeneous excess Helmholtz free energy functional.Physics and Chemistry of Liquids 01/2012; 51(2):1-19. DOI:10.1080/00319104.2012.708872 · 0.52 Impact Factor