Assessment of the Wolf method using the Stillinger-Lovett sum rules: From strong electrolytes to weakly charged colloidal dispersions.

The Ewald method has been the cornerstone in molecular simulations for modeling electrostatic interactions of charge-stabilized many-body systems. In the late 1990s, Wolf and collaborators developed an alternative route to describe the long-range nature of electrostatic interactions; from a computational perspective, this method provides a more efficient and straightforward way to implement long-range electrostatic interactions than the Ewald method. Despite these advantages, the validity of the Wolf potential to account for the electrostatic contribution in charged fluids remains controversial. To alleviate this situation, in this contribution, we implement the Wolf summation method to both electrolyte solutions and charged colloids with moderate size and charge asymmetries in order to assess the accuracy and validity of the method. To this end, we verify that the proper selection of parameters within the Wolf method leads to results that are in good agreement with those obtained through the standard Ewald method and the theory of integral equations of simple liquids within the so-called hypernetted chain approximation. Furthermore, we show that the results obtained with the original Wolf method do satisfy the moment conditions described by the Stillinger-Lovett sum rules, which are directly related to the local electroneutrality condition and the electrostatic screening in the Debye-Hückel regime. Hence, the fact that the solution provided by the Wolf method satisfies the first and second moments of Stillinger-Lovett proves, for the first time, the reliability of the method to correctly incorporate the electrostatic contribution in charge-stabilized fluids. This makes the Wolf method a powerful alternative compared to more demanding computational approaches.

Molecular dynamics simulation study of the effect of halothane on mixed DPPC/DPPE phospholipid membranes.

We report results of a molecular dynamics simulation study of the effect of one general anesthetic, halothane, on some properties of mixed DPPC/DPPE phospholipid membranes. This is a suitable model for the study of simple, two-phospholipid membrane systems. From the simulation runs, we determined several membrane properties for five different molecular proportions of DPPC/DPPE. The effect of halothane on the studied membrane properties (area per lipid molecule, density of membrane, order parameter, etc.) was rather small. The distribution of halothane is not uniform through the bilayer thickness. Instead, there is a maximum of anesthetic concentration around 1.2 nm from the center of the membrane. The anesthetic molecule is located close to the phospholipid headgroups. The position of the halothane density maximum depends slightly on the DPPC/DPPE molar proportion. Snapshots taken over the plane of the membrane, as well as calculated two-dimensional radial distribution functions show that the anesthetic has no preference for either phospholipid (DPPC or DPPE). Our results indicate that this anesthetic molecule has only small effects on DPPC/DPPE mixed membranes. In addition, halothane displays no preferential location around DPPC or DPPE. This is probably due to the hydrophobic nature of halothane and to the fact that the chosen phospholipids have the same hydrophobic tails.

A common neighbor analysis of crystallization kinetics and excess entropy of charged spherical colloids.

The topological analysis tool known as the common neighbor analysis (CNA) is used for the first time in this work to analyze crystallization kinetics and excess entropy of charge-stabilized colloidal suspensions. For this purpose, Brownian dynamics computer simulations are implemented to investigate the crystallization kinetics of homogeneously melted colloidal crystals that are composed of hard-core-screened-Coulomb interacting particles. The results are in agreement with recent static structure factor measurements that could indicate the presence of icosahedral units in the metastable melt, and with the fact that weakly screened charged colloids crystallize into body-centered-cubic (bcc) ordering. A two-step crystallization pathway is found, in which the population of bcc-subunit CNA-pairs satisfactorily obeys a Verhulst model. Moreover, the CNA helped to unveil that the excess entropy obeys a quasi-universal functional form, relating the behavior of colloidal, molecular, and metallic liquid systems. The work contributes to the scientific understanding of the crystallization pathway of charged colloids, and to the development of new ways to assess the degree of crystalline order, starting from the excess entropy.

Long-time self-diffusion of charged spherical colloidal particles in parallel planar layers.

The long-time self-diffusion coefficient, D(L), of charged spherical colloidal particles in parallel planar layers is studied by means of Brownian dynamics computer simulations and mode-coupling theory. All particles (regardless which layer they are located on) interact with each other via the screened Coulomb potential and there is no particle transfer between layers. As a result of the geometrical constraint on particle positions, the simulation results show that D(L) is strongly controlled by the separation between layers. On the basis of the so-called contraction of the description formalism [C. Contreras-Aburto, J. M. Méndez-Alcaraz, and R. Castañeda-Priego, J. Chem. Phys. 132, 174111 (2010)], the effective potential between particles in a layer (the so-called observed layer) is obtained from integrating out the degrees of freedom of particles in the remaining layers. We have shown in a previous work that the effective potential performs well in describing the static structure of the observed layer (loc. cit.). In this work, we find that the D(L) values determined from the simulations of the observed layer, where the particles interact via the effective potential, do not agree with the exact values of D(L). Our findings confirm that even when an effective potential can perform well in describing the static properties, there is no guarantee that it will correctly describe the dynamic properties of colloidal systems.

A unifying mode-coupling theory for transport properties of electrolyte solutions. I. General scheme and limiting laws.

We develop a general method for calculating conduction-diffusion transport properties of strong electrolyte mixtures, including specific conductivities, steady-state electrophoretic mobilities, and self-diffusion coefficients. The ions are described as charged Brownian spheres, and the solvent-mediated hydrodynamic interactions (HIs) are also accounted for in the non-instantaneous ion atmosphere relaxation effect. A linear response expression relating long-time partial mobilities to associated dynamic structure factors is employed in our derivation of a general mode coupling theory (MCT) method for the conduction-diffusion properties. A simplified solution scheme for the MCT method is discussed. Analytic results are obtained for transport coefficients of pointlike ions which, for very low ion concentrations, reduce to the Deby-Falkenhagen-Onsager-Fuoss limiting law expressions. As an application, an unusual non-monotonic concentration dependence of the polyion electrophoretic mobility in a mixture of two binary electrolytes is discussed. In addition, leading-order extensions of the limiting law results are derived with HIs included. The present method complements a related MCT method by the authors for the electrolyte viscosity and shear relaxation function [C. Contreras-Aburto and G. Nägele, J. Phys.: Condens. Matter 24, 464108 (2012)], so that a unifying scheme for conduction-diffusion and viscoelastic properties is obtained. We present here the general framework of the method, illustrating its versatility for conditions where fully analytic results are obtainable. Numerical results for conduction-diffusion properties and the viscosity of concentrated electrolytes are presented in Paper II [C. Contreras Aburto and G. Nägele, J. Chem. Phys. 139, 134110 (2013)].

Viscosity of electrolyte solutions: a mode-coupling theory.

We present a versatile theoretical method for calculating the steady-state viscosity and shear relaxation function of strong electrolyte solutions. In this method, the ions are described on a primitive model level as charged Brownian spheres, and the essential ion-ion hydrodynamic interactions (HIs) are accounted for in the shear relaxation effect of the ionic atmosphere. The method combines a many-component mode-coupling theory (MCT) approach by Nägele et al (1998 J. Chem. Phys. 108 9893) with a simplified solution scheme, leading to an analytic expression for the shear relaxation contribution to the viscosity. This expression accounts for both the excluded volumes of the ions and their HIs. We show that the limiting law results for the viscosity of electrolyte mixtures by Falkenhagen and by Onsager and Fuoss are recovered at very low concentrations, and we discuss HIs corrections appearing at higher concentrations. Our numerical results for a 1:1 electrolyte reveal a strong enlargement of the viscosity caused by the HIs. The high-frequency viscosity gives the largest contribution to the total viscosity at higher concentrations.