Torbjörn Åkesson

Electric double layer forces in the presence of polyelectrolytes

An electric double layer is studied by means of Monte Carlo simulations and mean‐field theory. The counterions of the uniformly charged surfaces are modeled as flexible polyelectrolytes. For this particular model system it turns out that the traditional double layer repulsion becomes attractive for a wide range of systems. The main reason for this attraction is an entropically driven bridging mechanism, and its magnitude is significant compared to ordinary double layer or van der Waals forces. The polyelectrolyte Poisson–Boltzmann theory developed here behaves in a qualitatively correct manner, also predicting an attractive interaction extending over several nanometers. These results may have some relevance to technical and biological systems, where sometimes puzzling force behavior is seen in the presence of polyelectrolytes.

Anisotropic Interactions in Protein Mixtures: Self Assembly and Phase Behavior in Aqueous Solution

Recent experimental studies show that oppositely charged proteins can self-assemble to form seemingly stable microspheres in aqueous salt solutions. We here use parallel tempering Monte Carlo simulations to study protein phase separation of lysozyme/α-lactalbumin mixtures and show that anisotropic electrostatic interactions are important for driving protein self-assembly. In both dilute and concentrated protein phases, the proteins strongly align according to their charge distribution. While this alignment can be greatly diminished by a single point mutation, phase separation is completely suppressed when neglecting electrostatic anisotropy. The results highlight the importance of subtle electrostatic interactions even in crowded biomolecular environments where other short-ranged forces are often thought to dominate.

On the simulation of thermodynamic and structural properties of simple liquids

This work, which is purely methodological, demonstrates new applications of perturbation methods in computer simulations of simple liquids. Most applications are based on the calculation of bulk and local excess chemical potentials of one or several inserted test particles, using a Widom technique in the canonical ensemble. This gives a powerful tool for obtaining distribution functions, some of which are virtually impossible to determine with other techniques. Results are also presented for single‐ion activity coefficients and Donnan potentials. A perturbation approach is used to calculate thermodynamic response functions with respect to particle number, temperature, and volume changes. The applicability is exemplified by studies of hard‐sphere fluids, uniform and nonuniform electrolyte solutions within the primitive model, and screened Coulomb systems.

ION-ION CORRELATIONS IN ELECTRIC DOUBLE LAYERS FROM MONTE CARLO SIMULATIONS AND INTEGRAL EQUATION CALCULATIONS. PART 2. CASE OF ADDED SALT

Anisotropic ion–ion distribution functions gij (R 1, R 2), where R 1 and R 2 are the positions of ions of species i and j respectively, and various other properties of electric double layer systems have been calculated by simulation and integral equation methods. The system is composed of a 1 : 1 or 2 : 1 electrolyte solution between two planar walls with rather high surface charge density (0·267 Cm-2) and in equilibrium with a bulk electrolyte solution with a certain concentration (1·0 M or 2·0 M). In the integral equation calculations, the hypernetted chain (HNC) or the reference hypernetted chain (RHNC) closure has been applied for the ion-ion distribution functions in the inhomogeneous electrolyte (the so-called anisotropic HNC or RHNC approximations). The Widom test particle technique has been used to calculate the ion–ion distribution functions in the simulations. The results of the anisotropic RHNC calculations and the simulations are in almost perfect agreement throughout (they virtually coincide)...

Polyelectrolyte adsorption on solid surfaces: theoretical predictions and experimental measurements.

This work utilizes a combination of theory and experiments to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces at pH 9. Both polymers, poly(diallyldimethylammonium chloride), PDADMAC, and poly(4-vinyl N-methylpyridinium iodide), PVNP, are highly charged and highly soluble in water. Another important aspect is that a silica surface carries a relatively high surface charge density at this pH level. This means that we have specifically chosen to investigate adsorption under conditions where electrostatics can be expected to dominate the interactions. Of specific focus in this work is the response of the adsorption to the addition of simple salt (i.e., a process where electrostatics is gradually screened out). Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes are evaluated by direct quantitative comparisons with corresponding experimental data, as obtained by ellipsometry measurements. We find that, at low concentrations of simple salt, the adsorption increases with ionic strength, reaching a maximum at intermediate levels (about 200 mM). The adsorption then drops but retains a finite level even at very high salt concentrations, indicating the presence of nonelectrostatic contributions to the adsorption. In the theoretical treatment, the strength of this relatively modest but otherwise largely unknown nonelectrostatic surface affinity was estimated by matching predicted and experimental slopes of adsorption curves at high ionic strength. Given these estimates for the nonelectrostatic part, experimental adsorption data are essentially captured with quantitative accuracy by the classical density functional theory.

Molecular evidence of stereo-specific lactoferrin dimers in solution.

Gathering experimental evidence suggests that bovine as well as human lactoferrin self-associate in aqueous solution. Still, a molecular level explanation is unavailable. Using force field based molecular modeling of the protein-protein interaction free energy we demonstrate (1) that lactoferrin forms highly stereo-specific dimers at neutral pH and (2) that the self-association is driven by a high charge complementarity across the contact surface of the proteins. Our theoretical predictions of dimer formation are verified by electrophoretic mobility and N-terminal sequence analysis on bovine lactoferrin.

Depletion and bridging forces in polymer systems: Monte Carlo simulations at constant chemical potential

A new Monte Carlo simulation method designed for polymer solutions confined to planar slits is presented. The slit is in equilibrium with a surrounding bulk solution and the method allows a variation of the slit width while maintaining the polymer chemical potential constant. This is achieved by changing the tangential pressure as a function of slit width. An analysis of chain parameters and monomer distribution within the slit has been carried out. The model system used is supposed to mimick a macromolecular solution whose stability is manipulated by addition of adsorbing and/or nonadsorbing polymers. Generally, for the nonadsorbing polymer an attractive depletion force is found. At high volume fractions the attraction is reduced and a repulsive force appears at short separations. The depletion force can also be extinguished in the case of an adsorption potential of intermediate strength, while strong adsorption gives rise to a significant attraction due to polymer bridges.

The ionic correlation contribution to the free energy of spherical double layers

An estimate of the ion fluctuation contribution to the free energy for a dispersion of spherical macroions, without additional salt, is made using the cell model. The Poisson–Boltzmann approximation is used to estimate the linear response in a given cell, which together with second order perturbation theory allow us to calculate the free energy of interaction between a pair of cells. As well, an infinite order theory is developed. We find excellent agreement with simulations for monovalent counterions, the comparison becoming poorer for more strongly coupled systems. A cubic lattice of cells is studied, modeling a macroionic dispersion. The fluctuation contribution is shown to be several orders of magnitude larger than the usual quantum dispersion forces. However, it plays a minor role in the thermodynamics of the lattice.

Brownian dynamics simulation of interacting particles

The dynamics of the ions in an electrical double layer has been studied using stochastic dynamics simulation methods. The theory of a brownian particle in an external force field is discussed and a modified form of the fluctuation-dissipation theorem is derived. With slowly varying external forces the simulations can be restricted to configuration space and the correction term to the fluctuation-dissipation theorem, arising from the external force, can be neglected. It is shown that the diffusion coefficient of a brownian particle can be calculated from the force correlation function. In comparison with calculations from mean-square displacements the former way turns out to be much more efficient. Finally, a relation between the external force () and the random force () is derived such that = -2 .

Ion-ion correlations in electric double layers from Monte Carlo simulations and integral equation calculations

The anisotropic ion-ion correlation function g(R 1, R 2), where R 1 and R 2 are the coordinates of ions 1 and 2, is calculated for double layer systems composed of counterions between two charged, planar walls. The primitive model of the electrolyte is used. The Widom formula for the excess chemical potential is applied in the neighbourhood of a confined particle in the double layer to obtain g(R 1, R 2) from Monte Carlo (MC) simulations. The pair correlations are also calculated in the anisotropic RHNC approximation, where the reference hypernetted chain (RHNC) closure is applied at the pair (ion-ion) level and where the anisotropic bridge function is taken from an inhomogeneous reference system composed of a hard sphere fluid with the same density profile as the double-layer system. The results of the MC simulations and RHNC calculations are generally in very good agreement with each other. This demonstrates the fact that the anisotropic RHNC approximation is a very accurate method for calculating the v...