Long-Ranged Attractive Forces between Neutral Surfaces Due to Amphiphile Aggregation

Overcharging and Free Energy Barriers for Equally Charged Surfaces Immersed in Salt Solutions

The stability of dispersions containing charged particles may obviously be regulated by salt. In some systems, the effective charge, as measured by the potential some small distance away from the particles, can have a sign opposite to the bare surface charge. If charge reversal takes place, there is typically a salt concentration regime within which colloidal stability increases with added salt. These experimental findings on dispersions have been corroborated by atomic force microscopy investigations, where an attraction is found at short separations. This attraction is stronger than expected from standard DLVO theory, and there has been considerable debate concerning its origin. In this work, we use simple coarse-grained models of these systems, where the bare surfaces carry a uniform charge density, and ion-specific adsorption is absent. Our hypothesis is that these experimental observations can be explained by such a simplistic pure Coulomb based model. Our approach entails grand canonical Metropolis Monte Carlo (MC) simulations as well as correlation-corrected Poisson-Boltzmann (cPB) calculations. In the former case, all ions have a common size, while the cPB utilizes a point-like model. We devote significant attention on apparent surface charge densities and interactions between large flat model surfaces immersed in either a 2:1 salt or a 3:1 salt. In contrast to most of the previous theoretical efforts in this area, we mainly focus on the weak long-ranged repulsion and its connection to an effective surface charge. We find a charge reversal and a concomitant development of a free energy barrier for both salts. The experimentally observed nonmonotonic dependence of colloidal stability on the salt concentration is reproduced using MC simulations as well as cPB calculations. A strong attraction is observed at short range for all investigated cases. We argue that in our model, all non-DLVO aspects can be traced to ion-ion correlations.

Long-ranged attractive forces induced by adsorbed dendrimers: direct force measurements and computer simulations

Interaction forces between charged interfaces in the presence of oppositely charged dendrimers are studied by experiment and simulation. The experiments involve direct force measurements with an atomic force microscope (AFM) between two negatively charged colloidal particles in the presence of adsorbed, positively charged globular dendrimers. The simulations are carried out by treating the macroions explicitly, while the small salt ions are treated implicitly through the Debye-Huckel approximation. The system undergoes overcharging, and at the isoelectric point long-ranged attractive electrostatic forces are present. The range of the attraction is on the order of half the Debye length at high salt concentration, but it becomes smaller at low salt concentration. Away from the isoelectric point, repulsive electrostatic forces are observed due to diffuse layer overlap. A semiquantitative agreement between experiment and simulation is obtained, despite the fact that the simple theoretical model does not involve any adjustable parameters. This study provides for the first time detailed comparison between experimental and simulation data of interaction forces between colloidal particles in the presence of multivalent macroions and monovalent salt ions.

Simulations of latex particles immersed in dendrimer solutions

In this work, we present Monte Carlo simulations of mixtures containing negatively charged latex particles and positively charged dendrimers. We focus on the interaction between two latex particles as salt concentration, dendrimer dose, and generation number are varied. Interaction free energies and corresponding stability ratios are calculated. Minimal stability is found near the isoelectric point, i.e., where the amount of adsorbed dendrimer charge matches the charge of the latex particles. Away from the isoelectric point, the stability increases as the latex particles get more and more under- or overcompensated, an increase that is more steep on the overcompensated side. Increasing the dendrimer generation leads to a more "patchy" surface. This heterogeneity is particularly relevant close to the isoelectric point. Given the relative simplicity of the model, the simulation results are in surprisingly good agreement with the experimental data.

Long-range attraction between charge-mosaic surfaces across water

We have measured directly the forces across water between hydrophilic surfaces covered with a random mosaic of positive and negative charged domains. We find a strong, long-ranged attraction between them at a surface separation comparable with the charge domain size (many tens of nanometers). This attraction persists at higher salt concentration, but its range then becomes comparable to the Debye screening length. We attribute the attraction to correlation between negative and positive regions on opposing surfaces, facilitated by the lateral mobility of the charge patches on the surfaces.

IONICEFFECTSBEYONDPOISSON-BOLTZMANNTHEORY

Polyelectrolytes are electrolytes asymmetric both in charge and size. Their properties in solution are dominated by Coulombic forces, and without a detailed understanding of these interactions, no interpretation of experimental data is possible. This paper is a review of recent developments in the theory of highly asymmetric electrolytes of spherical shape resembling surfactant micelles. Three different models are discussed: (a) the cell model, which is focused on the small ion-macroion interaction; (b) the model that treats the solution as an effective one-component fluid of macroions; and (c) the isotropic model, where the solution is represented as a mixture of charged spheres. Traditionally, the electrostatic interactions are accounted for via the solution of the Poisson-Boltzmann equation. This theory, however, ignores the fluctuations around the most probable distribution and may yield poor results for systems with multivalent ions. This paper focuses on developments beyond the Poisson-Boltzmann theory; the results of computer simulations and integral equation theories represent the major part of the review.