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

Abstract

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.

Interactions between conducting surfaces in salt solutions

“…As stated in the Introduction, similar behaviour has been observed also for non-conducting surfaces, experimentally, 13,35,36 as well as theoretically. 21–24 At long range, there will be a double-layer repulsion between surfaces that are effectively negative, or positive, depending on whether the surfaces are overcharged, or “undercharged”. However, at the threshold concentration (about 1.4 mM in our case), the effective surface charge is close to zero, which essentially eliminates the double-layer repulsion.…”

“…Given the extra adsorption of multivalent ions that such surfaces generate, the threshold concentration will be lower than for non-conducting surfaces. 24…”

“…Thus, at some intermediate critical concentration value, there is a “perfect surface neutralization” point, whereby the long-range repulsive barrier is negligible. 12–16,21–24 Experimentally, this manifests itself as a specific salt concentration where the charged particle dispersion has minimal stability. The neutralization point can also be experimentally identified through a vanishing electrophoretic mobility.…”

Theoretical Interpretation of pH and Salinity Effect on Oil-in-Water Emulsion Stability Based on Interfacial Chemistry and Implications for Produced Water Demulsification

“…The figure shows that as the pH increases, the surface charge density on the droplets of Moutray Crude Oil in oil-in-water emulsion systems will increase. Moreover, salinity has a positive effect on surface charge density, implying that increasing salinity increases the surface charge density [95,122,123]. From Table 1, Crude Oil A has the lowest isoelectric point after Moutray Oil, with the potential to develop a surface charge over a wider range of pH.…”

“…surface charge density [95,122,123]. From Table 1, Crude Oil A has the lowest isoelectric point after Moutray Oil, with the potential to develop a surface charge over a wider range of pH.…”