On the origin of oscillatory interactions between surfaces mediated by polyelectrolyte solution

Abstract

We use a numerical implementation of polymer classical density functional theory with an incompressibility condition to study the system consisting of nonadsorbing polyelectrolytes confined by two planar surfaces and quantify the effective interaction between the two planar surfaces as a function of the polyelectrolyte and salt concentrations. Our results indicate that for the uncharged surfaces (and weakly charged surfaces), the effective interaction primarily consists of a short-range attraction due to the depletion followed by repulsion due to the electric double layer overlapping and electrostatic correlations. For salt-free and low salt concentration systems, the magnitude of the repulsion barrier is determined by the overlap between the electric double layers, while at relatively high salt concentrations, the magnitude of the repulsion barrier is determined by the electrostatic correlations. Due to the competition between the electric double layer and the electrostatic correlations, the magnitude of the repulsion barrier varies nonmonotonically. In contrast, a mean-field Poisson-Boltzmann treatment of the electrostatics predicts a monotonically decreasing repulsion barrier with increasing salt concentration. At moderate salt concentrations, our theory predicts oscillatory interaction profiles. A comparison with the mean-field Poisson-Boltzmann treatment of electrostatics suggests that the oscillations are due primarily to electrostatic correlations.

Interfacial Structure and Tension of Polyelectrolyte Complex Coacervates

“…We thus speculate that the formation of the oscillatory electric double-layer structure at small ϕ s ,0 is likely a synergistic effect of electrostatic correlation, chain connectivity, excluded volume interaction, and the charge separation due to asymmetry between oppositely charged species. We note that a similar oscillatory behavior was also observed in other systems, such as concentrated ionic liquid , and polyelectrolyte solution confined between neutral surfaces …”

“…We note that a similar oscillatory behavior was also observed in other systems, such as concentrated ionic liquid 96,97 and polyelectrolyte solution confined between neutral surfaces. 98 In Figure 3b, we show the mean electrostatic potential ψ(x) corresponding to the charge density profiles shown in Figure 3a. For small ϕ s,0 (e.g., the black curve), when going from the supernatant to the coacervate phase, ψ(x) first decreases slightly and then increases rapidly to Ψ G , the value of the Galvani potential, after a slight overshoot.…”

Nonelectrostatic Adsorption of Polyelectrolytes and Mediated Interactions between Solid Surfaces

“…Note that the densities are normalized relative to the bulk concentration. The thickness of the polyanion depletion layer decreases as the bulk density increases due to increased screening of the surface and the increased osmotic pressure of the polyanion . For low bulk densities, a double layer forms near the surface due to the strong adsorption of counterions near the surface to compensate the surface charge.…”

“…In this work, we use a previously developed cDFT framework to characterize the effect of the strength of nonelectrostatic adsorption, salt addition, and polyelectrolyte charge density on the polyelectrolyte adsorption and the corresponding mediated interaction. The cDFT framework has two main advantages over self-consistent field theory (SCFT) approaches from previous works: ,, cDFT is better able to describe density profiles even in the absence of electrostatics, and cDFT takes into account electrostatic correlations not present in mean-field treatment of electrostatics. ,,, Previous works have not focused specifically on the role of nonelectrostatic attraction in relation to both adsorption and polyelectrolyte-mediated interactions. We focus on the surfaces that carry the same charge as the polymer to reflect common systems where nonelectrostatic adsorption likely plays a prominent role.…”

“…The cDFT framework has two main advantages over self-consistent field theory (SCFT) approaches from previous works: 21,38,56 cDFT is better able to describe density profiles even in the absence of electrostatics, 57 and cDFT takes into account electrostatic correlations not present in mean-field treatment of electrostatics. 25,29,55,58 Previous works have not focused specifically on the role of nonelectrostatic attraction in relation to both adsorption and polyelectrolyte-mediated interactions. We focus on the surfaces that carry the same charge as the polymer to reflect common systems where nonelectrostatic adsorption likely plays a prominent role.…”