Failure of Debye-Hückel Screening in Low-Charge Colloidal Suspensions

Abstract

Derjaguin–Landau–Verwey–Overbeek (DLVO) theory remains the cornerstone of colloid stability. Electrostatic interactions dominate van der Waals attractions at large colloid-colloid separations h, unless strongly screened. Under these conditions, the potential U ( h ) between charged colloids is expected to be exponentially screened, U ( h ) ∼ exp ( − κ h ) / h , with κ − 1 = λ D where λ D is the classical Debye-Hückel screening length. By measuring the force between individual charged particles at dilute electrolyte concentrations (<mM) using optical tweezers, we tested experimentally the prediction κ − 1 = λ D in a nonpolar solvent. At low salt concentrations, we found close agreement between the directly-measured decay length κ − 1 and Debye-Hückel predictions. However, above a critical electrolyte concentration (≈450 μ M), we obtained significant discrepancies between measured and predicted screening lengths, with κ − 1 ≫ λ D . In marked contrast to expectations, we found that the measured screening length κ − 1 appears to grow as the ionic strength of the solution is increased. The origin of this discrepancy is discussed and the importance of considering the surface is highlighted.

Effect of the ultrastructure of chitosan nanoparticles in colloidal stability, quorum quenching and antibacterial activities

“…The same trend was observed in 85 mM NaCl, although the measured -potential values were considerably lower in this medium, namely +16.4 ± 0.8 mV and +10.5 ± 0.9 mV for IC-NPs and CC-NPs, respectively. Reduction of the -potential at 85 mM NaCl was expected due to increased salt-induced screening of charges at higher ionic strength, together with the known strong dependence of surface charge densities on ionic strength (Debye-Hückel prediction) [13,56,75].…”

Screening length for finite-size ions in concentrated electrolytes

“…Although these length scales are expected to coincide in the limit of large surface separations, surface effects may prevent a clear identification of the measured screening length with the predicted bulk correlation length for charge fluctuations. For example, opticaltweezers experiments on charged colloids in nonpolar solvents [51] suggest that surface charge-regulation may be the origin of the nonmonotonic behavior of the electrostatic decay length observed in that system. We note that future scattering experiments may provide a more direct measure of the bulk correlation length without complications due to surface effects in SFA.…”

A suspension of conducting particles in a magnetic field – the particle stress

“…If the Debye length is much smaller than the particle radius, the magnetic moment due to particle rotation is nearly balanced by that due to the rotation of the counter-ion layer; the ratio of the difference in the magnetic moment of the two and the moment due to the surface charges is comparable to the ratio of the Debye layer thickness and the particle size. In contrast, in non-polar solvents, there is very little screening, and the Debye layer thickness could be larger than the particle radius (Hsu, Dufresne & Weitz 2005; Waggett, Shafiq & Bartlett 2018). In such cases, it should be possible to observe rheology modification or secondary flow due to the effect of the magnetic field on a charged particle.…”