Sedimentation of charged colloids: The primitive model and the effective one-component approach

Abstract

Sedimentation-diffusion equilibrium density profiles of suspensions of charge-stabilized colloids are calculated theoretically and by Monte Carlo (MC) simulations, both for a one-component model of colloidal particles interacting through pairwise screened-Coulomb repulsions and for a three-component model of colloids, cations, and anions with unscreened-Coulomb interactions. We focus on a state point for which experimental measurements are available [C. P. Royall, J. Phys.: Condens Matter 17, 2315 (2005)]. Despite the apparently different picture that emerges from the one- and three-component models (repelling colloids pushing each other to high altitude in the former, versus a self-generated electric field that pushes the colloids up in the latter), we find similar colloidal density profiles for both models from theory as well as simulation, thereby suggesting that these pictures represent different viewpoints of the same phenomenon. The sedimentation profiles obtained from an effective one-component model by MC simulations and theory, together with MC simulations of the multicomponent primitive model are consistent among themselves, but differ quantitatively from the results of a theoretical multicomponent description at the Poisson-Boltzmann level. We find that for small and moderate colloid charge the Poisson-Boltzmann theory gives profiles in excellent agreement with the effective one-component theory if a smaller effective charge is used. We attribute this discrepancy to the poor treatment of correlations in the Poisson-Boltzmann theory.

Equilibrium sedimentation profile of dilute, salt-free charged colloids

“…If local charge neutrality is assumed, the slope becomes a factor of 1 / ͑1+Z / z͒ smaller. [16][17][18][19][20][21][22][23][24] Therefore, as the salt concentration increases, both the colloid-colloid repulsion and colloid-ion attraction become weak and the system tends to display the behavior of regime ͑i͒, that is, sedimentation profile for colloids is barometric and the counterions most likely distribute uniformly in the solution. 2 ͑for z =1͒, the slope for the linear curve of regime ͑i͒ is about 1 / 13 and the slope for the linear region of regime ͑iii͒ is about 1 / 140 which is larger than the expected value ͑1 / 13ϫ 1 / 16͒.…”

The polydisperse cell model: Nonlinear screening and charge renormalization in colloidal mixtures

“…On the other hand, for small values of the structural charges, Z ren i ≃ Z i . As a consequence, expressions for the effective interaction potentials derived from linear theories, e.g., DLVO theory, are expected to be sufficient to describe the thermodynamic properties of the suspension, at least in the regime where pairwise additivity is expected [19,20].…”

Long ranged interactions in computer simulations and for quasi-2D systems

“…In plasma physics, in particular in dusty plasmas [450][451][452][453][454][455][456][457][458]81,[459][460][461][462], also in colloidal systems [9,10,[463][464][465][466][467][468][469][470][471][472], a lot of experimental set-up may be represented as quasi-2D geometries. For these systems it is necessary to take correctly into account the long ranged feature of these interactions [161,473,178].…”