Equilibrium sedimentation profile of dilute, salt-free charged colloids

Abstract

The sedimentation profile of a dilute colloidal solution follows the barometric distribution owing to the balance between gravitational force and thermal fluctuation. However, the electrostatic interactions may lead to significant deviation even in the low volume fraction limit (e.g., 10(-5)). On the basis of Monte Carlo simulations for a dilute, salt-free colloidal dispersion, five regimes can be identified through the resulting colloidal sedimentation profile and the counterion distribution. The electrostatic interactions depends on the Coulomb strength E(c) defined as the ratio of the Bjerrum length to the colloid size. At weak colloid-ion attractions (small E(c)), counterions tend to distribute uniformly in the container. However, both barometric and inflated profiles of colloids can be observed. On the contrary, at strong colloid-ion attraction (large E(c)), counterions accumulate in the vicinity of the colloids. Significant counterion condensation effectively decreases the strength of colloid-colloid repulsion and barometric profile of colloids can be obtained as well. As a result, the sedimentation profile and counterion distribution are indicative of the strength of effective colloid-colloid and colloid-ion interactions. It is also found that local electroneutrality condition is generally not satisfied and charge separation (or internal electric field) is neither a sufficient nor necessary condition for nonbarometric distributions.

Structure of Polyelectrolyte Brushes Subject to Normal Electric Fields

“…When | E | > | E *|, nearly all counterions reside within the chain layer (we will return to this point later) and ϕ ion ( z ) develops a peak near the substrate surface. This peak value increases with the field intensity and when | E | ≥ 0.15, the profile of ϕ ion ( z ) approaches an exponential distribution, similar to what we expect for the sedimentation of ions driven by a strong electric field. , …”

Electrical potentials of two identical particles with fixed surface charge density in a salt-free medium

“…These include, for example, electrical potential distribution [10][11][12][13][14], electrophoresis [15][16][17][18][19], electrical double layer [20,21], equilibrium sedimentation [22], and critical coagulation concentration or stability [23][24][25][26].…”