| dc.description.abstract | The effective charge is often invoked to account for the accumulation of counterions near the colloid with intrinsic charge . Although the ion concentrations are not uniform in the solution due to the presence of the charged particle, their chemical potentials are uniform everywhere. Thus, on the basis of ion chemical potential, effective ion concentrations, which can be experimentally measured by potentiometry, are defined with the pure salt solution as the reference state. The effective charge associated with the charged particle can then be determined by the global electroneutrality condition. Monte Carlo simulations are performed in a spherical Wigner-Seitz cell to obtain the effective charge of the colloid. In terms of the charge ratio , the effects of salt free concentration, added salt concentration, counterion valency, and particle charge are examined. The effective charge declines with increasing salt concentration and the multivalent salt is much more efficient in reducing the effective charge of the colloidal solution. Moreover, the extent of effective charge reduction is decreased with increasing intrinsic charge for a given concentration of added salt. Besides the colloid system, the effective charge of polyelectrolytes is also investigated in a salt free solution.
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). 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, , defined as the ratio of the Bjerrum length to the colloid size. At weak colloid-ion attractions (small ), 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 ), 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 non-barometric distributions.
Donnan equilibrium of a salt-free colloidal dispersion has been investigated by Monte Carlo simulations. The influences of colloidal characteristics, including particle size R, intrinsic particle charge Z, couterion valency , and concentration , on Donnan potential and effective charge are directly calculated by considering two chambers separated by a semipermeable, fictitious membrane. Donnan potential is increased by increasing and and by decreasing dielectric constant . In principle, is determined by the ratio of net charge density to dielectric constant in a chamber and charge density distribution. The former reveals that the existence of net charge is responsible for Donnan potential, while the latter illustrates the influence of colloid-ion interaction, which is associated with colloidal characteristics. | en_US |