Equilibrium sedimentation profile of dilute, salt-free charged colloids

Tzu Yu Wang, Hsien Tsung Li, Yu Jane Sheng, Heng Kwong Tsao

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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 Ec defined as the ratio of the Bjerrum length to the colloid size. At weak colloid-ion attractions (small Ec), 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 Ec), 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.

Original languageEnglish
Article number204504
JournalJournal of Chemical Physics
Volume129
Issue number20
DOIs
StatePublished - 2008

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