Sedimentation Equilibrium of Mixtures of Charged Colloids

Appendix BImportant length scalesIn the following sections systems are considered that are a prelude to colloidalsystems. Often they are easier to describe, because they only includeone type of interaction, have simple geometries and less objects. They provideuseful parameters, and insights for the more complex cases, the colloidalsuspensions. Some length scales will be used frequently in this thesis, andare explained in the following sections using the MFA, LDA (see 2.3.2) respectively:B.1 The Debye screening length κ −1 and the Bjerrum length λ B , a considerationof a charged plane in an ionic solution.B.2 The gravitational length, an ideal gas in the gravity field of the earth.B.3 The colloidal diameter.B.1 The Debye screening length κ −1This section is about κ −1 , or as it is called, the (bare) Debye screening length.It will turn out to be a convenient parameter for the theory in the previouschapters. When colloidal particles are suspended in some polar medium,they will often release or acquire micro-ions, and acquire some surface charge.This will have effect on the micro-ion density in the solvent. Equally chargedmicro-ions, called co-ions, tend to move away from the colloids, whereas oppositelycharged micro-ions, called counter-ions, are attracted to the colloids.These ions will form some distribution after balance is found between theelectrostatic forces and entropy. The counter-ions form what can be calleda layer around the colloids, that screens the colloidal charge. The typicalwidth of the layer will turn out to be κ −1 . Hence, κ −1 was called screening63