Chapter 6: Polymer Depletion: Recent Progress for Polymer/Colloid Phase Diagrams

A new theory for the phase behaviour of a mixture of colloids and nonadsorbing polymer is outlined. In such a mixture there is a depletion zone (thickness δ_s) around the colloids (radius a) which is void of polymer. Existing theory assumes δ_s to be equal to the radius R of the polymer coils. This is a fair approximation for the colloid limit (small q = R/a) but fails in the protein limit (large q = R/a) where δ_s becomes equal to the concentration-dependent correlation length ξ in semidilute solutions. The new theory includes a correct description for both limits and applies to intermediate size ratios (q ≈ 1) as well.