Segment-sphere size ratio influence on the stability of a polymer-colloid mixture

The influence of the sphere to segment size ratio on the effective depletion layer thickness around a sphere and the resulting phase behaviour of a colloid-polymer mixture is considered. An analytical expression for the excluded volume between an ideal freely jointed chain and a sphere [L. Lue, J. Colloid Interface Sci. 202, 558 (1998)] allows the effect of polymer characteristics (chain length, segment/sphere size ratio) on the depletion interaction in polymer-colloid mixtures to be calculated. For both small and intermediate polymer-to-colloid size ratios the respectively gas-solid and gas-liquid coexistence curves shift to larger polymer coil volume fractions as the relative segment size increases. It is further demonstrated that the conditions for optimal (protein) crystallization are affected when the segment to sphere size is altered.