Colloid–polymer mixtures in the presence of quenched disorder: a theoretical and computersimulation study

Abstract

We use theory and computer simulation to study the structure and phase behavior ofcolloid–polymer mixtures in the presence of quenched disorder. The Asakura–Oosawamodel (AO) (Asakura and Oosawa 1954 J. Chem. Phys. 22 1255) is used to describe thecolloid–colloid, colloid–polymer, and polymer–polymer pair interactions. We theninvestigate the behavior of this model in the presence of frozen-in (quenched) obstacles.The obstacles will be placed according to two different scenarios, both of which areexperimentally feasible. In the first scenario, polymers are distributed at positions drawnfrom an ideal gas configuration. In the second scenario, colloidal particles aredistributed at positions drawn from an equilibrium hard sphere configuration. Weinvestigate how the unmixing transition of the AO model is affected by the type ofquenched disorder. The theoretical formalism is based on the replica methodof Given and Stell (1994 Physica A 209 495). Our foremost aim is to test theaccuracy of three common closures to the replica Ornstein–Zernike equations,namely the hypernetted chain, the Percus–Yevick, and the Martinov–Sarkisovequations. The accuracy is determined by comparison with grand canonical MonteCarlo simulations. We find that, for quenched polymer disorder, all three closuresperform remarkably well. However, when quenched colloid disorder is considered,i.e. the second mentioned scenario, the predictions of all three closures worsendramatically.