Structure and thermodynamics of colloid-polymer mixtures : A macromolecular approach

– The change of the structure of concentrated colloidal suspensions upon addition of non-adsorbing polymer is studied within a two-component, Ornstein-Zernicke–based liquid state approach. The polymers’ conformational degrees of freedom are considered and excluded volume is enforced at the segment level. The polymer correlation hole, depletion layer, and excess chemical potentials are described in agreement with polymer physics theory in contrast to models treating the macromolecules as effective spheres. Known depletion attraction effects are recovered for low particle density, while at higher densities novel many-body effects emerge which become dominant for large polymers. Colloid-polymer mixtures (CPM) play an important role among dispersion systems for quite different reasons. On the one hand, their technological use has long been realized and the specific colloid-colloid interaction caused by free polymer, the depletion attraction [1, 2], is exploited to induce flocculation or phase separation in dispersions. On the other hand, this system allows one to experimentally address the fundamental question about the requirements on the pair potential for a liquid phase to exist [2, 3]. For even more complex systems CPM serves as a model system in order to address protein crystallization [4,5] and other phenomena involving spherical nanoparticles. Recent field-theoretic considerations of a few colloidal particles in dilute polymer solutions [6], and scaling arguments for semidilute solutions [7], have clarified the polymer structure close to particles and provided a fundamental understanding of the origins of the depletion attraction. The phase diagram of sterically stabilized hard-sphere–like colloidal particles in polymer solutions close to their Θ-point has been mapped out in detail [3], and a rather successful mean-field–like theory for it exists [8]. The thermodynamic approach of Lekkerkerker et al. maps the CPM onto a binary hard-sphere mixture with non-additive radii, allowing the effective polymer spheres (EPS) to overlap, but excluding them from the colloids. Simulation studies for a closely related model support some of the theoretical predictions [9]. So far little, however, is known about the structure of the CPM, nor how depletion phenomena change when particles are small compared to polymers. Whereas neutron scattering