Phase behaviour of proteins and colloid-polymer mixtures

In this thesis, we present a theoretical and experimental study of the equilibrium phase behaviour of colloidal dispersions and protein solutions. The thesis consists of two theoretical parts on the phase diagram of lysozyme solutions, and mixtures of charge-stabilised colloids and neutral polymers, respectively, and an experimental part on the stability of mixtures of charged colloids and non-adsorbing polymers, including a theoretical model of the aggregation kinetics. In the first part, the phase behaviour of lysozyme solutions is calculated using thermodynamic perturbation theory applied to a patchy colloidal model with an anisotropic interaction part that incorporates screened Coulomb repulsions. The interaction parameters in this model are obtained from experimental data sets. The experimental phases are consistently described by our model of protein interactions, and nearly quantitative agreement is found for the metastable gasliquid and the fluid-solid coexistence lines. We show that the phase behaviour of lysozyme strongly depends on the anisotropic hydrophobic interactions. The range of attraction obtained with our model is in excellent agreement with previous surface force measurements. In addition, a consistent description is obtained for the structure factor at small wave numbers. The second part addresses the depletion interaction and the phase behaviour of mixtures of charged spherical colloids and neutral polymer chains under good and θ-solvent conditions. To predict the phase behaviour, we have derived a generalised free-volume theory, which is in good agreement with computer simulation data of the binodal for polymer-to-colloid size ratios q ≤ 1. For q > 1, our theory captures the qualitative trends. We study the effect of weak electrostatic repulsions and the influence of the solvent quality, and show that the colloid charge stabilises the homogeneous phase against demixing. The phase stability is enhanced with increasing q and solvent quality. Our theory predicts that the solvent quality strongly affects the location of the gas-liquid coexistence curve in case of weakly charged colloids. For a θ-solvent and q > 1, the gas-liquid coexistence curve is almost unaffected by electrostatic repulsions. In the third part, the stability and phase behaviour of an aqueous mixture of charged, nanosized spheres and polysaccharide chains of roughly equal sizes is investigated by visual inspection and photon correlation spectroscopy. We explore the interplay between charge-induced repulsion and polymer-induced colloid attraction. We find that even weak electrostatic screening, or small amounts of polymers enhance colloid aggregation. Good agreement is observed between the experimental cluster growth rates and the predictions of doublet formation theory with hydrodynamic interactions, where the polymer-induced attractions are well described by the Asakura-Oosawa-Vrij potential. We compare the non-equilibrium phases caused by macroscopic phase separation, with predictions from a generalised free-volume theory on equilibrium phases, and observe similar trends regarding the salt influence