Bulk and confinement-induced phase transitions in colloidal suspensions

Abstract

In the thesis, we have presented computer simulation results on the bulk and interfacial phase behaviour of colloidal suspensions. In the first part, we have developed and tested a simulation technique to calculate the free energy of hard-core systems. This technique was used to calculate the interfacial free energy of colloidal hard spheres and with the addition of non-adsorbing polymer coils. Good agreement was found between the simulation results and those from density functional theory. Furthermore, we have determined the equilibrium phases of a system of hard spheres confined between two parallel hard walls for plate separations from one to five hard-sphere diameters. We found a fluid-solid transition, which corresponded to either capillary freezing or melting depending on the plate separation. The coexisting solid phase consisted of crystalline layers with either triangular or square symmetry. At high densities, intermediate structures, e.g., prism, buckled, and rhombic phases, were found. In addition we have analysed colloid-polymer mixtures confined between two parallel plates. We have considered different types of confinement, namely either through two hard walls or through two semi-permeable walls that repel colloids but allowed polymers to freely penetrate. For hard walls we found capillary condensation, while for semi-permeable walls we found capillary evaporation. In the second part of the thesis we have analysed the bulk behaviour of colloidal suspensions. We have studied the stability of mixtures of highly screened repulsive charged spheres and non-adsorbing ideal polymer chains. We found that the screened-Coulomb repulsion counteracts the effect of the effective polymer-mediated attraction. For mixtures of small polymers and relatively large charged colloidal spheres, the fluid-crystal transition shifted to significantly larger polymer concentrations with increasing range of the screened-Coulomb repulsion. For relatively large polymers, the effect of the screened-Coulomb repulsion was found to be weaker. The resulting fluid-fluid binodal was only slightly shifted towards larger polymer concentrations upon increasing the range of the screened-Coulomb repulsion. In addition, we have studied the phase behaviour and the interfacial tension of the screened Coulomb (Yukawa) restricted primitive model (YRPM) of oppositely charged hard spheres. The critical temperature decreased upon increasing the screening of the interaction (decreasing the range of the interaction), while the interfacial tension decreased upon increasing the range of the interaction. Finally, we have studied a mixture of monodisperse colloidal hard spheres and ideal polymers described by an effective one-component system. The equilibrium phase diagram was divided in different kinetic regimes. We carried out Brownian dynamics simulations to study the dynamic evolution of the different kinetic pathways. We found a fluid of `long-lived' clusters in the binodal regime at low colloid packing fractions. In the spinodal regime, we observed at low colloid packing fractions a phase of crowded clusters that could merge and break-up again, and a kinetic arrested spinodal decomposition at higher packing fractions. At even higher colloid densities, a homogeneous gel phase was observed. The structure of the clusters were crystal-like at low attractive interactions and glassy at high attraction strengths.