Phase behaviour of charged colloidal sphere dispersions with added polymer chains

Abstract

We study the stability of mixtures of highly screened repulsive charged spheres andnon-adsorbing ideal polymer chains in a common solvent using free volume theory. Theeffective interaction between charged colloids in an aqueous salt solution is described by ascreened Coulomb pair potential, which supplements the pure hard-sphere interaction. Theideal polymer chains are treated as spheres that are excluded from the colloids by ahard-core interaction, whereas the interaction between two ideal chains is set tozero. In addition, we investigate the phase behaviour of charged colloid–polymermixtures in computer simulations, using the two-body (Asakura–Oosawa pairpotential) approximation to the effective one-component Hamiltonian of the chargedcolloids. Both our results obtained from simulations and from free volume theoryshow similar trends. We find that the screened Coulomb repulsion counteractsthe effect of the effective polymer-mediated attraction. For mixtures of smallpolymers and relatively large charged colloidal spheres, the fluid–crystal transitionshifts to significantly larger polymer concentrations with increasing range of thescreened Coulomb repulsion. For relatively large polymers, the effect of the screenedCoulomb repulsion is weaker. The resulting fluid–fluid binodal is only slightly shiftedtowards larger polymer concentrations upon increasing the range of the screenedCoulomb repulsion. In conclusion, our results show that the miscibility of dispersionscontaining charged colloids and neutral non-adsorbing polymers increases uponincreasing the range of the screened Coulomb repulsion, or upon lowering the saltconcentration, especially when the polymers are small compared to the colloids.