Knott, Michael; (2001) Phases and phase transitions in charged colloidal suspensions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

We study the stability and phase behaviour of charged colloidal suspensions theoretically, starting from fundamental thermodynamics and electrostatics. The linearised Poisson-Boltzmann equation is solved, subject to justifiable approximations, for a suspension containing a large number of identical spherical macroions under conditions of constant surface charge. The electrostatic term in the Helmholtz free energy is cohesive, and this is opposed by a repulsive counterion ideal gas term. We argue that the stability and phase behaviour of a charged colloidal suspension result from competition between these two terms in the free energy, and that the DLVO theory is inappropriate for the description of these phenomena. In a system containing no added salt, a van der Waals loop can appear in the pV diagram, indicating phase coexistence between phases with different macroion densities. This occurs when the ratio of the macroion charge to the macroion radius is greater than a critical value. Theoretical phase diagrams for systems containing salt show phase separation when the macroion charge is high and the salt concentration is low, in agreement with experimental results. Calculation of the partition of salt between coexisting phases having different macroion densities reveals a 'reverse Donnan effect': at sufficiently high values of the macroion charge and mean salt concentration, the salt is densest in the macroion-rich region. We show that phase coexistence persists when the macroion charge is allowed to vary, using a simple model for the ion dissociation at the surfaces of the macroions. The square gradient approximation is used to calculate the surface tension between two colloidal phases of differing density, and the results are compared with evidence from various colloidal systems. The nucleation rate of a colloidal liquid cluster from a metastable colloidal gas is estimated using a version of classical nucleation theory. We explain the recently described 'Swiss Cheese effect' in terms of nucleation phenomena, and argue that it shows evidence both of homogeneous and of heterogeneous nucleation. Metastability is likely to be very important in colloidal systems, and therefore the consideration of nucleation rates is essential to the study of phase behaviour in such systems.

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