Bulk and surface properties of charge stabilized colloids

Under certain circumstances, charge-stabilized aqueous suspensions of monodisperse spherical colloidal particles exhibit a phase transition between a concentrated `liquid' or flocculated phase and a `gas' disperse phase. We investigate the interface between the two phases for the limit of a weakly charged system interacting via the DLVO interparticle potential, using both a microscopic version of Cahn-Hilliard theory and a mean field density functional approach. The DLVO potential between the colloidal particles has a short ranged Hamaker attraction and a medium range Debye-Huckel screened charge repulsion and we find that the competition between these parts can cause Cahn-Hilliard theory to break down. We also find that mean field calculations show sharp spontaneous oscillations in the interface between the two phases. We show that when this happens, the `liquid-gas' critical point is preempted by a finite wave vector critical point and the uniform colloid becomes unstable with respect to a density wave. We investigate this critical region further using a Landau type theory and find a new phase diagram which not only contains `liquid' and `gas' phases but also intermediate density, macrocrystal, incommensurate b.c.c. phases and layered phases similar to those found in magnetic and 2-D amphiphilic systems. Lastly, we investigate the behaviour of the colloid in this new critical region, when it is placed between hard walls. We have again used a mean field density functional approach. We predict that the amplitude of the density fluctuations of the inhomogeneous phases will be increased for wall separations commensurate with the period of the density fluctuations.