Phase Behavior of Charged Colloids and the Effect of External Fields

Abstract

Colloids are particles with a size in the nano- to micrometer range that are dispersed in a solvent, and that due to collisions from the solvent molecules, undergo Brownian motion. In most cases, the surface of the colloid acquires a net charge due to dissociation of chemical groups at the surface. In this thesis, we present results of computer simulations on the phase behavior of charged colloidal suspensions using various levels of description: Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, DLVO theory with effective many-body interactions, and the primitive model.

We make two attempts to amend the DLVO theory to include many-body interactions: (i) density-dependent truncation and (ii) three-body interactions. In the case of three-body interactions, we find at low salt concentration a very broad coexistence between a fluid and a dense face-centered-cubic (fcc) phase, while at intermediate salt concentration a broad body-centered-cubic (bcc)-fcc coexistence regime appears. However, in our primitive model calculations, we did not find any broad coexistence regions or any other manifestations of three-body interactions. Thus, the effective Hamiltonian of charged colloids seems to be better described by the standard pairwise DLVO potential, than by a Hamiltonian including a density-dependent truncation or three-body interactions.

We study the gas-liquid critical point of asymmetric electrolyte mixtures consisting of large multivalent macroions and small monovalent co- and counterions. The system can be seen as a binary mixture of colloids with their counterions and salt at strong electrostatic coupling. We calculate the critical point locus that connects the salt-free state consisting of macroions and counterions with the pure salt state.

We calculate the ground-state phase diagram of a mixture of large and small (size ratio 0.31) oppositely charged colloids. The phase diagram displays novel structures, but also colloidal analogs of simple-salt structures and of doped fullerene C60 structures. Three of the predicted structures were also observed experimentally. We also calculate the phase diagrams of (i) the restricted primitive model (RPM) and (ii) screened Coulomb particles. We show that the two phase diagrams are qualitatively similar, and more importantly that both contain a new experimentally observed solid phase, which is a colloidal analogue of the CuAu structure.

We also look at the effect of external electric or magnetic fields on the phase behavior of charged colloids and study charged colloids in gravity. The phase diagram charged colloids in an external electric or magnetic field shows fluid, fcc, hexagonal-close-packed (hcp), body-centered-orthorhombic (bco), and body-centered-tetragonal (bct) phases. The phase diagram is in agreement with the experimental phase diagram. We show that gravity gives rise to a macroscopic charge separation of colloids and microions, provided the added salt concentration is low enough. The mechanism is due to the intricate balance between colloidal and ionic entropy, potential energy, and electrostatic energy. The electric field that is generated by the charge separation is shown to be such that it largely compensates the gravitational force on the colloids. Therefore, the colloids are lifted to altitudes much larger than their gravitational length and one observes highly non-barometric sedimentation profiles.