Anisotropic Nanocolloids: self-assembly, interfacial adsorption, and electrostatic screening

Abstract

In this thesis we consider the influence of anisotropy on the behaviour of colloids using theory and simulations. The recent increase in the ability to synthesize anisotropic particles (cubes, caps, octapods, etc.) has led to samples of sufficient quality to perform self-assembly experiments. Our investigation is therefore particularly relevant to current and future experimental studies of colloids. We examine several topics for which shape anisotropy plays an important role: •Interfacial adsorption. We introduced the triangular-tessellation technique to approximate the surface areas and line length which are associated with a plane-particle intersection. Our method allowed us to determine the free energy of adsorption for a single irregular colloid with heterogeneous surface properties adsorbed at a flat liquid-liquid interface in the Pieranski approximation. Ellipsoids only adsorbed at the interface perpendicular to the interfacial normal. However, for cylinders we could find a metastable adsorption minimum corresponding to parallel adsorption. We also considered the possible time dependence of the adsorption process using simple dynamics. Finally, we studied the adsorption of truncated nanocubes with a contact-angle surface pattern and we observed that there are three prototypical equilibrium adsorption configurations for these particles. •Crystal-structure prediction. We extended an existing crystal-structure-prediction algorithm to predict structures for systems comprised of irregular hard particles. Using this technique we examined the high-density crystal structures for 17 irregular nonconvex shapes and we confirmed several mathematical conjectures for the packings of a large set of 142 convex polyhedra. We also proved that we have obtained the densest configurations for rhombicuboctahedra and rhombic enneacontrahedra, respectively. Moreover, we considered a family of truncated cubes, which interpolates between a cube and an octahedron, for which we obtained a fascinating richness in crystal structures. For the octahedron we determined the equation of state and we obtained a liquid, a (metastable) body-centred-cubic rotator phase, and a crystal phase. •Octapod hierarchical self-assembly. We analysed the recently observed hierarchical self-assembly of octapod-shaped nanocrystals (octapods) into three-dimensional (3D) superstructures. We constructed an empirical simulation model capable of reproducing the initial chain-formation step of the self-assembly. The van-der-Waals (vdW) interactions between octapods suspended in an (a)polar medium were obtained by means of a Hamaker-de-Boer-type integration and the nature of these interactions allowed us to justify elements of our empirical model. We used the theoretical vdW calculation, together with the experimental and simulation results, to formulate a mechanism which explained the observed self-assembly in terms of the solvent-dependence and directionality of the octapod-octapod interactions. •Ionic screening of charged Janus particles. We studied the screening of charged Janus particles in an electrolyte by primitive-model Monte Carlo (MC) simulations for a wide variety of parameters. We also introduced a method to compare these results to the predictions of nonlinear Poisson-Boltzmann (PB) theory. The comparison of MC and PB results allowed us to probe the range of validity of the PB approximation. This range of validity corresponds well to the range that was predicted by field-theoretical studies of homogeneously charged flat surfaces