Crystallization of Binary Mixtures of Charged Colloidal Silica Spheres

Abstract

Crystallization is a process ubiquitous in nature and of utmost technological importance for mankind. It is of general interest to understand the solidification of pure metals and alloys, since most metallic materials are produced from the liquid state as their parent phase. However, solidification of metallic melts is difficult to study in situ, due to the spatial and temporal scales involved. This difficulty may be surmounted by using suitable model systems, among them charged colloidal dispersions are of prominent relevance. The particles of such dispersions are ranging between 100 nm to some µm in size which are dissolved in an optically transparent carrier fluid. Hence, they are accessible for visual observation by proper optical means and their relaxation times are much more sluggish than those for atomic or molecular systems. Monodisperse charged colloidal silica particles are specially suited for studies by ultra-small angle X-ray diffraction (USAX) using synchrotron radiation, because they show a large scattering contrast. USAX experiments have demonstrated that colloidal silica suspensions exhibit a similar phase and crystallization behaviour as melts of pure metals. Both physically different systems of monodisperse charged silica suspensions and pure metals develop an icosahedral short-range order in the undercooled regime of the metastable fluid state. Colloidal systems of charged particles interact via a Yukawa-type potential, i.e. they crystallize as a result of mutual repulsion, which may be altered via a fine-tuning of the particles surface charge, e.g. by means of deprotonation of acidic surface groups, or its respective screening by additional electrolytes. This allows for a systematic study of their solidification under different conditions and for a complete characterization of their phase behaviour and fundamental parameters regarding their crystallization kinetics. Results of nucleation studies will be presented and are critically analysed within classical nucleation theory. They are discussed in relation to nucleation in undercooled melts of pure metals and the predictions of molecular dynamic simulation of nucleation processes. We present some results obtained from unary systems, first investigations of nucleation in the binary regime and give an outlook for experiments employing mixtures of particles with different X-ray scattering contrast, which shall be analyzed using a solvent contrast variation technique setup within an ultra-small angle X-ray scattering experiment.