Gel Texture, Nanoparticles, and Electric Effects in Periodic Precipitation Systems

Abstract

Self-organization in diffusion-precipitation systems has long been a phenomenon of interest, with wide applications in nature. The formation of a precipitate in a reaction-diffusion gel system displays a stratification of parallel bands in 1D or rings in 2D. The scenario depends on the precipitation kinetics coupled to transport processes, essentially diffusion governed by a steep concentration gradient. Such periodic precipitation, coined a Liesegang pattern results from the diffusion of an outer electrolyte into a gel medium containing a homogenously distributed inner electrolyte hosting a co-precipitate ion. The spatiotemporal pattern formation is essentially governed by three empirical laws: spacing law (distances between the bands), width law, and time law. Several models have been proposed to explain the mechanism of Liesegang pattern formation. The function of the gel can modify the pattern morphology and dynamics; thus, it warrants further study, from the viewpoint of composition, purity, structure and eventual crosslinking. This thesis is an extended study of periodic precipitation (Liesegang banding), which explores new aspects of this rich phenomenon. In the first part, we study the effect of different gel types on the morphology and dynamics of Liesegang patterns, with a focus on the cobalt hydroxide system. We aim to correlate the gel-precipitate texture and microstructure to the type of macroscopic pattern obtained. Four different gelling systems are used: Difco agar, Sigma agarose, Difco gelatin, and Sigma gelatin. The investigation will be based on SEM analysis, as well as viscosity measurements. In the second part, we shift our attention to the Ag2Cr2O7 system, to investigate the effect of silver nanoparticles on the formation of Ag2Cr2O7 Liesegang patterns. We synthesize monodisperse silver nanoparticles using different methods, and measure their sizes using dynamic light scattering and SEM. The latter are added to the gel, and the experiments are carried out as usual, under both light and dark conditions. SEM images of different regions in the tube were obtained to gain more insight into the colloid/aggregation present. In the third part, we investigate the effect of electric field using the magnesium hydroxide Liesegang system in 1D. At first, we test two gel types: Difco agar and Difco gelatin, using magnesium chloride hexahydrate as the inner electrolyte and ammonium hydroxide as the outer one. The tests pointed out to Difco agar as a more suitable medium. We apply a DC electric field on the magnesium hydroxide system, and compare field free, positive field, and negative field.