Using PXRD to Investigate the Crystallization of Highly Concentrated Emulsions of NH₄NO₃

Abstract

The process of crystallization of highly concentrated emulsions of ammonium nitrate can be studied using powder X-ray diffraction. The dispersed particles comprise a supercooled aqueous solution of the ammonium nitrate salt and are dispersed in a paraffin-based oil. This results in a thermodynamically unstable system that ‘ages’ with time resulting in changes in rheological properties and its phase composition where the collapse of the supercooled aqueous solution forms the crystallized salt. The crystallization processes of these emulsions are kinetically slow and can take up to a few months to crystallize completely. The general approach to this type of analysis is to determine the change in crystalline diffraction peak intensities relative to the halo due to the amorphous content. However, there are a number of problems associated with this method which are addressed by using Rietveld refinement methods which can take into account factors such as preferred orientation, crystallite size variations and mixtures of solid phases. The study showed that the ammonium nitrate emulsions kept at room temperature slowly crystallize predominantly to the room temperature solid ammonium nitrate phase IV. However, depending on the formulations used some samples showed crystallization to the high temperature ammonium nitrate phase II before changing to phase IV. The crystallization change could be modelled by the well-known JMAK kinetic relationship.

Keywords:  Ammonium nitrate emulsion, crystallinity, X-ray diffraction