Using PXRD and PONKCS to Determine the Kinetics of Crystallisation of Highly Concentrated NH4NO3 Emulsions

Abstract

A technique that is known as PONKCS (Partial or no known crystal structure) has become useful in the quantitative determination of solid phases in powder samples that have no known crystal structure, or that have poorly determined crystal structures. This technique was shown to be useful with commercial software, such as Topas®, to quantitatively determine the amorphous phase content of powdered samples that contain a number of crystalline phases within the bulk material. In this study, the method of analysis was applied to the crystallization process of highly concentrated emulsions of ammonium nitrate (HCE of AN) composed of a supersaturated salt solution within a continuous hydrocarbon phase that was stabilized with a variety of surfactants and additives. These emulsions are thermodynamically unstable, and would with time crystallize into the crystalline room temperature phase IV of AN. It was found that by applying slightly different mixing times during the preparation steps, emulsions with different droplets sizes would be obtained. Powder X-ray diffraction was shown to be a useful tool in studying the crystallization process of HCE of AN, where the fresh emulsion would have a characteristic amorphous halo that over time, would then show typical diffraction peaks to appear that correspond with the solid phases of crystalline AN. An assumption was made that the emulsion crystallizes close to about 80 % solid content, thereby allowing for a relative quantification to be made over time. The change with time in the degree of crystallization of the emulsions prepared with different droplet sizes could then be modeled with the well-known Johnson–Mehl–Avrami–Kolmogorov kinetic equation. A crystallization time-related constant and an empirical exponent that reflects the type of nucleation that occurred during the process was determined. The results showed that for a set of emulsions that were prepared by varying only the mixing times and their effective droplet sizes, that the emulsion with the smaller droplet had higher crystallization stability.