Browsing by Author "Volkwyn, Alexandra Lemisha"

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    Crystallisation of multicomponent crystals by sublimation: effect of experimental conditions
    (Stellenbosch : Stellenbosch University, 2023-10) Volkwyn, Alexandra Lemisha; Haynes, Delia Ann; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: This thesis aimed to develop a greater understanding of important principles behind the technique of sublimation and demonstrate how we can gain control over the technique to selectively form multicomponent crystal forms, such as hydrates, co-crystals, and salts. This aim was attained by carrying out two studies. The first study focused on investigating whether hydrates of oxalic acid (1), isonicotinamide (2), theophylline (3), caffeine (4), and 1,4-diazabicyclo[2.2.2]octane (5) can be grown by sublimation. Compounds 1-5 were sublimed in the presence and absence of water. An increase in the quantity of water added to the sublimation vessel resulted in an increase in the water content of sublimed crystals. Hydrates of 1 and 5 were easily crystallised from the gas phase in the presence of water. Competition experiments were carried out by co- subliming an anhydrous material and a hydrate, and water transfer took place from the hydrate to the anhydrous material. This study has shown that materials which have the most favourable interactions between the molecule in question and water within the hydrate crystal structure are more likely to crystallise as the hydrate from sublimation with water. The second study focused on investigating the effect of sublimation time, vacuum pressure, temperature, separation apparatus, mass scale, additives, and co-former polymorph on the co-sublimation of two systems: system 1 = succinic acid + hexamethylenetetramine and system 2 = oxalic acid + 4,4'-bipyridine. A salt formed under higher temperature and lower pressure conditions, and co-crystal was favoured under a reduced temperature and higher pressure. Separating the acid and base components prior to sublimation allowed for more control over the concentration of components in the gas phase. Sublimation conditions under which the concentration of acid and base component in the gas phase is maximised are more likely to result in salt formation. In the presence of additives, co-crystal formation was favoured during co-sublimations of system 1. In system 2, salt formation was favoured in the presence of methanol. Results suggest that the type and quantity of additive added to the sublimation vessel can be used to control the outcome of co-sublimations. These studies have shown that simple experimental conditions can be used to gain control over the method of sublimation, and can be used tomanipulate the concentration of components in the gas phase to selectively form salts or co-crystals. The effect of polymorph used during the co-sublimation of system 1 was studied. Sublimation with the β polymorph of succinic acid favoured the formation of co-crystal. It is clear that the use of a specific co-former polymorph can be used to direct the co-sublimation outcome. Overall, this thesis has provided a good foundational understanding of the important principles of the technique of sublimation, and has successfully demonstrated the importance of simple experimental conditions in multicomponent crystal growth from the gas phase. We believe that this simple approach towards gaining control over a crystallisation technique to form a desired product has broadened the scope of sublimation as a whole and will aid the development and design of new materials.