Hurley, Liam Anthony; (1993) Precipitation of cyanazine. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Large proportions of crop protection agents are currently formulated as emulsifiable concentrates. These aromatic hydrocarbon based solutions, which form emulsions when diluted with water, are coming under increasing pressure from environmental agencies. Potential alternatives are dispersible concentrates which precipitate the active ingredient when water is added. The key to developing commercially acceptable dispersion concentrates is dependent upon the control of this precipitation process. The commercial herbicide cyanazine was selected from candidate substances as a suitable active ingredient to study such a precipitation using aqueous ethanol solutions. Basic solubility data was then obtained followed by structural and kinetic studies. Investigations into the precipitation process revealed that cyanazine can crystallise in two different forms, needles and platelets. X-ray crystallography and differential scanning calorimetry showed that cyanazine platelets were metastable monohydrates of cyanazine while the needles were anhydrous. Optical microscopic analysis showed platelets to fragment and recrystallise as stable anhydrous needles. Needles were the only crystal form which agglomerated. Water weight fraction, supersaturation and temperature each had dramatic effects on crystal form. In general, increases in water weight fraction and supersaturation, and decreases in temperature increased the percentage number of platelets formed. The results of induction period and contact angle measurements suggest that primary nucleation followed by diffusion-controlled or polynuclear growth are the controlling mechanisms of crystallisation when the water weight fraction is between X=0.65-0.95. This conclusion is supported by dissolution and overall growth measurements at an initial supersaturation of S=3, which show a gradually increasing influence of diffusion on the two-step crystal growth process as water weight fraction is increased. Optical and scanning electron microscopy of single crystals reveal that the surface integration mechanism is controlled by layer by layer adsorption at high supersaturation and water weight fraction. As these parameters decrease the surface integration mechanism is influenced by screw dislocations. Finally, the commercial implications of this work are discussed.

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