MICRO-PARTICLE STUDIES IN POWDER SYSTEMS

Abstract

A study of the electrical and physico-chemical properties of comminuted micro-particles of sucrose has been made leading to a revised model of the sucrose molecule and crystal being postulated. The comminuted particles were prepared by milling crystals in either a rolling or vibrating ballmill. Ballmilling resulted in breakage predominantly along cleavage planes, producing particles of positive mono-poles, whilst vibromilling resulted in breakage predominantly across cleavage planes, giving rise to dipoles. However particles prepared by homogeneous nucleation in methylcellosolve or ethanol were found to be non-polar. These electrical properties of sucrose particles wore studied using a micro electrophoretic cell with voltage gradient up to 2000 volts/ cm and in a disperse medium of silicone oil. Size analyses of the micro-particles were achieved by dispersing them in either n-butanol or methylcellosolve and then using a light extinction principle built into the Hitachi Particle Size Analyser. The sizes were found to range between 2-1l0 microns. A more detailed study of the physico-chemical properties of the particle surfaces was made with a micro-calorimeter which gave heats of adsorption for various polar organic liquid adsorbates. Experimental results ranging from 100-200 microcalories/cm2 for this exothermic system confirmed the positive monopole on the ballmilled surface and the dipole on the vibromilled surface, and that small unpulverised crystals up to 300 microns behaved predominantly as positive monopoles. Electron micrographs of the particle faces cleaved by ballmilling or vibromilling wore seen to consist largely of a new fabric resulting from fusion and recrystallisation under stress, its anisotropy seeming to depend upon the degree of inhomogeneity of the strain, and to have resulted in alignment of the polar sucrose molecules in the comminuted particles, Also, under stress the crystals do not degenerate into an amorphous mass typical of brittle substances, but show plastic behaviour is found by studying the stress-strain behaviour of a bed of crystals under compression. By systematic studies of this character a model of a polar sucrose molecule and crystal has been postulated. The model provides or eight cationic sites on the molecules due to polarisation of the eight hydroxyl groups resulting in protective cationic sheath over the molecule. A crystal grows by attachment of a sucrose molecule, to the active anionic sites on the crystal surface. These eight anionic sites are normally protected but randomly exposed during frequent oscillations of the cationic sheath. A molecule in the solution phase could also experience anionic site exposure resulting in attachment to a crystal cationic site or in secondary nucleation if the frequencies of exposure are high enough in the solution phase, The water molecules present in a saturated solution provide competition for active sites on the crystal and at temperatures below 30°C may form a perpetuating sheath of water molecules through intermolecular hydrogen bonding, or a non-perpetuating monolayer as inclusion, and at the extreme, hydrates with sucrose. According to the model the solubility behaviour of sucrose in various polar organic solvents would be interpreted in terms of a polar and stereochemical interaction between solute and solvent molecules. The present study of the behaviour of the sucrose molecule in the solid and liquid phases also indicates possibilities for crystallisation control.