Abstract
Ultrasound is known to promote crystal nucleation, but despite significant research there remains uncertainty about how the mechanisms are affected. Despite the proposal of various primary nucleation theories, most studies provide no way to quantify or observe the extent to which primary nucleation is taking place, leaving open the possibility that sonocrystallisation is occurring by a secondary nucleation-driven mechanism. By utilising the widely reported enantiomeric properties of sodium chlorate, the extent to which ultrasound can induce primary nucleation can clearly be observed. It was demonstrated during seeded cooling crystallisation that when stirring the seed similarity was 99.3% on average, indicating secondary nucleation had almost exclusively taken place. The application of ultrasound however, decreased the seed similarity to 85.8% and 92.4% when applying 98 kHz and 200 kHz ultrasound respectively, clearly showing that primary nucleation had been induced and indicating the frequency dependency of the induced primary nucleation. This frequency dependency suggests a link between crystal nucleation and high intensity cavitation collisions and collapses, and the potential existence of a collapse/collision intensity threshold required to induce primary nucleation. In addition, secondary nucleation rate was investigated using anti-solvent crystallisation and was observed to increase with the application of ultrasound, though it appeared frequency independent (between 98 kHz & 200 kHz), suggesting that higher energy cavitational events are less important in inducing secondary nucleation or that a lower cavitation intensity threshold exists compared to primary nucleation.
Highlights
Crystallisation is a commonly used purification and separation technique utilised across the pharmaceutical, food and fine chemical industries [1,2,3], whereby solid solute particles precipitate out of a so lution as the system tries to reach an equilibrium saturated state
This acoustic cavitation causes the phenomenon of sonoluminescence [11,12,13,14,15,16], the process by which the intense collapse of the cavitation bubbles causes a short burst of light to be emitted alongside high temperatures and pressures [17]
It is clearly demonstrated that a much higher multibubble sonoluminescence (MBSL) is observed with the sodium chlorate solution compared to water for both frequencies
Summary
Crystallisation is a commonly used purification and separation technique utilised across the pharmaceutical, food and fine chemical industries [1,2,3], whereby solid solute particles precipitate out of a so lution as the system tries to reach an equilibrium saturated state. The driving force for crystallisation is supersaturation of solute, which can be generated by cooling or evaporating solvent from a saturated solution or by the addition of an antisolvent This involves two steps, namely crystal nucleation (the “birth” of new crystals) and crystal growth [4,5]. The effects caused by sonocrystallisation are a result of the acoustic cavitation phenomenon induced by ultrasound [10], in which oscillating pressures created by the sound waves cause the growth and collapse of microbubbles [11] This acoustic cavitation causes the phenomenon of sonoluminescence [11,12,13,14,15,16], the process by which the intense collapse of the cavitation bubbles causes a short burst of light to be emitted alongside high temperatures and pressures [17]. This has become a useful analytical tool to quantify the amount of acoustic cavitation taking place [10]
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