Sonocrystallisation is the application of ultrasound to the crystallisation process. The benefits obtained by sonication have been widely studied since the beginning of the 20th century and so far it is clear that ultrasound can be a very useful tool for enhancing crystallisation and controlling the properties of the final product. Crystal size, polymorphs, purity, process repeatability and lower induction time are only some of the advantages of sonocrystallisation. Even though the effects of sonication on crystallisation are quite clear, the physical explanation of the phenomena involved is still lacking. Is the presence of cavitation necessary for the process? Or is only the bubbles surface responsible for enhancing crystallisation? Are the strong local increases in pressure and temperature induced by cavitation the main cause of all the observed effects? Or is it the strong turbulence induced in the system, instead? Many questions still remain and can only be appreciated with an understanding of the complexity behind the individual processes of crystallisation and acoustic cavitation. Therefore, this review will first summarise the theories behind crystallisation and acoustic cavitation, followed by a description of all the current proposed sonocrystallisation mechanisms, and conclude with an overview on future prospects of sonocrystallisation applications.
Crystallisation by precipitation is a very common technique in industry, however the crystals produced are often of poor quality, characterised by broad particle size distribution and morphological inhomogeneities. These problems can be overcome by the use of ultrasound. Sonocrystallisation, or ultrasound-assisted crystallisation, has already demonstrated its general benefits in terms of reduction in crystallisation time, smaller and more homogeneous crystals and repeatability. However, there are many issues still unaddressed, such as poor knowledge of the mechanism of sonocrystallisation and of the governing parameters of the whole system. This is largely attributed to lack of systematic studies, varying frequency and power while using the same equipment, as well as difficulties in comparing results from different systems. Furthermore, ultrasound is still considered uneconomical in industrial scale, due to its high energy demand. Therefore, there is the necessity of optimising the use of ultrasound for guaranteeing a high-quality product with the lowest energy consumption.
This thesis therefore focused on studying the effects of sonocrystallisation for two different crystallisation systems by precipitation: the antisolvent crystallisation of sodium chloride and the reactive crystallisation of ZIF-8, a type of Metal-Organic Frameworks (MOFs). In both systems, frequency, power and sonication time was systematically varied, and the effects on the product obtained investigated. The effect of changing supersaturation was also studied. The comparison between a simpler and a more complex system could help on obtaining general conclusions useful for clarifying the mechanism of sonocrystallisation.
The antisolvent sonocrystallisation of sodium chloride revealed that sonication caused a reduction of the crystal size up to 10 times with respect to unsonicated conditions. This was achieved within a few seconds of sonication (5 s for high supersaturation and 15 s for low supersaturation), indicating that the effects of ultrasound were on the nucleation, causing the rapid formation of small and regular crystals. Phenomena of sono-fracture was excluded because the crystals formed were morphologically regular, very different from fragments. Adding a subsequent step in silent conditions or using intermittent ultrasound bursts coupled with silent conditions showed a limited crystal growth. Hence, ultrasound uses the majority of the solute forming new small crystals and leaving in the solution only a small amount of sodium chloride. The effects of different frequencies and powers were more evident at lower supersaturations, with lower frequency being more effective at reducing crystal size. However, under these conditions, although a small average crystal size was obtained, the particle size distributions were bimodal at low power. At high supersaturation, this bimodality disappeared even at low powers.
For the reactive crystallisation of ZIF-8 at high excess of ligand, it was confirmed that the action of ultrasound caused a reduction of the crystal size up to nanometre. Furthermore, it was demonstrated again that the action of sonication was on the early stages of the crystallisation. However, the effects of ultrasound resembled a lot the results obtained when only mixing was applied, suggesting that sonication was not directly influencing the chemistry of the reaction. In addition, the BET surface area appeared to be related to the power used, reaching a maximum and decreasing beyond a certain power. This suggests that a competition between micromixing, causing an increase of the BET surface area, and shockwaves, responsible of breaking the framework during its formation. On the other hand, when a low excess of ligand was used, it was revealed that the formation of by-products was accelerated when mixing or sonication were applied. However, if shorter reaction time was applied, it was possible to obtain a reasonable quality of ZIF-8 at low