Influence of Cavitation Activity on the Sonocrystallization\nof <i>p</i>‑Aminobenzoic Acid

A systematic study\non the batch cooling sonocrystallization of <i>p</i>-aminobenzoic\nacid (pABA) in ethanol was investigated under\ndifferent ultrasound frequencies (22 kHz to 1 MHz), powers (1–40\nW), and sonication times (1.73–31.85 min), with the crystal\nlength, width, aspect ratio (AR), and induction time recorded for\neach experiment. These measured variables were related to sonoluminescence\n(SL) values to quantify the cavitation activity. For the first time,\nthe crystal length, aspect ratio, and induction time were observed\nto decrease with the logarithm of integrated SL intensity per acoustic\ncycle, irrespective of frequency. No such correlations were observed\nwith power. 200 kHz ultrasound produced crystals with the lowest mean\naspect ratio of 5.17, a 68% reduction from the silent case. However,\n22 kHz was the most efficient frequency at reducing the induction\ntime (by 95%), crystal length (by 76%), width (by 67%), and aspect\nratio (by 26%) at the lowest applied power of 1 W, while increasing\nthe power had no further significant impact. Applying 22 kHz ultrasound\nbursts (1–10 min duration) before nucleation reduced the induction\ntime by up to 75%, with burst duration and power (5 and 10 W) having\nno observable effect on the crystal shapes. When applying ultrasound\nto crystals in a slurry, fragmentation was observed with only 22 kHz\nultrasound. It is hypothesized that the mechanism behind sonication\nunder 22 kHz is related to both crystal nucleation and fragmentation\ncaused by unstable, transient cavitation. This differs from the mechanism\nfor 200 kHz and 1 MHz where the cavitation is mainly stable and produces\na weaker physical force that does not cause severe crystal fragmentation\nbut enhances crystal nucleation.