Influence of Cavitation Activity on the Sonocrystallization of <i>p</i>-Aminobenzoic Acid

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