Development of Sodium Chloride Crystal Size during Antisolvent Crystallization under Different Sonication Modes

This paper reports for the first time the development in the size and shape of sodium chloride crystals during antisolvent crystallization in ethanol under different sonication modes. Sonication using 98 kHz and a calorimetric power of 6 W was applied either continuously for a range of crystallization times (5-90 s) or intermittently (5 s pulse). Under silent conditions, a crystallization time of 90 s generated crystals with an average size of 73.8 +/- 6.9 mu m, in comparison to 8.7 +/- 2.8 mu m under 90 s of continuous sonication. However, it was observed that within the first 5 s of sonication at the beginning of the crystallization, the average crystal size was already reduced to 7.0 +/- 3.3 mu m. If the system was left to crystallize further to 90 s without ultrasound, the crystal size grew only slightly to 8.2 +/- 1.4 mu m. When a 5 s burst of ultrasound was applied during the crystallization process, a bimodal distribution of small (from sonication) and large crystals (from the silent period) was obtained. These results imply that the major influence of sonication is crystal nucleation rather than fragmentation, and equilibrium is reached with 5 s sonication by precipitating most of the crystals in solution.