Experimental study of sono-crystallisation of ZnSO4·7H2O, and interpretation by the segregation theory

Abstract

Power ultrasound is known to enhance crystals nucleation, and nucleation times can be reduced by one up to three orders of magnitude for several organic or inorganic crystals. The precise physics involved in this phenomenon still remains unclear, and various mechanisms involving the action of inertial cavitation bubbles have been proposed. In this paper, two of these mechanisms, pressure and segregation effects, are examined. The first one concerns the variations of supersaturation induced by the high pressures appearing in the neighbourhood of a collapsing bubble, and the second one results from the modification of clusters distribution in the vicinity of bubble. Crystallisation experiments were performed on zinc sulphate heptahydrate ZnSO(4)·7H(2)O, which has been chosen for its pressure-independent solubility, so that pressure variations have no effect on supersaturation. As observed in past studies on other species, induction times were found lower under insonification than under silent conditions at low supersaturations, which casts some doubts on a pure pressure effect. The interfacial energy between the solid and the solution was estimated from induction times obtained in silent conditions, and, using classical nucleation theory, the steady-state distribution of the clusters was calculated. Segregation theory was then applied to calculate the over-concentrations of n-sized clusters at the end of the collapse of a 4 μm bubble driven at 20 kHz by different acoustic pressures. The over-concentration of clusters close to the critical size near a collapsing bubble was found to reach more than one order of magnitude, which may favour the direct attachment process between such clusters, and enhance the global nucleation kinetics.

Antisolvent Sonocrystallisation of Sodium Chloride and the Evaluation of the Ultrasound Energy Using Modified Classical Nucleation Theory

“…The influence of sonication on the induction time is shown in Figure 5, which agrees with what has been reported in the literature [3,6,24,25]: Sonication can significantly reduce the induction time and this reduction increases with decreasing supersaturation ratio. The effect of different sonication powers is more pronounced when the data is plotted in terms of ln(t ind ) as a function of 1/ln 2 (S) ( Figure 6).…”

“…However, this approach sometimes suffers from long induction times, broad particle size distributions, asymmetrical crystal morphology and product purity [2]. Ultrasound has been shown to facilitate the nucleation of crystals, reduce the induction time (t ind ) in a controlled and reproducible way to generate narrower size distribution products, and selectively crystallise a specific polymorph [3][4][5][6][7][8][9][10][11]. However, the mechanism behind ultrasound-induced nucleation is still contentious [12][13][14] as studies have shown that ultrasound increases induction time [15] and promotes crystal growth [16].…”

Ultrasound Assisted Particle Size Control by Continuous Seed Generation and Batch Growth

“…As a result, these clusters become supersaturated near the bubble wall for a very short time (ca. 1 ns) and their direct collisions are promoted and nucleation is enhanced [17,43]. In addition to these hypotheses which focus on primary nucleation, secondary nucleation was also observed in the literature upon sonication [44].…”

Ultrasound‐assisted emerging technologies for chemical processes

“…In addition, the mechanism behind ultrasound‐assisted nucleation is not known today. Several hypotheses exist to explain the effect of ultrasound on nucleation . These hypotheses relate the effects of ultrasound to the size of the bubbles, the speed of the bubble implosions or the flows introduced by the cavitation bubbles.…”

“…Several hypotheses exist to explain the effect of ultrasound on nucleation. [41][42][43] These hypotheses relate the effects of ultrasound to the size of the bubbles, the speed of the bubble implosions or the flows introduced by the cavitation bubbles. All these effects are optimal at low ultrasonic frequencies between 20 and 100 kHz, but the exact optimal ultrasound parameters can differ between the different effects.…”