The rate enhancing effect of ultrasound by inducing supersaturation in a solid-liquid system

Ultrasound has been found to induce the supersaturation of a sparingly soluble solid in a liquid system. Such supersaturation was observed with two different types of solid-liquid systems: sodium sulfide in acetonitrile and calcium citrate in water, with the former being investigated in greater detail. In the presence of ultrasound, the concentrations of these solutes in their respective solvents were increased at least 1.4 times the corresponding equilibrium saturation values. One possible mechanism may be an increased solubility of the solute in the hot-spot region where the solvent exists in a supercritical state. This increased solubility is retained long after the hot spot dissipates and the region returns to ambient conditions. Another mechanism may be the Gibbs-Thompson effect, i.e., the enhanced solubility of very small (< 1.0 mu m) particles due to pressure differences between the curved interface of a particle and the bulk solution. Here, again, the enhanced solubility of each system is retained after the sonication is discontinued. The resulting enhancement of the overall rate of reaction is a distinct supplement to the well-known contributions of the rate constant and interfacial area to the enhancement. (C) 2000 Elsevier Science Ltd. All rights reserved.