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 solu ble solid in a liquid system. Such supersaturation was observed with two di fferent types of solid-liquid systems: sodium sulfide in acetonitrile and c alcium citrate in water, with the former being investigated in greater deta il. In the presence of ultrasound, the concentrations of these solutes in t heir respective solvents were increased at least 1.4 times the correspondin g equilibrium saturation values. One possible mechanism may be an increased solubility of the solute in the hot-spot region where the solvent exists i n a supercritical state. This increased solubility is retained long after t he hot spot dissipates and the region returns to ambient conditions. Anothe r 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 t he curved interface of a particle and the bulk solution. Here, again, the e nhanced solubility of each system is retained after the sonication is disco ntinued. The resulting enhancement of the overall rate of reaction is a dis tinct supplement to the well-known contributions of the rate constant and i nterfacial area to the enhancement. (C) 2000 Elsevier Science Ltd. All righ ts reserved.