TEMPERATURE EFFECT ON THE SONOLYSIS OF METHANOL-WATER MIXTURES

Methanol/water mixtures with various concentrations of methanol (from pure water to pure methanol) were exposed to ultrasonic irradiation (724 kHz, 50 W) under an argon atmosphere at various bulk temperatures lying in a range from -30 to +58 degrees C. It was shown that changing the bulk temperature dramatically influenced the rate of the sonolytic formation of molecular hydrogen. With increasing temperature, the rate initially grows, reaches a maximum, then falls. Increasing the concentration of methanol in the mixture decreases the temperature at which the maximal H-2 formation rate is reached. The observed effect is explained in terms of two opposing factors, and relative contributions of those factors change with the alteration of the bulk temperature: it is suggested that saturating the cavitational bubbles with methanol vapors as the temperature of the mixture being sonicated increases (1) on one hand, accelerates the methanol vapor thermolysis in the cavitational hot spots during the collapse of the bubbles because methanol is a reactant, but (2) on the other hand, decreases the local temperature of the cavitational hot spots that, in turn, disfavor the thermolysis. The effect of the bulk solution temperature on a constant methanol concentration is compared with that of the concentration of methanol in the mixture when a constant bulk solution temperature is kept.