Effects of initial concentration of LASs on the rates of sonochemical degradation and cavitation efficiency

The effect of initial concentration of linear alkylbenzene sulfonate (LAS: p-octylbenzene sulfonate (LAS C₈), p-nonylbenzene sulfonate (LAS C₉), p-dodecylbenzene sulfonate (LAS C₁₂)) on the rate of sonochemical degradation was investigated over a wide concentration range under Ar atmosphere by 200 kHz ultrasonic irradiation. The degradation rate of each LAS increased with increasing initial concentration of LAS and then started to decrease with the different behavior depending on the types of LASs. This result indicated that the cavitation efficiency was gradually changed by their concentrations and the optimum LAS concentrations for their effective degradation existed. The maximum degradation rates were observed at 250 μM of LAS C₁₂, 1250 μM of LAS C₉, and 2500 μM of LAS C₈, respectively. These optimum concentrations were found to be about four to six times smaller than these critical micelle concentrations (CMCs). It was also found that the maximum degradation rates at the optimum concentrations were observed to be almost linearly correlated with their CMCs. Based on the obtained results, it could be suggested that the micelle formation occurs in the interfacial region of cavitation bubbles during rectified diffusion and this phenomenon reduces the cavitation efficiency. In addition, from the results of the rate of the sonochemical degradation of LASs and the yield of hydrogen peroxide, the existence of thermal gradient in the interfacial region of cavitation bubbles was also confirmed.