Abstract
The application of pulsed ultrasound for the degradation of the nonvolatile surfactants sodium 4-octylbenzene sulfonate (OBS) and sodium dodecylbenzenesulfonate (DBS) was investigated at a frequency of 354 kHz. By comparing the degradation rate constants with those of continuous wave (CW) ultrasound, observed pulse enhancements were found to be dependent on the pulse length, pulse ratio, initial concentration, and surface activity of the surfactants. For a pulse length of 100 ms and a pulse ratio of 1:1 (equal on/off times), the degradation rate constant of 1 mM OBS was nearly twice the value for CW. Furthermore, the degradation rate constant for 1 mM DBS increased significantly when sonicated under a pulse length of 100 ms and a pulse on/off ratio of 1:50. However, the degradation rate of 0.1 mM OBS increased by only 30% with a 100 ms pulse length and pulse ratio of 1:1 as compared to CW, indicating concentration dependence. The enhanced degradation of surfactants by pulsed ultrasound was attributed to the accumulation of surfactants on cavitation bubble surfaces. In addition, as compared to shorter pulse intervals, longer pulse intervals enhanced DBS degradation, indicating that DBS, a more surface active compound, accumulated and equilibrated with the bubble interface more slowly.
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