Effects of pulsed ultrasound on the adsorption of n-alkyl anionic surfactants at the gas/solution interface of cavitation bubbles.

Sonolysis of argon-saturated aqueous solutions of the nonvolatile surfactants sodium dodecyl sulfate (SDS) and sodium 1-pentanesulfonate (SPSo) was investigated at three ultrasonic frequencies under both continuous wave (CW) and pulsed ultrasound. Secondary carbon-centered radicals were detected by spin trapping using 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and electron paramagnetic resonance (EPR) spectroscopy. Following sonolysis, -*CH- radicals were observed for both surfactants under both sonication modes. Under CW at 354 kHz, the maximum plateau -*CH- radical yield was higher for SPSo than for SDS, indicating that SDS, which is more surface active under equilibrium conditions, accumulates at the gas/solution interface of cavitation bubbles to a lesser degree, compared with the less surface active surfactant, SPSo. However, after sonolysis (354 kHz) under pulsed ultrasound with a pulse length of 100 ms and an interval of 500 ms, the -*CH- radical yield at the plateau concentrations was higher for SDS than for SPSo due to increased amounts of SDS accumulation on the bubble surfaces. In contrast to the findings following sonolysis at 354 kHz, sonolysis of aqueous surfactant solutions at 620 kHz and 803 kHz showed a higher -*CH- radical yield for SDS compared with SPSo under CW but lower -*CH- radical yield with increasing pulsing interval, indicating a frequency dependence on accumulation. Results indicate that pulsing the ultrasonic wave has a significant effect on the relative adsorption ability of n-alkyl surfactants at the gas/solution surface of cavitation bubbles.