Sonoluminescence from aqueous alcohol and surfactant solutions

The sonoluminescence generated in water with pulsed 515 kHz ultrasound has been studied in the presence of different chain length (C-1-C-5) aliphatic alcohols and the surfactants sodium dodecyl sulfate (SDS), dodecyltrimethylammonium chloride (DTAC), and N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DAPS). The ultrasound pulse widths used ranged from 1 to 10 ms, with duty cycles (on/off ratios) of 1:3 to 1:9. It was found that the sonoluminescence from the initial pulses was very low but increased in intensity and reached a maximum after 20-50 pulses, for all systems studied, depending on the purse width and duty cycle. In the presence of alcohol the maximum signal decreased with increasing alcohol concentration, and the signal decline was more pronounced with increasing chain length of the alcohol. A good correlation was found to exist between the decline in the sonoluminescence signal and the Gibbs surface excess of the alcohol at the air/water interface. In the presence of SDS (an anionic surfactant) and DTAC (a cationic surfactant), quite different behavior was observed. At low concentrations of these two surfactants the maximum signal was significantly enhanced over that obtained in pure water, reaching a maximum at about 1 mM of surfactant. At higher concentrations the signal decreased again reaching a limiting value similar to that obtained in pure water. The sonoluminescence signal in DAPS (a zwitterionic surfactant) solutions remained much the same as in pure water. On the addition of 0.1 M NaCl to the three different types of surfactin solutions, the intensities of the emission signals obtained were essentially the same as in pure water. Possible mechanisms responsible for the different behavior in the sonoluminescence signal in the presence of the alcohols and surfactants are discussed.