Effect of surfactants on single-bubble sonoluminescence

The effect of surfactants on single-bubble sonoluminescence (SBSL) is studied theoretically based on the hot-spot model that a SBSL bubble collapses quasiadiabatically and that the quasi-thermal radiation is the origin of the light emission. Stottlemyer and Apfel [J. Acoust. Soc. Am. 102, 1418 (1997)] reported that the surfactant called Triton X-100, which provides free interfacial motion, reduced the magnitude of the light pulse from the bubble. It is clarified by the present study that the effect of the surfactant is caused by the inhibition of condensation of water vapor at the bubble wall at the collapse, which results in lowering the achieved temperature inside a bubble due to the enhancement of the amount of vapor that undergoes endothermal chemical reactions. It is predicted, based on the hot-spot model, that the radiation is not thermalized inside a bubble in the case of SBSL in a solution of the surfactant in water and that the spectrum of SBSL may deviate from the blackbody spectrum and may have some characteristic lines such as the OH line (310 nm). It is suggested that surfactants can be used to enhance the chemical reactions of vapor in sonochemistry. It is also suggested that some of the surfactants are dissociated by the extremely high temperature at the bubble wall at the collapse.