POSITRONIUM FORMATION IN AQUEOUS MICELLAR SOLUTIONS OF SODIUM DODECYLSULFATE

Positron annihilation lifetime measurements on aqueous solutions of sodium dodecylsulfate (NaLS) were performed over a wide range of temperatures and surfactant concentrations. The sensitivity of the positronium formation process toward changes in physical properties accompanying micellization is demonstrated. The critical micelle concentration (CMC) is associated with a sharp decrease in positronium formation. The CMC of 8.4±0.5 mM at 20°C in this system is in agreement with literature values. At higher temperatures the CMC is shifted to slightly higher surfactant concentrations (10.4±0.7 mM at 90°C). The formation probability for thermal positronium as expressed by I2, the intensity of the long-lived component in the lifetime spectra, gradually decreases above the CMC with increasing surfactant concentration reaching a limiting value. A model is suggested involving the reactions of hot" positrons or positronium atoms with micellar aggregates leading to the observed reduction of thermalized positronium atoms. © 1979 American Institute of Physics."