19F NMR Study of Molecular Aggregation of Lithium Perfluorooctylsulfonate in Water at Temperatures from 30 to 250 °C

Abstract

19F NMR chemical shifts have been measured to determine the critical micelle concentration (cmc) and aggregation number (m) of lithium perfluorooctylsulfonate (LiFOS) in water at temperatures ranging from 30 to 250 °C under the saturation pressure. The cmc increases moderately in the temperature range from 30 to 100 °C, whereas above ∼150 °C the increase is steep; cmc is 6.5, 13.2, and 161 mmol kg-1 at 30, 100, and 250 °C, respectively. The aggregation number, estimated on the basis of a mass action model, exhibits a rapid decrease in the temperature range of 30−150 °C and becomes almost constant at higher temperatures; m is 36, 6.8, and 2.4 at 30, 100, and 250 °C, respectively. Thermodynamic parameters obtained indicate that the micellization is entropy driven at ambient temperatures and energy driven at high temperatures above 150 °C.