F-19 NMR study of molecular aggregation of lithium perfluorooctylsulfonatein water at temperatures from 30 to 250 degrees C

F-19 NMR chemical shifts have been measured to determine the critical micel le concentration (cmc) and aggregation number (m) of lithium perfluorooctyl sulfonate (LiFOS) in water at temperatures ranging from 30 to 250 degrees C under the saturation pressure. The cmc increases moderately in the tempera ture range from 30 to 100 degrees C, whereas above similar to 150 degrees C the increase is steep; cmc is 6.5, 13.2, and 161 mmol kg(-1) at 30, 100, a nd 250 degrees C, respectively. The aggregation number, estimated on the ba sis of a mass action model, exhibits a rapid decrease in the temperature ra nge of 30-150 degrees C and becomes almost constant at higher temperatures; m is 36, 6.8, and 2.4 at 30, 100, and 250 degrees C, respectively. Thermod ynamic parameters obtained indicate that the micellization is entropy drive n at ambient temperatures and energy driven at high temperatures above 150 degrees C.