$^{19}$F NMR Study of Molecular Aggregation of Lithium Perfluorooctylsulfonate in Water at Temperatures from 30 to 250 \r{ }C

$^{19}$F 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 \r{ }C under the saturation pressure. The cmc slightly increases in the temperature range from 30 to 100 \r{ }C, whereas above 125 \r{ }C the increase is much steeper; cmc is 6.5, 13.2, and 161 mmol kg$^{-1}$ at 30, 100, and 250 \r{ }C, respectively. The aggregation number, estimated on the basis of a mass action model exhibits a rapid decrease in the temperature range of 30 - 125 \r{ }C and becomes almost constant at higher temperatures; $m$ is 36, 6.8, and 2.4 at 30, 100, and 250 \r{ }C, respectively. Thermodynamic parameters indicate similar values for the free energy at all temperatures, and constant values for the enthalpy and entropy of aggregation at high temperatures.