Volumes and Heat Capacities of Sodium Perfluoroalkanoates in Water

The apparent molar volumes and heat capacities of sodium perfluoroalkanoates, NaCF3(CF2) x COO, for x equal to 0, 1, 2, 4, and 6, were measured in water at 25 °C, and also at 5 and 55 °C in the case of NaCF3(CF2)6COO, from densities and heat capacities per unit volume. The CF2 contribution to the standard partial molar volumes and heat capacities are (23.57 ± 0.05) cm3·mol-1 and (124 ± 1) J·K-1·mol-1, respectively. The data for NaCF3(CF2)4COO and NaCF3(CF2)6COO were analyzed with a mass-action model to obtain the thermodynamic properties of micellization of the surfactants. The volumes and heat capacities of micellization at 25 °C are respectively (8.8 and 11.2) cm3·mol-1 and (−443 and −587) J·K-1·mol-1. At 5 °C, NaCF3(CF2)6COO is too close to the Krafft temperature to obtain data for the surfactant in the micellized form. At 55 °C, the volume and heat capacity of micellization are significantly smaller in magnitude than those at 25 °C.