Thermodynamics of micelle formation of alkali-metal perfluorononanates in water. Comparison with hydrocarbon analogues

Abstract

The enthalpy and heat capacity changes for micelle formation of lithium perfluorononanate in water have been determined from calorimetric measurements of differential enthalpies of dilution of concentrated surfactant solution. The final concentrations varied between 0.001 and 0.025 mol dm^–3 and measurements were made at four temperatures between 15 and 43.5 °C. The enthalpy of micelle formation of sodium perfluorononanate was determined at 30 °C from measurements of differential enthalpies of solution of the crystalline surfactant. The volume change accompanying micelle formation of sodium perfluorononanate at 30 °C was determined from density measurements. In evaluating the experimental results the concentrations of surfactant in monomer and micellar forms in solutions above the c.m.c. were computed using the thermodynamic model for the association of ionic amphiphiles proposed by Jönsson and Wennerström. At ambient temperatures the perfluorononanate salts give closely the same enthalpy and volume changes of micelle formation as the hydrocarbon analogues. However, the heat capacity change is larger, probably stemming from a larger partial molar heat capacity contribution of the perfluoroalkyl group in the aqueous, monomeric amphiphile. The larger size of the perfluoroalkyl group means that there will be a larger number of water molecules in the first hydration layer around a perfluoroalkyl group than around the analogous alkyl group. Thus the stronger hydrophobic character of perfluorocarbon surfactants compared to the hydrocarbon analogues may be ascribed to the difference in size between the two types of hydrophobic groups.