Volumetric and thermochemical properties of ionic surfactants: sodium decanoate and octylamine hydrobromide in water

The heat capacities per unit volume and the densities of sodium decanoate (NaDec) in water were measured as a function of concentration and temperature in the range 0.5 to 45 °C. Similar data were obtained for octylamine hydrobromide at 5 and 15 °C. Enthalpies of dilution of NaDec in water were measured at 25 °C. From these and previously published data the apparent and partial molal heat capacities, expansibilities, volumes, and relative enthalpies were determined as a function of temperature and concentration. The concentration dependence of [Formula: see text] and [Formula: see text], is similar to that of other surfactants such as alkyltrimethylammonium bromides and sodium dodecyl sulfate, but in contrast with the other surfactants, [Formula: see text] goes through a sharp maximum in the micellization region at the lower temperatures. These thermodynamic properties can be analysed with a phase-separation model to give the changes in volume, heat capacity, and enthalpy during micellization as a function of temperature. The limitations of this model are also discussed.