Equilibrium and dynamic surface tension behavior of aqueous soaps: Sodium octanoate and sodium dodecanoate (sodium laurate)

The dynamic surface tension behavior of sodium salts of fatty acids having eight (sodium octanoate) and twelve (sodium laurate) carbon atoms in pure water or in 0.1 N aqueous NaOH has been studied. The primary reason for this study was to obtain fundamental knowledge of the tension behavior of these molecules which are present in several consumer products. Tension data were obtained at a concentration of 30 mM for octanoate and 0.5-10 mM for dodecanoate at 25 and 37 degrees C with the pulsating bubble surfactometer at constant or pulsating areas. In addition, the spinning bubble tensiometer and the Wilhelmy plate/Langmuir trough methods were used for better establishing reliable values of equilibrium tensions. An oscillating jet apparatus was used for obtaining estimates of tensions at small time-scales (3-10 ms). From these measurements, the most important factors affecting the dynamic tension behavior were the soap solubility, chain length, the aqueous medium, and the concentration. The equilibrium tension of aqueous sodium octanoate was determined to be about 40 mN m(-1). Sodium octanoate was found to be very slow to equilibrate, with a time-scale of about 1000 s. Aqueous sodium laurate has a longer chain length and is much faster to adsorb and more surface-active. The equilibrium tension was 23 mN m(-1), and the time-scale of equilibration was very short. When tested in 0.1 N aqueous NaOH, sodium laurate showed much higher tensions. The tensions decreased with increasing concentration, as expected, and more strongly at the higher temperature.