Dynamic surface tension measurement of dodecylamine solutions—a pendant bubble tensiometer with modified cell system

A pendant bubble tensiometer was modified for the dynamic surface tension (ST) measurement of a dodecylamine (DDA) surfactant solution. The dynamic STs of DDA solutions showed a peculiar relaxation: the ST relaxed from the ST of the solvent, reached a minimum, and then increased to ∼72 mN/m. The pH values of aqueous DDA solutions indicated that the DDA was partially ionized, with an ionization degree between 2 and 7%. The partially protonated DDA molecules probably prefer to adsorbed on the quartz surface of the sample cell and the metal needle surface, thereby depleting the bulk concentration and causing the desorption of the previously adsorbed DDA molecules at the air–solution interface. A coated quartz cell and a continuous pumping system were implemented in the pendant bubble tensiometer. The dynamic STs of DDA in the 0, 50, and 500 mM NaCl solutions smoothly relaxed from the ST of the solvent, reached the equilibrium ST (γ_e) and remained at γ_e for a long time without any increase. This pendant bubble tensiometer with modified cell system is capable of measuring the dynamic ST of the partially ionized dodecylamine surfactant solution.