Surface tension measurements with the pulsating bubble method

The bubble method for determining equilibrium and dynamic surface tensions from the measured pressure difference across the air/water interface is described. Data for either constant surface area or pulsating area conditions are presented for three sets of systems. For dipalmitoylphosphatidylcholine (DPPC)-hexadecanol dispersions in saline at 37°C, the dynamic tension depends strongly on the protocol of dispersion preparation. The two surfactants show strong synergistic effects in lowering tension. For aqueous salts of fatty acid soaps (carbon number n = 8-18) at 37°C in saline/buffer, the surface activity increases (tension is lower and equilibration time is shorter) up to n = 12, and then decreases for n ≤ 14, as these soaps are less soluble. For aqueous sodium dodecylsulfate (SDS) and octanol in water at 25°C, the dynamic tension depends on the total concentration, on the molar ratio, as expected, and on the pulsation period relative to the timescale for tension equilibration. The results are relevant in the formulation of surfactant mixtures for cleaning products, foaming, and lung surfactant replacement drugs.