Effect of the surface expansion and wettability of the capillary on the dynamic surface tension measured by the maximum bubble pressure method

The dynamic surface tension (DST) of sodium dodecyl sulfate solutions in the presence of sodium chloride is studied by the maximum bubble pressure method. The pressure oscillations are measured with a pressure transducer, while the change of the bubble area with time is determined by means of a video system. The role of the wettability of the capillary is studied by means of measurements with hydrophilic and hydrophobic capillaries. A strong effect of wettability of the capillary on the bubble growth and the DST is observed. The DST data are interpreted with a model for diffusion-controlled adsorption assuming different laws of bubble expansion. The real law of expansion is found to be important for correct interpretation in the case of the hydrophobic capillary. However, the surface expansion is not of primary importance for interpretation of the DST data obtained with the hydrophilic capillary. It is proved that the maximum pressure does not correspond to the hemispherical shape of the bubble in the presence of surfactant. Neglecting this effect does not lead to a significant error in the DST for bubbling periods smaller than several seconds.