A study on the dynamic surface tension of surfactant solutions at dilute concentrations

The dynamic surface tension (ST) for Aerosol-OT (AOT) molecules adsorbing onto a freshly created bubble surface in a 10 mM NaCl_(aq) solution was measured at extremely dilute concentrations using a pendant bubble tensiometer. An air bubble was formed one after another inside the same uniform solution at two different states of the free surface on the top of the solution in quartz cell: (i, bubble #1) a nearly clean air-water interface and (ii, bubble #2) a thermodynamic equilibrated air-water interface. Whilst the dynamic ST of bubble #1 decreased noticeably faster than that of bubble #2, both these bubbles reached the same equilibrium ST. AOT molecules kept adsorbing onto the free surface during the ST measurement of bubble #1, which caused the concentration of AOT in bulk to decrease significantly at dilute concentrations. During the measurement of bubble #2, the bulk concentration remained constant. Subsequently, the dynamic ST data of bubble #2 precisely described the adsorption of AOT molecules onto a freshly created air-water interface. Consequently, the adsorption onto the free surface played an important role in the dynamic ST measurement for surfactant solutions at extremely dilute concentrations. This new experimental procedure was capable of precisely measuring the dynamic ST of surfactant solution at extremely low concentrations.