Film tension and film rupture of alkanes at the air-water interface using axisymmetric drop shape analysis

In connection with interfacial tension measurements for n-hexadecane-water under conditions of mutual saturation, we accidentally found an extremely high tension value for the aqueous phase of approximately 80 mJ/m(2), using axisymmetric drop shape analysis-profile (ADSA-P) for an air bubble formed in the water phase. This high tension value is that of an alkane film at the air-water interface. From static measurements (the drop surface area kept constant) at 25 +/- 0.2 degrees C, we observed this film tension value to be 80.67 +/- 0.06 and 77.20 +/- 0.02 mJ/m(2) at the 95% confidence level for hexadecane and dodecane films at the air-water interface, respectively. These film tension values were also confirmed in dynamic measurements (the drop surface area continuously increased) for both alkane systems. In addition, film rupture has been demonstrated both in the sudden change in the bubble profile and in the abrupt decrease in the tension value. The alkane film tension measured is approximately equal to the sum of the interfacial tensions of air-alkane and alkane-water. The method presented provides a new way to measure film tension; it utilizes axisymmetric drops or bubbles. This is less restrictive than other approaches that require the drop or bubble to be spherical. The measurements for film tension can be carried out under both static and dynamic conditions, and information about the surface area, drop, or bubble volume as well as the radius of curvature can all be obtained simultaneously.