Oil Lenses on Water and the Nature of Monomolecular Expanded Films

A higher hydrocarbon, such as tetradecane, on the surface of water forms a circular lens. The condition for the formation of such stable lenses is that the spreading coefficient FS = γ1 — γ2 — γ12 shall be negative. As the volume of the oil lens is increased, the thickness t at the center slowly approaches a limiting value given by t∞2 = —2Fsρ1/gρ2(ρ1 — ρ2). Equations are given by which FS can be accurately determined from measurements of the radii of large lenses of known volume. The magnitude of the linear tension at the lens boundary is also calculated. Experiments with tetradecane on water at 25°C give FS = —6.2 dynes cm—1 and γ = 26.9. When hydrophilic molecules are introduced into the interface between the lens and the water, they give a spreading force F12 which causes a decrease in — FS and in t. When t<0.1 mm gravitational effects are negligible and the lens degenerates into a duplex film. If such a film is confined by a movable barrier (piston), the force per unit length F exerted on the barrier...