Oil Lenses on Water and the Nature of Monomolecular Expanded Films

Abstract

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 = —2F s ρ 1 /gρ 2 (ρ 1 — ρ 2 ). Equations are given by which F S 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 F S = —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 F 12 which causes a decrease in — F S 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 is equal to F S . If the adsorbed molecules at the interface give a force F 12 in accord with the gas laws, then (F — F 0)(a — a 0) = kT gives the equation of state of the duplex film, where F 0 is the value of F S in the absence of hydrophilic substances. It is shown that this equation applies to expanded oil films on water and that these are properly regarded as duplex films although they are monomolecular. The presence of a kink in the observed F — a curves is attributed to the sudden appearance of micelles, each containing β‐molecules when F 12 rises to a critical value. For the fatty acids on dilute HCl, β = 13. This small value is due to the crowding apart of the heads because of hydration. The esters show little or no hydration and β = 60. The heat of formation of the micelles in the fatty acids is found to be 5.5+0.24n kg cal. per mole where n is the number of carbon atoms. The values of F 0 and a 0 for various substances are determined from data given by Adam.