Diffusion Dynamics in Uniformly Thick Surfactant Films Formed on Hydrocarbon Liquids

Abstract

Diffusion through free-standing, micelle-laden, surfactant films is not well understood due to rapidly decreasing and nonuniform film thickness. Films formed on hydrocarbon liquids are flat, with slowly decreasing thickness. However, the film’s stability and vapor diffusivity have not been fully quantified and understood. We applied a fluorosurfactant solution gently on a hydrocarbon pool so that it fully covers the pool surface. The film is inherently unstable as it floats on a lower-density liquid. We have quantified the hydrocarbon diffusion rate, diffusivity, and film lifetime up to 3 h. The new measurements show that the film suppresses the diffusion rate significantly, but not completely, and quickly as soon as the film fully covers the pool. Thus, a quasi-steady-state diffusion is established quickly. After that, suppression decreases with time very slowly and eventually reduces to zero as the film is no longer an effective diffusion barrier. During this period, bubbles form, grow, and rupture on the film surface depending on the hydrocarbon volatility. The film becomes ineffective because its thickness decreases slowly due to slow evaporation of water and slow emulsification with the hydrocarbon. We also show that lifetime and suppression increase linearly with the spreading coefficient but not with decreasing vapor pressure of the hydrocarbons. As the spreading coefficient increases, it can spread a larger amount of solution quickly to form a thicker film that lasts for a longer time. We also show that an increase in surfactant concentration increases suppression and the film’s lifetime, while increased pool temperature decreases both. The film becomes ineffective as a result of a diffusion barrier at pool temperatures encountered under fire conditions.