Impact of Phase Separation on Marangoni Synergism in Binary Surfactant Mixtures

Linda T. Liu^1,2, Stephen Garoff³, Robert D. Tilton^1,2, Department of Chemical Engineering¹, Department of Biomedical Engineering², Department of Physics³

Carnegie Mellon University, Pittsburgh, PA 15213

Surfactants are molecules that reduce the surface tension of a liquid by adsorbing to the liquid surface. A non-uniform distribution of surfactants on a liquid surface causes a surface tension gradient that drives Marangoni spreading from low surface tension to high surface tension regions. This flow plays an important role in a variety of surfactant applications such as pulmonary aerosol drug delivery, coatings, and agrochemical sprays. Many of these products are formulated with mixtures of surfactants to take advantage of synergistic surface tension reduction effects. A spreading phenomenon recently discovered in cationic and anionic surfactant mixtures is Marangoni synergism. This occurs when a binary surfactant mixture prompts a greater Marangoni spreading velocity than the individual surfactants. It remains to be determined how widespread Marangoni synergism may be.

Many mixtures of cationic and anionic exhibit phase separation. This could impact Marangoni spreading. This project will focus on Marangoni spreading in binary surfactant mixtures that exhibit phase separation. Spreading experiments were performed with binary surfactant samples at various ratios and total concentrations, where each mixture exhibited a different extent of phase separation. The spreading velocities were tracked for each sample and allowed for evidence about the role of phase separation in Marangoni spreading. This work showed that systems that exhibited the most extensive phase separation were not synergistic, but in fact antagonistic. The Marangoni spreading velocity was lower than either of the individual surfactants by themselves. This can be attributed to the inadequate mass transfer of surfactants from the aggregate phase to the air/water interface to maintain the necessary surface tension gradient. This work may improve the design of complex fluid products that rely on spreading at fluid interfaces.