Spreading of aqueous trisiloxane surfactant solutions over liquid hydrophobic substrates

Systematic studies of trisiloxane surfactant aqueous solutions spreading over hydrophobic liquids are performed by videomicroscopy. Three trisiloxane nonionic surfactants M(D'E-n)M with different ethoxy-chain lengths are investigated. In addition to surfactant concentration, the spreading rate strongly depends on surfactant solubility in the organic liquid substrate, even when the initial spreading coefficient is essentially constant. In contrast to previously studied DDAB solutions' spreading (Svitova, T.; Hill, R.; Radke, C. Langmuir 1999, 15 (21), 7392), two different spreading regimes are found. At relatively low surfactant concentrations, the spreading rate is limited by surfactant mass transport toward stretching interfaces, as in the case of DDAB solutions. At concentrations greater than 0.5 wt %, however, the maximum rate increases to a level for which viscous dissipation by the substrate starts to play an important role, along with surfactant mass transport limitations. For all liquid substrates studied, the spreading lenses of the trisiloxane surfactant solutions retract after a maximum area is reached because of surfactant dissolution into the oil phase. Intensive surfactant dissolution into the substrate leads to fast lens retraction, and it overrides the gain due to less viscous drag when the substrate liquid is of lower viscosity. The presence of surface concentration gradients and the resulting Marangoni flow is established by monitoring the movement of Teflon particles at the upper surface of the spreading lens.