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When placed on the surface of water, alkanes longer than 6 carbons collect in lens-like droplets, and do not spread. However, mM bulk concentrations of a surfactant drive the alkane into a pseudo-partial wetting regime, where a 2D alkane monolayer coexists on the surface with the 3D alkane droplets. This Langmuir-Gibbs film, which consists of a mixture of alkanes and surfactant tails, is stabilized by the balance between a positive spreading pressure and the disjoining pressure across the monolayer. We have studied the phase diagram and film structure of these Langmuir-Gibbs films for a range of temperatures (T) and surfactant tail (m), and alkane (n), lengths using surface tensiometry and surface-specific x-ray measurements (1). A first-order surface freezing transition was found leading to a crystalline monolayer of hexagonally-packed surface-normal molecules (for m > n+1) or a bilayer (for m > n+1). A related effect was found at the bulk alkane/solution interface, and studied by the same metrhods. Here, however, the interface-frozen monolayer is formed for m > n-1. The surface-tension-derived entropy loss, and the x-ray reflectivity measurements of the frozen monolayer indicate a uniform, dense monolayer, corresponding well to a close-packed, ordered, solid monolayer of surface-normal molecules (2).
The (T,n,m) phase diagram of the layer at both interfaces is well described by a simple mixtures-based thermodynamic theory, where the n/m length mismatch exchange interaction dominates the transition.
Finally, at surfactant concentrations close to the cmc a novel , T-tunable, reversible, spontaneous emulsification effect is observed by light microscopy. Here the interfacial freezing transition causes the alkane/solution tension to vanish at low enough T. Since there is no free energy cost for interface formation, emulsion droplets are formed spontaneously. Moreover, we also observe spontaneous droplet fragmentation, non-spherical droplet formation, and the appearance of dynamically diminishing "pearl-necklace" chains.
(1) "Wetting, mixing and phase transitions in Langmuir-Gibbs films", E. Sloutskin et al., Phys. Rev. Lett. 99, 0136102 (2007) (2) "Surfactant induced interfacial freezing", L. Tamam et al., (submitted). |
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