Long-chain alkanes do not spread on water, but form a lens in equilibrium with a dilute 2-D gas. Adding surfactant to the water causes a first-order wetting transition to a state known as ‘pseudo-partial wetting’ in which a lens is in equilibrium with a mixed monolayer of surfactant and oil. The composition of the monolayer and the thermodynamics of the phase transition have been determined by a combination of tensiometry and ellipsometry. A lattice model gives quantitative agreement with experiment. This work was carried out in collaboration with Prof. Aratono at Kyushu University.
Cooling of a mixed monolayer of alkane and surfactant leads to another first-order phase transition from a liquid to a solid monolayer. This freezing transition can occur at temperatures up to 30°C above the bulk melting point of the alkane. Sum-frequency spectroscopy and ellipsometry have been used to characterise both the liquid and solid phases.[4,5] These measurements are consistent with an upright rotator II phase, similar to that formed by monolayers of alcohols on water. X-ray reflectometry and grazing incidence X-ray diffraction experiments are planned to characterise the electron density profile and translational order in the solid phase.
Almost all known materials show surface melting, in which the surface melts at a lower temperature than the bulk. Alkanes are almost unique in showing surface freezing. No surface freezing has been observed at the interface between pure alkanes and water. Surfactants can, however, be used to induce surface freezing at the oil-water interface. The mole fraction of the surfactant in the frozen monolayer can be as low as 0.1. One consequence of surface freezing is the vanishing of the interfacial tension above the bulk freezing point of the oil, which leads to bizarre and wonderful behaviour. Ellipsometry and interfacial tensiometry are the principal techniques used to characterise the interface, though neutron and X-ray scattering experiments are planned. (In collaboration with M. Deutsch at Bar-Ilan University.)
1. H. Matsubara, N. Ikeda, T. Takiue, M. Aratono and C. D. Bain "Interfacial Films and Wetting Behavior of Hexadecane on Aqueous Solutions of Dodecyltrimethylammonium Bromide" Langmuir 2003, 19, 2249–2253 (DOI).
2. K. M. Wilkinson, C. D. Bain, H. Matsubara and M. Aratono "Wetting of Surfactant Solutions by Alkanes" ChemPhysChem 2005, 6, 547–555 (DOI).
3. H. Matsubara, M. Aratono, K. M. Wilkinson and C. D. Bain "Lattice Model for the Wetting Transition of Alkanes on Surfactant Solutions" Langmuir, in press.
4. C. E. McKenna, M. M. Knock and C. D. Bain "First-Order Phase Transition in Mixed Monolayers of Hexadecyltrimethylammonium Bromide and Tetradecane at the Air–Water Interface" Langmuir 2000, 16, 5853–5855 (DOI).
5. K. M. Wilkinson, Q. Lei and C. D. Bain "Freezing Transitions in Mixed Surfactant/Alkane Monolayers at the Air–Solution Interface" Soft Matter 2006, 2, 66–76 (DOI).
6. Q. Lei and C. D. Bain "Surfactant-Induced Surface Freezing at the Alkane–Water Interface" Physical Review Letters 2004, 92, 176103/1–4 (DOI).
7. E. Sloutskin, C. D. Bain, B. M. Ocko and M. Deutsch "Surface Freezing of Chain Molecules at the Liquid–Liquid and Liquid–Air Interfaces" Faraday Discussions 2005, 129, 339–352 (DOI).