By: Eduardo J. M. Filipe
From: Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa
At: Instituto de Investigação Interdisciplinar, B2-01
(Note that exceptionally, this seminar will take place in room B2-01)
Mixtures of alkanes and perfluoroalkanes are highly non-ideal systems in spite of the apparent similarity between these substances. They exhibit large regions of liquidâ€“liquid immiscibility, large positive deviations from Raoultâ€™s law and large positive excess properties a clear indication of weak unlike interactions. In recent years, highly fluorinated compounds have become key fluids in a wide range of fields due to their chemical inertness, biocompatibility andpeculiar physical properties. These range from medical applications, as oxygen carriers in blood substitute formulationsor as fluids in eye surgery, to technological applications as solvents for biphasic synthesis, fire-extinguishers or lubricants.
Hydrogenated and fluorinated chains display very different thermophysical and mechanical properties. Perfluorinated chains (-CF2CF2CF2-) are not only highly hydrophobic, but also lyophobic, i.e. they segregate hydrogenated chains. Taking advantage of the natural "antipathy" between hydrogenated and perfluorinated chains, molecular films are produced using mixtures of hydrogenated and fluorinated long-chain alcohols (n= 12 to 18) which are structured at the nanoscale. The organization of the film is the result of a delicate balance of interactions and depends on factors such as the length and structure of hydrogenated and fluorinated chains, the presence of hydrophilic groups and the experimental conditions of surface pressure and temperature.
Additionally, the interfacial and bulk properties of mixtures of hydrogenated and fluorinated short-chain alcohols (n=2 to 6) is also studied: surface tensions as a function of composition and densities as a function of composition, temperature and pressure have been experimentally measured, and molecular dynamics simulations have been performed in order to gain a better understanding of the intermolecular interactions in these systems. The peculiar behaviour of these mixtures seems to originate in the balance between the unfavourable unlike dispersion interactions and the favourable unlike hydrogen bonding. The understanding of these phenomena at the molecular level will also ultimately allow the prediction and fine-tuning of the molecular films of the longer alcohols, both in terms of structure as in terms of surface properties.