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Multiple Surfactant Lamellar (Lα) Phases Might be Commonplace – Attractive?
Prof. Gordon TIDDY
School of Chemical Engineering and Analytical Sciences, University of Manchester
Wed, Jan. 23rd 2008, 11:00
NIMBE Bât.125, p.157, CEA-Saclay
The lamellar phase (Lα) is the most common surfactant liquid crystal, occurring widely when biological surfactants (lipids) or commercial materials used in cleaning products are dispersed in water. Any surfactant (minimum requirement is non-polar chain attached to polar head group) can be used to form a lamellar phase, provided that the non-polar group is designed to give disc micelles from “packing constraints” theory. Uncharged surfactants usually form lamellar phases that can swell to a certain extent on addition of water, but above a certain concentration (typically 40-60 wt% water) additional water forms a separate phase. The forces responsible for this behaviour are thought to be a short-range steric/“hydration” repulsion, balanced by a medium range attraction. The origin of the latter is still a matter of debate. With ionic surfactants the strong osmotic repulsion due to univalent counterions gives rise to a long range force that allows the incorporation of large amounts of water (>95%). But, with some ionic surfactants (pure, single components) at high concentrations (30-60wt% surfactant) there is a coexistence of two lamellar phases with different water concentrations. Frequently such systems exhibit an upper consolute loop, where above a critical temperature, only a single, continuously-swelling phase occurs. Both anionic and cationic surfactants show this behaviour. We have examined several examples where this phenomenon occurs, using microscopy, NMR, X-rays and other measurements to determine both the structural differences between the coexisting phases (if any!) and the molecular interactions within the phases. We also have measurements on complex multi-component mixtures where multiple lamellar phases (five or more) coexist. A molecular mechanism for the behaviour will be discussed. The same mechanism is likely to occur in biological systems such as membrane lipids, hence it could provide an explanation for the occurrence of “lipid patches”.


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