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Vesicles in Ionic Liquids

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journal contribution
posted on 16.08.2011 by Florence Gayet, Jean-Daniel Marty, Annie Brûlet, Nancy Lauth-de Viguerie
The formation of vesicles from 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) in several room-temperature ionic liquids, namely, 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2), and N-benzylpyridinium bis(trifluoromethylsulfonyl)imide (BnPyNTf2), as well as in a water/BmimBF4 mixture, was investigated. In pure ionic liquids, observations by staining transmission electron microscopy demonstrated clearly the formation of spherical structures with diameters of 200–400 nm. The morphological characteristics of these vesicles in ionic liquids, in particular, the membrane thicknesses, were first investigated by small-angle neutron scattering measurements. The mean bilayer thickness was found to be ∼63 ± 1 Å in a deuterated ionic liquid (BnPyNTf2-d). This value was similar to that observed in water. The effect of ILs on the modification of the phase physical properties of multilamellar vesicles (MLVs) was then investigated by differential scanning calorimetry. In pure IL as in water, DPPC exhibited an endothermic pretransition followed by the main transition. These transition temperatures and the associated enthalpies in ILs were higher than those in water because of a reduction of the electrostatic repulsion between zwitterionic head groups. To better understand the effect of ionic liquid on the formation of multilamellar vesicles, mixtures of BmimBF4 and water, which are miscible in all proportions, were analyzed (BmimBF4/water ratio from 0% to 100%). SANS and DSC experiments demonstrated that the bilayer structure and stability were strongly modified by the IL content. Moreover, matching SANS experiments showed that BmimBF4 molecules prefer to be located inside the DPPC membrane rather than in water.