Impact of the spacer on the condensing effect of fluorinated chains investigated by grazing incidence X-ray diffraction on ultrathin Langmuir monolayers.

Langmuir monolayers of double perfluoroalkyl(alkyl) chain amphiphiles fitted with a monomorpholinophosphate polar head, [C(n)F(2n+1)(CH(2))(m)O](2)P(O)[N(CH(2)CH(2))(2)O] (di(FnHm)MP with n = 6, 8, or 9; and m = 1 or 2), were investigated by surface pressure (π)/molecular area (A(0)) compression isotherms for temperatures ranging from 15 to 50 °C, and by grazing-incidence X-ray diffraction (GIXD) at 25 °C. Ultrathin monolayers were obtained for these short surfactants. Though the hydrocarbon spacer is short, it has a remarkable impact on the monolayer's organization. At 25 °C, whereas di(F8H2)MP monolayer presents a liquid expanded (LE)/liquid condensed (LC) transition, simply replacing one CH(2) by a CF(2) in the latter compound's structure at constant chain length, i.e. shortening the spacer from 2 to 1 CH(2) (as in di(F9H1)MP), suppresses the LE phase. At 25°, GIXD established that for both di(F8H2)MP and di(F9H1)MP, the chains form an hexagonal lattice in the LC phase. The collective tilt of the two compounds is close to zero. The lattice of the dense phase can be compressed, as assessed by the continuous linear decrease of the d spacing with increasing pressure. This indicates that the azimuthal distribution of the molecular tilts is progressively reduced upon compression. The d value for di(F9H1)MP is significantly lower than that of di(F8H2)MP, providing evidence for strong condensing effect of the fluorinated chains. Molecular areas were determined directly from the compression curves and also from the X-ray data, the latter allowing reconstruction of the compression isotherms. The calculated lattice compressibilities are ~30% and 50% of the macroscopic compressibilities for di(F9H1)MP and di(F8H2)MP, respectively. Comparison with the experimentally determined isotherms shows that the monolayer of di(F9H1)MP is more stable than that of di(F8H2)MP. The enthalpies and entropies determined for di(F9H1)MP and di(F8H2)MP, derived from the Clausius-Clapeyron equation, confirm that the observed transitions are both of the LE/LC type, although the triple point temperatures are strikingly different (27 °C vs -18 °C); this large difference further illustrates the stabilizing effect of the fluorinated chains. Disorder is hindered by the fluorinated chains and facilitated by a hydrocarbon spacer when larger than 1 CH(2).

Measurement of elastic forces between iron colloidal particles in a nematic liquid crystal.

The forces that arise between two iron particles in a nematic liquid crystal with a strong homeotropic anchoring were studied. For the first time, the short range repulsive force resulting from the presence of a hedgehog defect between two particles was precisely determined thanks to application of a small magnetic field and observation of the equilibrium position resulting from the balance between the elastic and magnetic forces. Above a given threshold force, the particles stuck together whereas the hedgehog defect was expelled and transformed into a Saturn ring located between the particles. The attractive part of the interparticle force was determined with the same method on the entire range of separation distances; we found that the equilibrium distance between two particles was r = 1.19 +/- 0.05 ( was the average diameter of the pair of particles).

Small-Angle Scattering and Electron Microscopy Investigation of Nanotubules Made from a Perfluoroalkylated Glucophospholipid.

Anionic glucophospholipids were recently reported as a new family of tubule-forming lipids. We report here investigations on the structure of nanotubules made from a glucophospholipid with a mixed fluorocarbon-hydrocarbon hydrophobe, using freeze fracture and cryo-transmission electron microscopy (TEM) and X-ray and neutron small angle scattering (SAXS, SANS). The hollow and regularly shaped tubules are very thin: they have an external radius of 140 Å and an internal radius of 35 Å on the average. Their 105 Å-thick wall appears to consist in three bilayers in which the glucophospholipid molecules are probably in a tilted and/or interdigitated configuration. Upon heating these nanotubes convert reversibly into vesicles; transformation is complete at 60 degrees C. Copyright 1999 Academic Press.