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)OP(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).