Laboratoire de Physique de la Matière Condensée (LPMC), UMR 7336, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France.
Langmuir. 2012 Aug 21;28(33):12022-9. doi: 10.1021/la302037n. Epub 2012 Aug 10.
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).
双全氟（烷）基（烷基）链两亲物的朗缪尔单层，其带有单吗啉磷酸盐极性头，C(n)F(2n+1)(CH(2))(m)OP(O)[N(CH(2)CH(2))(2)O]（二（FnHm）MP，其中 n = 6、8 或 9；m = 1 或 2），通过在 15 至 50°C 范围内的表面压力（π）/分子面积（A(0)）压缩等温线，以及在 25°C 下的掠入射 X 射线衍射（GIXD）进行了研究。对于这些短链表面活性剂，可以获得超薄的单层。尽管碳氢化合物间隔物很短，但它对单层的组织有显著影响。在 25°C 时，二（F8H2）MP 单层呈现出液体膨胀（LE）/液体凝聚（LC）转变，而在后者化合物的结构中，仅将一个 CH(2)替换为 CF(2)，在保持链长不变的情况下，即缩短间隔物从 2 个 CH(2)变为 1 个（如在二（F9H1）MP 中），会抑制 LE 相。在 25°C 时，GIXD 确定对于二（F8H2）MP 和二（F9H1）MP，在 LC 相中，链形成六方晶格。两种化合物的集体倾斜接近于零。通过增加压力，密集相的晶格可以被压缩，这可以通过 d 间距随压力的连续线性减小来评估。这表明，随着压缩，分子倾斜的方位分布逐渐减少。二（F9H1）MP 的 d 值明显低于二（F8H2）MP，这证明了氟代链的强烈凝结效应。分子面积直接从压缩曲线确定，也可以从 X 射线数据确定，后者允许重建压缩等温线。计算出的晶格压缩系数分别为二（F9H1）MP 和二（F8H2）MP 的宏观压缩系数的 30%和 50%。与实验确定的等温线的比较表明，二（F9H1）MP 的单层比二（F8H2）MP 的单层更稳定。从克劳修斯-克拉佩龙方程得出的二（F9H1）MP 和二（F8H2）MP 的焓和熵，证实了所观察到的转变均为 LE/LC 型，尽管三相点温度差异很大（27°C 与-18°C）；这种巨大的差异进一步说明了氟代链的稳定作用。当间隔物大于 1 CH(2)时，氟代链阻碍了无序的形成，而促进了无序的形成。
