Freezing transition of Langmuir-Gibbs alkane films on water

We show that adding CTAB (CTAB, hexadecyltrimethylammonium bromide) in sub-millimolar bulk concentrations to water reduces its surface tension (ST) to a level where spontaneous surface spreading of a monolayer of medium-sized alkane (C_nH_2n+2, 12 ≤ n ≤ 17) occurs. ST and X-ray reflectivity (XR) measurements are used to show that the quasi two-dimensional (2D) liquid monolayer can be driven through a reversible surface freezing phase transition upon cooling. Grazing incidence diffraction (GID) shows that the frozen monolayer is crystalline, hexagonally packed, with surface-normal molecules, and a crystalline coherence length of at least a few hundred Å, very similar to the structure of surface-frozen (SF) monolayers at the surface of similar-length alkane melts.