X-ray diffraction study of heneicosanol monolayers on the surface of water

We have studied Langmuir monolayers of CH₃(CH₂) ₂₀OH (heneicosanol) using x-ray diffraction. Over the temperature range 7-55°C, the monolayer isotherms show only one discontinuity, which may be either a "flat section" or a "kink;" we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted-hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next-nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a "tilted hexagonal" phase, even at temperatures where the untilted phase is hexagonal (rotator-II).