The structure of a Langmuir monolayer of methyl eicosanoate as determined by x-ray diffraction and Brewster angle microscopy

Relaxed Langmuir monolayers of methyl eicosanoate were studied using Brewster angle microscopy (BAM) and grazing incidence x-ray diffraction (GIXD). The structure of the various phases in this system was determined and compared to previous isotherm and microscopy measurements. At low pressure and low temperature, a crystalline phase with tilt toward nearest neighbor, I(L″₂), is observed. At a temperature of ∼ 14 °C and low pressure there is a transition to a phase with tilt toward next-nearest neighbor, F(L′₂). Finally, as the temperature continues to be raised at low pressure, there is a transition to a phase where the two F(L′²) peaks have the same values of Q_xy, here called the τ phase, with a Rotator IV-like structure. At high pressure and low temperature, an untilted, orthorhombic phase, U′ (CS), is observed. As the temperature is increased, a second untilted, orthorhombic phase, U(S), is observed, beginning at ∼ 12 °C. Above 21 °C and at high pressure, a Rotator-II-like phase is seen, with an undistorted, untilted structure. Finally, results from BAM measurements are used to monitor the texture of the film in the different phases.