Discontinuities in the Tilt Angle of Octadecanol Langmuir Monolayers As Observed with Brewster Angle Autocorrelation Spectroscopy

We have studied the tilt angle of Langmuir monolayers of octadecanol (CH 3 (CH 2 ) 17 OH) using Brewster angle autocorrelation spectroscopy. Measurements were carried out at the triple point of the next nearest neighbor tilted L 2 ‘, the untilted distorted hexagonal LS(Rot I), and the crystalline herringbone S phase and at the triple point of the next nearest neighbor tilted L 2 ‘, the untilted distorted hexagonal LS(Rot I), and the hexagonal undistorted LS(Rot II) phase. We show that the transition from the tilted phase to the untilted phases as a function of temperature changes from second order (L 2 ‘/S) to first order (L 2 ‘/LS(RotI)) to second order (L 2 ‘/LS(Rot II)) and that the S/LS(Rot I) and LS(Rot I)/LS(Rot II) phase transition lines at high pressure continue into the tilted L 2 ‘ phase. The L 2 ‘ phase actually consists of three different phases of different tilt angle with some similarity to the phase behavior of fatty acids. Strong superheating and supercooling effects prevent a direct observation of both phase transitions.