Using Brewster angle microscopy to study fatty acid Langmuir monolayers, Overbeck and Mobius (J. Phys. Chem. 1993, 97, 7999) have observed changes of texture indicating a new phase boundary not visible in monolayer isotherms. We have studied Langmuir monolayers of CH3(CH2)17COOH at 30°C using X-ray diffraction. We find that the monolayer undergoes a first-order phase transition from the known L2 phase in which molecules tilt toward a nearest neighbor to a phase with tilt toward a next-nearest neighbor. Both before and after the transition, the structures are hexagonal in the plane perpendicular to the molecules. Neither the intermolecular spacing in this plane (≈ 4.85 A, close to that seen in the Rotator II phase) nor the tilt magnitude has an observable discontinuity at this transition; only the tilt direction changes. As a result, the isotherm area/molecule is also continuous across the transition.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry