X-ray diffraction study of a recently identified phase transition in fatty acid Langmuir monolayers

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.