X-RAY-DIFFRACTION STUDY OF A RECENTLY IDENTIFIED PHASE-TRANSITION IN FATTY-ACID LANGMUIR MONOLAYERS

Using Brewster angle microscopy to study fatty acid Langmuir monolayer s, Overbeck and Mobius (J. Phys. Chem. 1993, 97, 7 999)have observed c hanges of texture indicating a new phase boundary not visible in monol ayer isotherms. We have studied Langmuir monolayers of CH3(CH2)(17)COO H at 30 degrees C using X-ray diffraction. We find that the monolayer undergoes a first-order phase transition from the known L(2) phase in which molecules tilt toward a nearest neighbor to a phase with tilt to ward a next-nearest neighbor. Both before and after the transition, th e structures are hexagonal in the plane perpendicular to the molecules . Neither the intermolecular spacing in this plane (approximate to 4.8 5 Angstrom, close to that seen in the Rotator II phase) nor the tilt m agnitude has an observable discontinuity at this transition; only the tilt direction changes. As a result, the isotherm area/molecule is als o continuous across the transition.