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

Journal Article · Thu Feb 17 04:00:00 EST 1994 · Journal of Physical Chemistry; (United States)

DOI:https://doi.org/10.1021/j100058a005· OSTI ID:5041920

Durbin, M K; Malik, A; Ghaskadvi, R; Dutta, P ^[1]; Shih, M C ^[2]; Zschack, P ^[3]

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1. Northwestern Univ., Evanston, IL (United States)
2. Princeton Univ., NJ (United States)
3. Oak Ridge National Lab., TN (United States)

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 CH[sub 3](CH[sub 2])[sub 17]COOH at 30[degrees]C using X-ray diffraction. We find that the monolayer undergoes a first-order phase transition from the known L[sub 2] 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 ([approx] 4.85 [angstrom], 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. 10 refs., 5 figs.

• DOE Contract Number: FG02-84ER45125
• OSTI ID: 5041920
• Journal Information: Journal of Physical Chemistry; (United States), Journal Name: Journal of Physical Chemistry; (United States) Vol. 98:7; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102 -- Chemical & Spectral Procedures
400201* -- Chemical & Physicochemical Properties
99 GENERAL AND MISCELLANEOUS
990200 -- Mathematics & Computers
CARBOXYLIC ACIDS
COHERENT SCATTERING
DIAGRAMS
DIFFRACTION
MATHEMATICAL MODELS
MOLECULES
ORGANIC ACIDS
ORGANIC COMPOUNDS
PHASE DIAGRAMS
PHASE TRANSFORMATIONS
SCATTERING
X-RAY DIFFRACTION