Journal of Lipid Research, Vol. 11, 190-194, May 1970
Copyright © 1970 by Lipid Research, Inc.
Stability of fatty acid monolayers and the relationship between equilibrium spreading pressure, phase transformations, and polymorphic crystal forms
Richard E. Heikkila , Chui N. Kwong , and David G. Cornwell
Department of Physiological Chemistry, The Ohio State University, Columbus, Ohio 43210
Force-area isotherms were obtained for hexadecanoic, octadecanoic, eicosanoic, and docosanoic acid monolayers at different compression rates. Equilibrium spreading pressures were determined both by monolayer collapse and by spreading from the bulk phase. Monolayers formed metastable phases at all pressures above their equilibrium spreading pressures and at all surface areas smaller than the surface areas at their equilibrium spreading pressures. These metastable phases collapsed to stable phases at the equilibrium spreading pressures of the fatty acids. Collapse phenomena and compression experiments at very slow compression rates suggested that a previously unrecognized phase transformation occurred at the equilibrium spreading pressure. The surface area at this phase transformation corresponded to the cross-sectional area of the C-form of the fatty acid crystal. Fatty acid monolayers at the phase transformations previously described by other workers had surface areas which were closely related to molecular areas in their A and B polymorphic crystal forms. These correlations indicated that molecular structure in saturated fatty acid monolayers was similar to molecular structure in fatty acid crystals.
Supplementary key words hexadecanoic • octadecanoic • eicosanoic • docosanoic • compression rate • collapse • surface area • molecular structure
Submitted on October 14, 1969
Accepted on January 7, 1970
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