Effect of unsaturation in fatty acids on the main characteristics of Langmuir monolayers

The main characteristics of unsaturated acid monolayers deviate considerably from those of the corresponding saturated fatty acids. The monolayers of an unsaturated cis and trans fatty acid with the same hydrocarbon chain length having the double bond at the same position (omega 9, Delta 13) are compared to obtain detailed information on the effect of the cis and trans double bonds. The studies were performed with erucic acid chosen for the cis form and brassidic acid chosen for the trans form. Some additional studies performed with the homologous cis-nervonic acid (omega 9, Delta 15) and trans-elaidic acid (omega 9, Delta 9) allow information about the effect of the hydrocarbon chain length. Phase behavior, domain morphology, and two-dimensional lattice structure of the cis and trans unsaturated fatty acid monolayers are compared, using surface pressure-molecular area (pi-A) isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) techniques. The pi-A isotherms of the unsaturated fatty acids studied have a fluid/condensed phase coexistence region (plateau) over the measured temperature range. The thermodynamic analysis, performed on the basis of the pi-A isotherms, shows that the introduction of unsaturation into the hydrocarbon chain affects the thermodynamic characteristics of the monolayer in a similar way as shortening the alkyl chain length, and it is much larger for the cis isomers rather than for the trans isomers. The morphology of the condense phase domains formed in the fluid/condensed coexistence region reveals considerable differences between the cis and the trans conformations at the same molecular structure of the unsaturated fatty acid. Completely different to the behavior of saturated fatty acid monolayers, for all cis and trans unsaturated fatty acids studied, only a centered rectangular lattice with alkyl chains tilted toward the nearest neighbor (NN) direction is found, and no indication for a phase transition between different condensed phases could be observed independent of different surface pressures and temperatures. The conformity of the cis and trans unsaturated fatty acid monolayers to only one lattice type having a rectangular centered unit cell with alkyl chains tilted in the NN direction correlates obviously with the obstructive effect of the double bond in the alkyl chain on the molecular ordering.