Isotherm and X-ray diffraction studies of mixed monolayers

We have studied Langmuir monolayers of saturated fatty acid-alcohol mixtures (same chain length) and acid-acid mixtures (different chain length) using isotherms and X-ray diffraction. The diffraction data confirm that the constituents are miscible, and that at zero pressure the mixed acid-alcohol monolayer areas are less than the areas of the pure components. We have determined the phase diagrams of a number of different mixtures. In any given phase, the intermolecular distances measured perpendicular to the molecules are independent of the pressure and of the constituents of the mixture. However, the location of some phase boundaries is a strong function of the composition of the mixture.