Lattice structures and molecular tilts in Langmuir monolayers of saturated fatty acid-alcohol mixtures

We have studied Langmuir monolayers composed of mixtures of heneicosanoic acid and heneicosanol using π-A isotherms and x-ray diffraction. We find that the two materials are miscible in monolayer form. We have determined how the phase diagram of the acid evolves to that of the alcohol as the composition varies. X-ray diffraction data along isotherms and isobars show that the head-head interactions, like the tail-tail interactions, are anisotropic. The "swiveling transition" between phases with different tilt directions is accompanied by a rotation of the molecules about their long axes.