LATTICE STRUCTURES AND MOLECULAR TILTS IN LANGMUIR MONOLAYERS OF SATURATED FATTY-ACID ALCOHOL MIXTURES

We have studied Langmuir monolayers composed of mixtures of heneicosan oic acid and heneicosanol using pi-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 a lcohol as the composition varies. X-ray diffraction data along isother ms and isobars show that the head-head interactions, like the tail-tai l interactions, are anisotropic. The ''swiveling transition'' between phases with different tilt directions is accompanied by a rotation of the molecules about their long axes.