Synchrotron x-ray studies of monolayers at air-water interfaces and on solid substrates

The authors have used X-ray diffraction to study the structures of Langmuir monolayers. They find that both heneicosanoic acid and heneicosanol monolayers show the same untilted structures at high monolayer pressure as do lamellar paraffins; in other words, the high-pressure untilted structures are determined by chain packing and not by the head group. At lower pressures, however, the structures, lattice spacings, and tilt angles and tilt directions all appear to be determined by competition between head group and tail group interactions. For example, they find that the effect of increasing subphase pH on heneicosanoic acid monolayers is to reduce the in-plane spacings at zero pressure, and thus reduce the tilt angles and move the phase transitions to lower pressure. Again, while acid monolayers show phases with both nearest-neighbor and next-nearest-neighbor tilts, alcohol monolayers show next-nearest-neighbor tilts only. Finally, they have studied the correlation between structures of phases on water and structures of films transferred from these phases to solid substrates; they find that the structures are not preserved, although there are small open-quotes memory effects.close quotes