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X-ray diffraction data from water-supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2’, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2’–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The distortions are related to symmetry of the phases and described by the order parameters responsible for tilt and herringbone ordering of the backbone planes of the molecules. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied Langmuir monolayers of CH3(CH2)20OH (heneicosanol) using x-ray diffraction. Over the temperature range 7–55 °C, the monolayer isotherms show only one discontinuity, which may be either a ‘‘flat section’’ or a ‘‘kink;’’ we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted-hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next-nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a ‘‘tilted hexagonal’’ phase, even at temperatures where the untilted phase is hexagonal (rotator-II). [ABSTRACT FROM AUTHOR]

We have studied the structure of a monolayer of C21H43OH on water, in the region near close packing, by grazing incidence in-plane x-ray diffraction. For all temperatures studied the isotherms in the πa plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for π=30 dyn/cm the transition temperature is in the range 16.3