Tilted phases of fatty acid monolayers

Abstract

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.

A determination of the phase diagram of relaxed Langmuir monolayers of Behenic acid

“…This might suggest that these two phases are hexatic. 48,49 The GIXS data does not conclusively support this suggestion, although generally the diffraction peaks corresponding to the F(L 2Ј ) phase, and almost always the degenerate peak of the I(L 2 ) phase, were observed to be broader than the resolution limit. In contrast, the diffraction peaks for the IЈ(L 2Љ ) phase always appeared resolution limited.…”

C 60 -propylamine adduct monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study

“…[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][32][33][34] Except for a few cases, which have hexatic order, most of these phases exhibit resolution-limited Bragg peaks characteristic of 2D crystalline phases. Figure 1͑b͒ contains a schematic illustration of the kinematics for the GID experiment.…”

“…1-3 A principal challenge in this general area of physics has been to identify real, well defined physical systems that are suitable for experimental studies. Examples of two-or quasi-twodimensional systems that have been investigated for this purpose in recent years include rare gases adsorbed on solid substrates, [3][4][5][6] freely suspended thin liquid crystal films, [7][8][9][10] and Langmuir monolayers 11,12 of simple long-chain molecules such as fatty acids, [13][14][15][16][17][18][19][20][21][22][23] alcohols, [24][25][26][27] esters, 28 etc. Related statistical phenomena at surfaces and interfaces that have been often studied for more practical motivations, include wetting, [29][30][31] premelting and reconstruction of crystalline surfaces, [32][33][34][35] and surface induced order at liquid surfaces.…”

Langmuir monolayers as disordered solids: Strain-tilt-backbone coupling and natural order parameters for the swiveling transitions

“…Is it the conformation of the individual alkane chains or their interaction and dynamics ͑or both͒ which govern the structures of the different phases of the molecular monolayers? In recent years there has been impressive progress in experimental structural determinations, 4 although there is still unresolved ambiguity in the assignment of phases as crystalline or mesophase. 5 The phases are distinguished by widths of diffraction peaks, but the interpretation of the broad peaks in some phases as hexatic ͑mesophase͒ is questioned based on finite resolution.…”

“…However, an important structural aspect, the distortion of the hexagonal lattice, was not uniquely defined nor considered. Advocating the liquid-crystalline approach, Kaganer et al 4 have recently characterized the distortions with respect to a hexagonal lattice using two concepts: the so-called distortion ellipse 11 and the ''projected unit cell.'' 12 With so many parameters, the theory not only became less transparent but also did not help us to understand and to describe phases observed in the generic phase diagram.…”