Domain Structures and Phase Transitions in Langmuir Monolayers

Abstract

A variety of recent measurements has shown that the phase diagrams of Langmuir monolayers of relatively simple amphiphiles such as fatty acids and their methyl and ethyl esters are remarkably complex. Nine condensed phases have been identified; their structures can be related to those of known smectic phases. A key distinction between phases is the orientation of the molecular tilt azimuth with respect to the local hexagonal order of the head groups. When monolayers are examined by fluorescence microscopy, regions of uniform tilt can be observed if the exciting radiation is polarized with respect to the surface normal. The tilt regions form patterns similar to those observed in freely suspended films of smectic liquid crystals. The patterns can be changed by compressing the film and by changing the temperature. Transitions between different phases can be observed.

The structure of a Langmuir monolayer of methyl eicosanoate as determined by x-ray diffraction and Brewster angle microscopy

“…The flux incident on the sample was typically 10 9 photons/cm 2 /sec. In this paper, a double notation is used to describe the LM phases: The first notation is based on that used to describe the symmetry of the ordered liquid crystal phases 17,20,25,36 and will be easier to interpret, at least for some readers, than the second notation that is more traditional to the field. 37 Structures observed here that have been observed previously in Langmuir monolayers include the F(L 2 Ј) phase, in which the alkane chains are tilted towards their next nearest neighbor and the I(L 2 Љ) phase, where the alkane chains are tilted toward the nearest neighbor.…”

“…The advent of synchrotron x-ray sources and the development of fluorescence and Brewster angle microscopy ͑BAM͒, provided the ability to gather microscopic structural information, as well as to directly image the domain texture of these films. 6,[12][13][14][15][16][17][18][19][20] The development of synchrotron based grazing incidence x-ray diffraction ͑GIXD͒ techniques for measuring the microscopic structure ͑i.e., lattice parameters and molecular tilt͒ of the LM phases with long range order is considered by some to be an important recent developments in condensed matter physics. [21][22][23] An important advantage this technique brings to the thermodynamics of LMs is that from a measure of the angular position of the Bragg peaks of the twodimensional LM lattice, it is possible to obtain the size and shape of the microscopic unit cell.…”

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

“…These phases have been labeled by adopting a nomenclature analogous to that used to describe smectic liquid crystal phases with the same in-plane structure; 21,22,33 however, to avoid confusion, the Stenhagen notation 7 has been included in parenthesis. As shown in Fig.…”

The Structure and Phases of a Relaxed Langmuir Monolayer of Methyl Eicosanoate as Determined by Grazing Incidence Xray Diffraction and Brewster Angle Microscopy

“…The sixth appears to not have been previously reported in any monolayer system. The previously described phases include two untilted phases with 2d orthorhombic unit cells (the U'(CS) and U(S) phases); one untilted rotator II-like phase in which the unit cell is hexagonal (R); an orthorhombic, tilted phase with tilt toward nearest neighbor (called I here, in analogy to the smectic I phase but also known as L 2 " for historical reasons) [10,11]; and an orthorhombic, tilted phase with tilt toward next nearest neighbor (called F here, in analogy to the smectic F phase but also known as L 2 '). The phase previously not reported is tilted, in that the maximum intensity of the GIXD peak is out of the surface plane, but the unit cell appears to be hexagonal.…”