Tilt stripe textures in Langmuir monolayers of fatty acids

doi:10.1016/0378-4371(94)90449-9

Physica A: Statistical Mechanics and its Applications

Volume 204, Issues 1–4, 1 March 1994, Pages 606-615

https://doi.org/10.1016/0378-4371(94)90449-9 Get rights and content

Abstract

Quantitative studies are described of striped textures in Langmuir monolayers of tetradecanoic and pentadecanoic acids. The textures, which are the result of modulations of the tilt azimuths of the molecules, have been visualized with polarized fluorescence microscopy. The stripe widths change with pressure and temperature. When the monolayer is compressed beyond a threshold pressure, the stripes become unstable and form a zig-zag pattern, which is likely the result of a crossover between the bend and splay elastic constants in the monolayer.

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Citation Excerpt :
An energy minimum results if an orientation change of 360° is subdivided in five jumps of each 60° and a continuous change of the orientation of 60° between the defect lines. Such domain textures were discussed for monolayers of myristic acid and pentadecanoic acid at low temperatures [35–38]. In pentadecanoic acid monolayers, domains were observed showing the pattern of molecular orientations corresponding to a boojum having a cusp in the domain boundary at the vertex of the boojum.
Knowledge of the mesoscopic morphology of condensed phase domains formed after the main phase transition in the two-phase coexistence region of Langmuir monolayers progressed rapidly with the development of the highly-sensitive imaging techniques, particularly by Brewster angle microscopy (BAM). Latest developments of commercial BAM instruments have been developed to a high technical level and allow upgrading to imaging ellipsometers which combine optical microscopy and ellipsometry and make the assessment of small layered structures or patterned thin films possible. A large variety of condensed phase domains different in mesoscopic sizes and shapes as well as their textural features has been observed which depend sensitively on the chemical structure of the amphiphilic monolayer and the system conditions, such as surface pressure and temperature. This unsuspected morphological variety of condensed phase domains has been proven not only in Langmuir monolayers but also in adsorbed monolayers (Gibbs monolayers), in Langmuir monolayers penetrated by dissolved surfactants or in adequate molecular recognition systems. The inner textures of domains can be explained on the basis of their geometry and the two-dimensional lattice in dependence of the tilt angle of the alkyl chains and gave rise to the development of a geometric concept on the basis of the molecular packing. New knowledge has been gained about non-equilibrium structures and their transition kinetics into the equilibrium state. Combined results obtained recently by BAM have enhanced the understanding of molecular organization in phase diagrams and binary mixtures. Recent advances in model studies about chiral discrimination effects and of the highly specific structural changes of host-monolayers by recognition of non-surface active guest-components have made progress. Semi-empirical quantum chemical methods have been used to gain insight into the role of different types of interactions involved in the main characteristics of mesoscopic length scale aggregates of mimetic systems.
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Recent progress in our understanding of the properties of monolayers is linked to the development of techniques for their direct visualization. The use of such methods for the determination of phase diagrams of Langmuir monolayers, both pure substances and mixtures, is described. Complex optical textures — stripes, spirals, stars — have been observed that are related to those found in liquid crystals. Textures can persist when the monolayers are transferred to solid supports, allowing them to be studied by scanning probe microscopy. The transfer procedure can be employed to produce stable patterned surfaces.
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^☆: Dedicated to Proffesor Kyozi Kawasaki on the occasion of his retirement.

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Tilt stripe textures in Langmuir monolayers of fatty acids☆

Abstract

Annu. Rev. Phys. Chem.

Chem. Scr.

J. Phys. Chem.

J. Phys. Chem.

Structure of Liquid Crystal Phases

Phys. Today

Phys. Rev. Lett.

Phys. Rev. Lett.

Mater. Res. Soc. Symp. Ser.

Phys. Rev. A