Morphologie et propriétés élastiques de phases hexatiques dans des films monomoléculaires d'acides gras

Abstract : This thesis presents a study of monomolecular films of long-chain fatty acid at the air-water interface, performed mainly with Brewster angle microscopy. this technique allows for a direct visualisation of phase coexistence during first order transitions; moreover, it is sensitive to a possible optical anisotropy of the films. The first part of this thesis consists of a study of the phase diagram of fatty acids which contains mesophases analogous to those found in smectic liquid crystal films. It is shown that, for the transition between two phases, the texture ( the molecular arrangement) of the phases is reversible. Also, information about the order of the phase transition is obtained . In addition, a very weak optical anisotropy due to the rectangular shape of the molecular lattice, is pointed out in two different phases composed of vertically aligned molecules. Then, observations are made of a film adsorbed at the free surface of an aqueous solution of sodium myristate. We study domains of a tilted mesophase containing defect lines of molecular orientation which fluctuate thermally. the tension of these defect lines is determined by measuring the amplitude of their fluctuations. Finally, we study the elastic constants which govern the equilibrium shape and texture of domains of mesophases. on the one hand, we determine the line tension of the interface between a liquid expanded phase and a tilted mesophase from the shape of the domains. Their shape results from the balance between the line tension, which favors circular domains, and dipoles which deforms the domains. surface potential measurements allow for the calculation of the dipolar forces. On the other hand we evaluate, from the texture of other domains of this mesophase, the ratio between the anisotropic line tension and the elasticity of curvature of the molecular orientation
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Data Analysis, Statistics and Probability. Université Pierre et Marie Curie - Paris VI, 1995. French


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Submitted on : Tuesday, February 19, 2002 - 3:22:05 PM
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Sophie Rivière-Cantin. Morphologie et propriétés élastiques de phases hexatiques dans des films monomoléculaires d'acides gras. Data Analysis, Statistics and Probability. Université Pierre et Marie Curie - Paris VI, 1995. French. <tel-00001108>

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