Phase transitions, elasticity and capillary waves in langmuir monolayers on water: An X-ray optical study

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Abstract
In this paper we investigate, by means of X-ray reflectivity at glancing angles, the phase transitions of Langmuir monolayers on water in different compounds (fatty acids and DPPC). We show that the determination of the profile of the electron density normal to the film provide two separate types of information: thicknesses and roughnesses. The thickness of the layer is directly related to the structure of the aliphatic medium and the roughness is due to thermally excited capillary waves. Our study clearly demonstrates that the transition between the so-called liquid-expanded to liquid-condensed phases is characterized by a significant discontinuity of the thickness of the aliphatic medium, whereas at the liquid-condensed to solid transition a strong decrease in the roughness is observed. This striking feature is attributed to the appearance of a rigidity constant of the monolayer in the solid phase.