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Journal Articles Journal de Physique II Year : 1991

Structural properties and elasticity of amphiphilics on water

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J. Daillant
  • Function : Author
B. Harzallah
  • Function : Author
J. Benattar
  • Function : Author

Abstract

Monolayers of amphiphilic molecules adsorbed at the air-water interface have been studied using X-ray reflectivity. This technique allows an accurate and independent determination of thicknesses (by the measurement of fringes of equal inclination) as well as densities and roughnesses (since this is an absolute intensity measurement); therefore a complete electron density profile can be obtained. The surface roughness is due to thermally excited capillary waves, and is thus related to the dynamics of the interface, which are conditionned by the elastic constants of the layer. The phase diagrams of the C15, C21 and C29 fatty acids and of the phospholipid L-α-DPPC have been investigated using this method, and an analysis of the phase transitions has been performed. From the structural point of view, a general feature of this class of compounds adsorbed on water is an unchanging aliphatic medium density within the liquid and solid phases. If follows that the pressure vs. film thickness curves mimic the pressure area curves. In particular evidence for the liquid-expanded to liquid-condensed transition is given by an abrupt increase of the aliphatic medium thickness. The data suggest that this transition should involve conformational changes. Furthermore the decrease in molecular area within the liquid condensed phase is revealed by a decreasing molecular tilt angle. The structure of this phase is also shown to be independent of the temperature and of the nature of the polar head-group. A transition to a multilayer structure is observed with the longest chain compound. We show that the dynamics of the interface are consistent in the liquid phases with a simple capillary wave model, with no adjustable parameter. A strong drop in the roughness at the transition to the solid state reveals the onset of a rigidity against bending. The high values of the rigidity modulus obtained (on the order of 100 $k_{\rm B}~T$) are sensitive to the aliphatic chain length and can be interpreted using a simple model for the elastic behavior of the solid monolayer. The scattering of X-ray by capillary waves in detailed for a rigid interface such as a monolayer.
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Dates and versions

jpa-00247504 , version 1 (01-01-1991)

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J. Daillant, Louis Bosio, B. Harzallah, J. Benattar. Structural properties and elasticity of amphiphilics on water. Journal de Physique II, 1991, 1 (2), pp.149-170. ⟨10.1051/jp2:1991102⟩. ⟨jpa-00247504⟩
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