| mots-clés : monocouches sur des sous-phases liquides – acides gras – phospholipides – acide eicosanoique – acide dipalmitoyl-phosphatidique (DPPA) – autoassemblage à basse température – transition de phase rotateur-cristalline – isothermes de Langmuir – diffraction X sous incidence |
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| Low Temperature Self-Assembly of Organic Monolayers |
| At ambient temperature monolayer phases of most alkane chain molecules exhibit phases, characterised by the rotation of the chain around the molecular axis. Consequently, these phases are only weakly ordered rather than crystalline. In order to achieve crystalline ordering, the energy of the system needs to be reduced. New cryo-protective liquid subphases extend the accessible range to much lower temperatures than the freezing threshold of water. Monolayers of common surfactants like fatty acids and phospholipids were prepared under constant volume/constant surface pressure conditions and their phase behaviour was studied upon cooling. First insights into film stability and evolution upon cooling were achieved by Langmuir isotherms and GIXOS, while the in-plane ordering was investigated by GIXD. The molecules were found to adapt crystalline phases by a purely entropy driven self-assembly process, reaching packing densities similar to three dimensional single crystals. The evolution of the structure parameters compare to those of bulk alkanes undergoing the rotator-crystalline phase transition. XPCS was employed to relate the surface dynamics of the system to the appearance of crystalline phases. The phase transition was found to be accompanied by a change of the dynamics from propagating to overdamped capillary waves. |
| mots-clés en anglais : monolayers on liquid subphases – fatty acids – phospholipids – eicosanoic acid – Dipalmitoyl Phosphatidic Acid (DPPA) – low temperature self-assembly – rotator-crystalline phase transition – structure factor calculations – Langmuir isotherms – grazing incidence X-ray diffraction (GIXD) – X-ray photon correlation spectroscopy (XPCS) |
