# Fluorescence microscopy evidence for two different LE-LC phase transitions in Langmuir monolayers of fatty acids

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Abstract : We have studied the liquid expanded LE-liquid condensed LC phase transition in Langmuir monolayers of myristic MYA and pentadecanoic PDA acids by extensive surface pressure measurements and fluorescence microscopy observations. Depending on the temperature T, the LE phase transforms through first order phase transitions into two different condensed phases called LC1 and LC2. The LE-LC1 transition is observed below a well-defined temperature T* of $24.0 \pm 0.5~^\circ$C for MYA and $35.6 \pm 0.5~^\circ$C for PDA. It is followed at high pressures by a LC1-LC2 transition which is signaled by a narrow plateau region in the isotherms. Above T*, only the LE-LC2 transition is observed. These results are in good agreement with previous measurements by Bibo and Peterson Adv. Mater. 2 1990 309. Fluorescence microscopy observations in the coexistence region show that the LC1 domains are circular and easily deformable, whereas the LC2 domains are highly ramified and evolve very slowly towards their equilibrium circular shape. The LC1 phase is clearly a low viscous fluid, whereas the LC2 phase is much more viscous and exhibits viscoelastic properties.

Autor: Silvère Akamatsu F. Rondelez

Fuente: https://hal.archives-ouvertes.fr/

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