UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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2024-11-22 2:06
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Conference: Bucharest University Faculty of Physics 2004 Meeting


Section: Atomic and Molecular Physics; Astrophysics


Title:
Electron Paramagnetic Resonance Investigations of Lyotropic Crystal Liquids


Authors:
M. A. Tarpan*, V. V. Grecu*, M. N. Grecu**, I. Zgura**, T. Beica**, R. Moldovan***


Affiliation:

*) Physics Faculty, University of Bucharest, **) National Institute for Materials Physics, Bucharest, ***)Romanian Academy Center of Advanced Studis in Physics, Bucharest


E-mail
adelatarpan@yahoo.com


Keywords:
EPR, liquid crystals


Abstract:
ELECTRON PARAMAGNETIC RESONANCE INVESTIGATIONS OF LYOTROPIC CRYSTAL LIQUIDS Electron Paramagnetic Resonance, EPR, of doped liquid crystals with nitroxide spin labels are efficiently used for studying the local ordering and dynamics of liquid crystals, LC. In their ordered phase the LC force the nitroxide spin label molecules to embrace their ordering. Besides this, depending on the local dynamics, the anisotropic interactions, Zeeman and hyperfine interactions, of the spin label are more or less averaged. From the experimental spectra one can obtain, after a proper analysis, infomations concerning the order parameter and the relaxation times. These data can be correlated with ordering structures of the LC, and with its motional freedom degrees. Results on lyotropic LC doped with fatty acid spin labels, FASL, are reported. The spectra are interpreted in a model of fast rotation around the length of the FASL, and slower rotations or oscillations around the other directions. A homogeneous ordering of the amphiphilic molecules is obtained by sucking the liquid crystalline phase in capillaries. Different spectra are obtained depending on the cylindrical, respectively, bilayer ordering; formation of nematic ordered phases of micelles is also put in evidence. Using FASL molecules labeled at different positions along their length one can differentiate the dynamical behaviors nearer to the hydrophobic or hydrophilic zone. Data are reported on three component mixtures, stearic acid with decanol and water. Depending on its composition the mixture shows different ordering properties. The EPR spectrum of FASL I(14,1) indicates a free mobile zone, and the I(3,12) FASL, located near the hydrophilic end, shows less mobility. Comments on the procedure used in spectra analysis are given.