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Current issue   Ukr. J. Phys. 2014, Vol. 58, N 7, p.636-645

    Paper

Gavrilko T.A.1, Puchkovska G.O.1, Styopkin V.I.1, Bezrodna T.V.1, Baran J.2, Drozd M.2

1 Institute of Physics, Nat. Acad. of Sci. of Ukraine
(46, Prosp. Nauky, Kyiv 03028, Ukraine; e-mail: gavrilko@iop.kiev.ua)
2 Institute of Low Temperatures and Structure Research, PAN
(2, Okolna Str., Wroclaw, Poland)

Molecular Dynamics and Phase Transitions Behavior of Binary Mixtures of Fatty Acids and Cetyltrimethylammonium Bromide as Studied Via Davydov Splitting of Molecular Vibrational Modes

Section: Soft matter
Original Author's Text: English

Abstract: The 1:1 solid phase complexes of stearic (SA) and behenic (BA) fatty acids (FA) with cationic surfactant cetyltrimethylammonium bromide (CTAB) are prepared from an equimolar ethanol solution of their binary mixtures. A supramolecular complexation between FA and CTAB molecules is proven with FTIR spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). A single-phase layered crystalline structure of both CTAB:SA and CTAB:BA complexes is revealed by XRD. The greatly enhanced thermal stability of the CTAB:FA complexes over the pure FA (by about 40–50C) is found with DSC along with a number of successive solid-solid phase transitions. The temperature-dependent FTIR study of the Davydov splitting for CH2 rocking (720–730 cm−1 ) vibrations revealed a significant difference in the conformational disorder of methylene chains and the molecular packing in successive solid phases of CTAB:SA and CTAB:BA complexes. Our research provides a molecular basis for a prospective application of such class of binary mixtures of oppositely charged cationic and anionic surfactants in thermo-sensitive supramolecular systems.

Key words: FTIR spectroscopy, X-ray diffraction, DSC, CTAB, fatty acids, supramolecular complex.