Molecular dynamics and phase transitions behavior of binary mixtures of fatty acids and cetyltrimethylammonium bromide as studied via Davydov splitting of molecular vibrational modes

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 40-50°C) 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. © T.A. Gavrilko, G.O. Puchkovska, V.I. Styopkin, T.V. Bezrodna, J. Baran, M. Drozd, 2013.