FTIR and DSC Studies of Binary Mixtures of Long-Chain Aliphatic Compounds: Lauric Acid – Cetyl-trimethylammonium Bromide

Abstract

Structural and thermal properties of a solid-state binary mixture of long-chain cationic and anionic surfactants (so-called catanionic complexes) composed of cetyltrimethyl-ammonium bromide, [H3C–(CH2)15–N+(CH3)3]Br−(CTAB), and saturated fatty acid (FA), CH3(CH2)12COOH (lauric acid, kC12), are studied. To clarify the effect of intermolecular interactions on the crystalline structure and phase transitions in this class of supramolecular compounds, the 1 : 1 kC12-CTAB binary mixture is investigated by means of X-ray diffraction, differential scanning calorimetry (DSC), and temperature-variable Fourier transform infrared spectroscopy (FTIR). Based on the comparison of the obtained results with those of other CTAB-FA binary mixtures with different alkyl chain length mismatches, the possible molecular packing in the crystal phase of CTAB-FA complexes and the mechanism of successive phase transitions in the condensed state are proposed.