Chain length effect on the structure and thermotropic phase transitions of solid catanionic complexes built of cetyltrimethylammonium bromide and saturatedn-fatty acids

Chain length effect on the structure and thermotropic properties of asymmetric solid catanionic complexes (CAC) built of cetyltrimethylammonium bromide (C(16)TABr) and saturatedn-fatty acids (C(n)SFA, CnH2nO2,n = 12, 18, 20, and 22) was investigated by XRD, DSC, and FTIR spectroscopy. All CAC show a lamellar crystalline structure and a series of solid-solid phase transitions below the melting point. The phase transitions are suggested to be caused by sequential melting of the methylene chains in the hydrophobic layer followed by a change in the crystalline structure. C(16)TABr stabilizes the bilayer structure of CAC and raises its melting point compared to initial C(n)SFA.