IR-study of thermotropic phase transitions in cetyltrimethylammonium bromide powder and film

The peculiarities of the molecule conformation and arrangement at the first-order phase transitions in the cetyltrimethylammonium bromide (CTAB) in different states have been investigated by DSC and IR-spectroscopy methods. The samples in the form of a powder and a thin film, obtained by the solvent evaporation-induced precipitation (SEIP) method have been studied and compared. DSC experiments have revealed the existence of two phase transitions in the CTAB powder at 75–78 and 106 °C temperatures. The IR-spectroscopic study, especially the analysis of the temperature dependence for Davydov’s splitting (DS) shows that a solid–solid phase transition in the CTAB thin film obtained by the SEIP method occurs at temperatures a few degrees lower than that in the powder. The difference can reach even 10°. This decrease in the phase transition temperature can result from some disorder in the molecule packing of the film. The disorder leads to an appearance of the molecules with gauche-conformations in a CTAB film at lower temperatures than in the powder; this is proved by IR-spectroscopy. The X-ray diffraction study of the CTAB powder and film at room temperature has revealed that a distance between the adjacent ionic layers is 5.25 nm for a powder and 5.18 nm for a film. This evidences about a bit larger distance between methylene chains of the CTAB neighboring molecules in the films compared to the powder, which is in a good agreement with the results of IR-spectroscopy experiments. Theoretical calculations of the DS temperature changes along with the experimental data show that this transition in the bulk and film CTAB samples can be described by three mechanisms: conformation changes (1), “hindered rotation” (2), and free rotation (3), and the orientational transformations of the CTAB molecules contributes mainly into the CTAB phase transition. The analysis of DS temperature dependence allows to suggest that the low temperature phase transition in the CTAB powder at T = 75–78 °C is connected with the change in the molecular packing from the monoclinic to the orthorhombic, while the orthorhombic Vand’s subcell for the methylene chain packing remains.