Fourier transform infrared spectroscopic study of the kinetics of a first-order phase transition in tridecanoic acid СH$_{3}$(CH$_{2}$)$_{11}$СООH

The structural changes in crystalline lamella cores of tridecanoic acid СH$_{3}$(CH$_{2}$)$_{11}$СООH during heating in the range from the temperature $T_{1}$ = 13.5$^\circ$C to $T_{2}>T_{m}$ = 41.6$^\circ$C have been investigated using Fourier transform infrared spectroscopy. The behavior of the bands of rocking (in the region of 720 cm$^{-1}$) and bending (in the region of 1470 cm$^{-1}$) vibrations of СН$_{2}$ groups in tridecanoic acid methylene segments has been analyzed. It has been shown that, in the first-order phase transition region ($T_{s-s}\sim$ 36$^\circ$C) within a narrow temperature range ($\Delta T_{1}\le$ 1 K), there is a gradual transformation of the initial triclinic subcell into the hexagonal subcell. The mechanism of this transition has been considered in terms of the theory of diffuse first-order phase transitions.