Fourier transform infrared spectroscopic study of the kinetics of a first-order phase transition in tridecanoic acid CH3(CH2)11COOH

The structural changes in crystalline lamella cores of tridecanoic acid CH3(CH2)11COOH during heating in the range from the temperature T1 = 13.5°C to T2 > Tm = 41.6°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 CH2 groups in tridecanoic acid methylene segments has been analyzed. It has been shown that, in the first-order phase transition region (Ts–s ~ 36°C) within a narrow temperature range (ΔT1 ≤ 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.