Band Analysis of Temperature-Dependent Near-Infrared Spectra of Oleic Acid in the Pure Liquid State by the Analytic Geometric Approach

The applicability of the band-stripping and complementary matching method has been demonstrated by the analysis of temperature-dependent near-infrared (NIR) absorption spectra in the 7500–6500 cm⁻¹ region of oleic acid (cis-9-octadecenoic acid) in the pure liquid state. This method is based on first derivative–second derivative pair (D1–D2) plots and a new concept called the complementary band, cBD _i , created by subtracting all the rest of the bands, exclusive of the ith estimated band, eBD _i , from an experimental spectrum. The degree of coincidence of both band shapes provides a suitable measure for the quality of fit for each individual component band. It has been confirmed from the present analysis of the NIR spectra of oleic acid measured over a temperature range of 16–79 °C that the change of the peak intensity of the component band at around 6915 cm⁻¹ due to the first overtone of an O–H stretching vibration of the monomer has two transition points around 35 and 55 °C. Moreover, the present study has provided new insight into the analysis of temperature-dependent spectral variations of oleic acid. Among the three temperature ranges, 16–35 °C, 35–55 °C, and 55–79 °C, in the first range the band near 6915 cm⁻¹ shows a slight increase and in the second range it has a linear intensity change with a slope of 0.002 a.u./°C. In the third range, a rapid increase of the peak intensity is observed. This band exists even at 15 °C (just below the melting point) and shows a shift from 6910 to 6915 cm⁻¹ and a band narrowing from 85 to 80 cm⁻¹ (full width at half-height) over a temperature range of 16 to 79 °C. Furthermore, it has been found that there are two broad bands at around 6835 and 6778 cm⁻¹.