POTENTIAL OF FOURIER-TRANSFORM NEAR-INFRARED SPECTROSCOPY IN STUDIES OF THE DISSOCIATION OF FATTY-ACIDS IN THE LIQUID-PHASE

Fourier transform near-infrared (FT-NIR) technique has been employed t o determine the degree of dissociation and the thermodynamic parameter s of octanoic acid in the liquid phase. FT-NIR absorption spectra of t he acid over a temperature range of 15 to 92-degrees-C were recorded. From the spectra, intensities of the first overtone of the OH stretchi ng mode due to the monomer form of the acid were extracted and were us ed to calculate the degree of dissociation and the thermodynamic param eters for the process of dissociation of dimeric octanoic acid into mo nomer species. In the 7400-6700 cm-1 region there appear many bands th at are heavily overlapped; the attempts to apply curve-fitting and dec onvolution algorithms to separate individual bands have failed. Here, a simple and reliable method of calculation of the intensities of the first overtone of the OH stretching mode due to the monomer is propose d. Also, an excellent procedure of determining the molar absorption co efficients (integrated and at peak position) through a series of measu rements in CCl4 solution at very low concentrations is presented. The bands due to CH vibrations were eliminated in two ways; one way involv ed taking the concentration as a reference, and the other used the ban d area of the second overtone of the CH stretching modes as a referenc e. In the operation the spectrum of neat octanoic acid at 15.1-degrees -C was employed. The coefficients obtained by both methods are compare d and discussed. The present studies indicate that both band areas and peak heights give results which are found to be in very good agreemen t.