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The self-associations of butan-1-ol, butan-2-ol, and 2-methylpropan-2-ol ( tert-butanol) in the pure liquid state and in carbon tetrachloride solutions have been studied mainly through near-infrared spectroscopic observation at various temperatures. A new analysis assuming a successive association process for the alcohol molecules was applied to the sharp band around 1410 nm (the first-overtone band of the OH stretching vibration mode attributed to free OH monomer and partly to OH polymer); it became clear that the mean association number for each alcohol increases with increasing concentration and decreases with increasing temperature. Comparisons of the association numbers at various constant temperatures for the three kinds of alcohols show that the association abilities are on the order butan-1-ol > butan-2-ol > 2-methylpropan-2-ol.

The first paper in a series devoted to self-association in neat butanols presents the results of two-dimensional (2D) near-infrared (NIR) correlation analysis of temperature-induced spectral variations of sec-butanol. By taking advantage of resolution enhancement in the 2D correlation spectra, it was possible to identify spectral features due to vibrations of the free and associated OH groups in the first-overtone region. On the basis of a few assumptions, band assignments of the various types of OH bonds have been proposed. The monomer band (near 7100 cm−1) can be resolved into three components; two of them are due to a rotational isomerism (7089 and 7116 cm−1), and the third one is attributed to the free terminal OH groups in linear polymers (7055 cm−1). The presence of the 7055 cm−1 band implies that the intensity of the monomer peak cannot be used as a measure of the concentration of the monomer species (except in very diluted solutions). Thus, previous estimations of equilibrium constants and thermodynamic parameters associated with hydrogen-bond dissociation have been subject to unacceptable error. At higher temperatures, a new band near 6550 cm−1 becomes visible. This band originates from bended OHO bond, mostly in the cyclic polymers. In order to obtain more detailed information on the complex mechanism of the thermal dissociation of hydrogen-bonded sec-butanol in the pure liquid phase, the entire experimental temperature range was divided into narrower ranges, and then 2D correlation analysis was performed for smaller data sets. It has been shown that the variations of population of the polymeric species and the cyclic dimers are faster than the corresponding changes for the monomers. At elevated temperatures an appreciable dissociation of the cyclic species takes place.

This paper demonstrates the usefulness of principal component analysis (PCA) in the study of the dissociation process of oleyl alcohol ( cis-9-octadecen-1-ol) as the pure liquid. Fourier transform near infrared (FT-NIR) spectra of the neat alcohol were measured over a temperature range of 6.5–90°C in the 11,500–6000 cm−1 region. The degree of dissociation of the alcohol was determined by the intensities of the first and second overtones of the OH stretching mode of the monomer. PCA on the obtained spectra were calculated over three different regions, the entire 11,500–6000 cm−1 region and the 11,500–9087 and 9087–6000 cm−1 regions. PCA clearly reveals that there exists a break point at around 45°C in the dissociation process of the alcohol.

The study of the infrared spectra of neat cis-9, cis-12-octadecadienoic acid in the liquid phase has been carried out over a temperature range of 31 to 110°C. In the region of 1710 cm−1 a strong absorption band accompanied by two weaker bands appears. The strong band is assigned to a C=O stretching vibration due to the dimeric form of the acid, and the weaker bands near 1750 and 1650 cm−1 are attributed to a C=O stretching vibration due to its monomer and to C=C stretching modes arising from both forms, respectively. The shape of the band envelope strongly depends on temperature. Making a curve fitting, one can determine spectral parameters of the individual bands, and from them it is possible to calculate the degree of dissociation of the acid. The obtained values of the degree of dissociation enable us to determine enthalpy (Δ H) and entropy (Δ S) for the process of dissociation of dimeric species into free molecules, and the results seem to be reasonable as compared with the literature data for other fatty acids.

Fourier transform near-infrared (FT-NIR) technique has been employed to determine the degree of dissociation and the thermodynamic parameters of octanoic acid in the liquid phase. FT-NIR absorption spectra of the acid over a temperature range of 15 to 92°C were recorded. From the spectra, intensities of the first overtone of the OH stretching mode due to the monomer form of the acid were extracted and were used to calculate the degree of dissociation and the thermodynamic parameters for the process of dissociation of dimeric octanoic acid into monomer species. In the 7400–6700 cm−1 region there appear many bands that are heavily overlapped; the attempts to apply curve-fitting and deconvolution 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 proposed. Also, an excellent procedure of determining the molar absorption coefficients (integrated and at peak position) through a series of measurements in CCl4 solution at very low concentrations is presented. The bands due to CH vibrations were eliminated in two ways; one way involved taking the concentration as a reference, and the other used the band area of the second overtone of the CH stretching modes as a reference. In the operation the spectrum of neat octanoic acid at 15.1°C was employed. The coefficients obtained by both methods are compared and discussed. The present studies indicate that both band areas and peak heights give results which are found to be in very good agreement.

Recently, Iwahashi et al. demonstrated that near-infrared (NIR) spectroscopy is a powerful technique in investigating the structure and thermodynamic properties of fatty acids and alcohols In order to extract useful information from NIR spectra, they examined temperature behaviors of the first overtone (~7000 cm−1) of the OH stretching mode of the monomeric forms of cis-9-octadecenoic acid and octan-1-ol. Since the first overtone bands are heavily overlapped with bands due to combination modes of CH3 and CH2 groups, they had to subtract the contributions from CH modes to obtain net spectra for the free and hydrogen-bonded OH groups. They adopted a computational method, which employed as a reference the peak heights of the second overtone of the CH stretching mode of decane to cancel the contributions from the CH modes of octan-1-ol. In a companion paper, we proposed an improved method, which eliminates the bands due to the CH vibrations by subtraction of the spectrum of the same sample measured at a temperature close to the melting point. At the same time, we showed that FT-NIR is more profitable than conventional NIR in the studies of such spectral changes; by using FT-NIR one can calculate more accurate difference spectra with a higher signal-to-noise ratio.