Extension of the molar absorption coefficient for non-ideal mixtures: an application to aqueous monovalent alcohol solutions.

The hydration state of the alcohols was investigated using the extended molar absorption coefficient, which redefines the molar absorption coefficient as a differential coefficient of concentration. The extended molar absorption coefficient is a function of the concentration calculated from the difference in absorbance, and is consistent with the conventional molar absorption coefficient, allowing a complete quantitative comparison. The quantitative performance was verified using IR and NIR absorption spectra of aqueous solutions of monovalent alcohols (methanol, ethanol, 1-propanol, 2-propanol, and tert-butanol) that were soluble in water at any mixing ratio. Extended molar absorption coefficient spectra were calculated for the combination bands of water, which were further separated by multivariate curve resolution-alternating least squares (MCR-ALS) into molecular species with different peak wavenumbers: strongly hydrogen-bonded (SHB), weakly hydrogen-bonded (WHB), and free OH species. The number of water species that change when one alcohol molecule increases, i.e., the perturbed hydration number (PHN), was calculated by comparison with the conventional molar absorption coefficient of pure water. The calculated PHN indicates that the numbers of SHB and WHB species are reversed at approximately 20 wt%, and that the free OH species increase at higher alcohol concentrations and are more pronounced for alcohols with bulky alkyl groups. These results provide a quantitative answer to the long-debated question of anomalies in water-alcohol mixing.