Hydrogen bonding of water in aqueous alcohol solution: Spectral analyses by an extended concept of molar absorption coefficient

Abstract

The hydration of monovalent alcohols (methanol, ethanol, 1-propanol, 2-propanol and tertbutanol), which are perfectly soluble in water, has been studied by using near infrared spectroscopy by focusing on stretching vibrations of each component at around 7000 cm-1. The absorbance of the band ordinarily does not obey Beer's law because of the presence of intermolecular interactions, e.g. hydrogen bonding. To estimate the spectra of non-ideal solutions, such as water-alcohol mixtures, partial molar absorptivity (PMA) is newly proposed as an extended concept of conventional molar absorption coefficient. A PMA is defined as the concentration differentiation of absorbance. Subsequently, the non-ideal spectral components, called excess partial molar absorptivity (EPMA), are calculated by subtraction between real and ideal molar absorption spectra. EPMA reveals the hidden absorption peaks assigned to different hydrogen-bonding species. By careful examination of PMA and EPMA spectra, strengthening of hydrogen bonding is found in low alcohol range less than a well-known breakpoint at around 25 mass% as a result of hydration to the solute molecules. In a concentration range higher than the breakpoint, water and alcohol tend to be separated into each small domain. These results are common in the monovalent alcohol-water binary mixtures. © 2010 The Japan Society for Analytical Chemistry.