ELUCIDATION OF SPECIES IN ALCOHOL-WATER MIXTURES USING NEAR-IR SPECTROSCOPY AND MULTIVARIATE-STATISTICS

High precision near-infrared spectroscopy in the wavelength region 1100-1800 nm was used to provide data on the intermolecular interactions of alcohols and water. This data can be used to further refine alcohol/water composition diagrams as an aid to understanding gradient elution in reversed-phase chromatography using an alcohol/water mixture as a mobile-phase system. The spectral data were analyzed using multivariate model-based regression. Within the approximation of the ideal associated solution model, a lower limit on the number of alcohol/water complexes was determined. Additionally, the spectra, stoichiometry, and equilibrium constants governing the interrelationships of the pure species and their complexes were obtained. The spectroscopic data were consistent with four species in the alcohol/water composition diagrams. While a 1:1 alcohol/water complex is observed at high alcohol concentrations, a complex involving a higher number of waters becomes important at low alcohol concentration. This latter species may explain the unusual elution behavior patterns obtained when using low alcohol concentrations in reversed-phase chromatography.