PMR studies on the structures of water-ethyl alcohol mixtures

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The PMR signals of each individual proton species present in water-ethanol mixtures were analyzed throughout the entire range of composition. Chemical shifts and line shapes were studied as a function of ethanol mole fraction ( f), temperature, and working frequency, from −50 to +80°C, at 60, 100 and 220 MHz. The experimental results are discussed in terms of possible mechanisms of molecular aggregation in the various regions of ethanol concentration. There is evidence that addition of small quantities of ethanol to water promotes H-bonding association among water molecules (at t = 20°C, f ≲ 0.08). At intermediate ethanol concentrations (at t = 20°C, 0.25 ≲ f ≲ 0.75), the linear dependence of f exhibited the chemical shift of the water signal evidences that the water structures are progressively disrupted by increasing alcohol concentration. At the higher ethanol concentration (at 20°C, f ≳ 0.8) the substantial independence of both ethanol and water O H chemical shifts from the concentration suggests that water molecules either coordinate or/and are incorporated into the linear aggregates peculiar to pure ethanol. The width of the f range in which the hydroxyl signals coalesce is progressively reduced with decreasing temperature. We qualitatively tested the applicability of an adiabatic two-site exchange model to the hydroxyl protons of these mixtures. This model is correctly applicable to solutions at both extremes of the concentration range, but it cannot describe the linewidth behaviour of the mixtures at intermediate concentrations.