Mesoscopic structuring and dynamics of alcohol/water solutions probed by terahertz time-domain spectroscopy and pulsed field gradient nuclear magnetic resonance.

Terahertz and PFG-NMR techniques are used to explore transitions in the structuring of binary alcohol/water mixtures. Three critical alcohol mole fractions (x1, x2, x3) are identified: methanol (10, 30, 70 mol %), ethanol (7, 15, 60 mol %), 1-propanol (2, 10, 50 mol %), and 2-propanol (2, 10, 50 mol %). Above compositions of x1 no isolated alcohol molecules exist, and below x1 the formation of large hydration shells around the hydrophobic moieties of the alcohol is favored. The maximum number of water molecules, N0, in the hydration shell surrounding a single alcohol molecule increases with the length of the carbon chain of the alcohol. At x2 the greatest nonideality of the liquid structure exists with the formation of extended hydrogen bonded networks between alcohol and water molecules. The terahertz data show the maximum absorption relative to that predicted for an ideal mixture at that composition, while the PFG-NMR data exhibit a minimum in the alkyl chain self-diffusivity at x2, showing that the alcohol has reached a minimum in diffusion when this extended alcohol-water network has reached the highest degree of structuring. At x3 an equivalence of the alkyl and alcohol hydroxyl diffusion coefficients is determined by PFG-NMR, suggesting that the molecular mobility of the alcohol molecules becomes independent of that of the water molecules.