Thermal Properties and Mixing State of Diol-Water Mixtures Studied by Calorimetry, Large-Angle X-Ray Scattering, and NMR Relaxation

Differential scanning calorimetry (DSC) has been performed on aqueous mixtures of three diols, which involve a linear carbon chain, HO-(CH2)(n)-OH (n = 3, 4, and 5), over the whole mole fraction range of diols. The DSC results have shown the alkyl chain parity for the freezing process of the aqueous mixtures: aqueous mixtures of 1,3-propanediol (PrD) and 1,5-pentanediol (PeD) are kept in the supercooled state or vitrified over a wide mole fraction range, while those of 1,4-butanediol (BuD) are easily crystallized. The structure of PrD-water mixtures has been elucidated by using the large-angle X-ray scattering (LAXS) technique. It has been suggested that the structural change of PrD-water mixtures occurs at PrD mole fractions of x(PrD) = 0.4 and 0.8: in the range of x(PrD) <= 0.4 where the tetrahedral-like structure of water predominates, in the range of 0.4 < x(PrD) < 0.8 where both PrD and water structures coexist, and in the range of x(PrD) >= 0.8 where the inherent structure of PrD is mainly formed. O-17 and H-1 NMR relaxation measurements have been made on aqueous mixtures of ethylene glycol (EG, n = 2), PrD, and BuD to clarify the dynamics of H,170 and diol molecules. The O-17 NMR relaxation rates have suggested that the rotational motion of water molecules is gradually retarded in the diol-water mixtures with increasing diol content and that the restriction of the motion is more remarkable in the order of EG < PrD < BuD. On the basis of all the results, together with comparison with those of methanol-water, ethanol-water, and 1-propanol-water mixtures previously reported, the mixing state of diol-water mixtures has been discussed at the molecular level.