Thermal properties, structure, and dynamics of supercooled water in porous silica of two different pore sizes (30 and 100 Å) have been investigated over a temperature range from 298 down to 193 K by differential scanning calorimetry (DSC), neutron diffraction, neutron quasi-elastic scattering, and proton NMR relaxation methods. Cooling curves by DSC showed that water in the 30 Å pores freezes at around 237 K, whereas water in the 100 Å pores does at 252 K. Neutron diffraction data for water in the 30 Å pores revealed that with lowering temperatures below 237 K hydrogen bond networks are gradually strengthened, the structure correlation being extended to 10 Å at 193 K. It has also been found that crystalline ice is not formed in the 30 Å pores in the temperature range investigated, whereas cubic ice (I c) crystallizes in the 100 Å pores at 238 K. The self-diffusion coefficients of water protons in both pores determined from the quasi-elastic neutron scattering measurements showed that the translational motion of water molecules is slower by a factor of two in the 30 Å pores than in the 100 Å pores, the motion of water molecules in the 100 Å pores being comparable with that of bulk water. The self-diffusion coefficients of water in both pores at different temperatures showed that the translational motion of water molecules is gradually restricted with decreasing temperature. The spin-lattice relaxation time (T 1) and the spin-spin relaxation time (T 2) data obtained by the proton NMR relaxation experiments suggested that the motions of water molecules in the 100 Å pores are faster by a factor of 2−3 than those of water molecules in the 30 Å pores. The peak area, the half-width at half maximum, the relaxation rates (T 1 -1 and T 2 -1) of water molecules at the various temperatures all showed an inflection point at 238 and 253 K for the 30 and 100 Å pores, respectively, suggesting the freezing of water in the pores.

The structure of clusters in ethanol–water binary solutions at xE (ethanol mole fraction) ≥ 0.2 was investigated by the mass-spectrometric analysis of clusters isolated from liquid droplets and X-ray diffraction measurements of the intact solution. The average number of water molecules (Na) of the ethanol m-mer hydrates (5 < m < 15) in the mixtures at 0.2 ≤ xE ≤ 0.8 was found to be expressed by a function of the water mole fraction (xw), Na = {(m/4.2) − 1}2 (xw + 0.85), suggesting that the fundamental structure of ethanol polymer-hydrates is invariable in this region. An inflection point of Na was found at xE ≈ 0.2. The radial distribution functions (RDFs) from X-ray measurements demonstrated a decrease of linear hydrogen bonds at 2.8 Å with increasing xE, while keeping the strongest peak at 4.8 Å for 0.4 ≤ xE ≤ 1.0. On the basis of a composition analysis of the mass spectra and the concentration dependence of RDFs, we propose the most likely model of ethanol–water binary clusters formed in the region 0.2 ≤ xE ≤ 0.8.

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Articles you may be interested inA hybrid neutron diffraction and computer simulation study on the solvation of N-methylformamide in dimethylsulfoxide J. Chem. Phys. 138, 044502 (2013); 10.1063/1.4773346Ion solvation dynamics in water-methanol and water-dimethylsulfoxide mixtures The solvation of chloride by methanol-surface versus interior cluster ion states Pulsed neutron diffraction measurements have been carried out on 8.6, 5.8, and 1.7 molar lithium chloride ͑LiCl͒ solutions in deuterated water (D 2 O), methanol-d 4 ͑MeOD͒, and N,N-dimethylformamide-d 7 ͑DMF͒, respectively. A first-order difference method with chlorine isotopes substitution was used to derive the Cl Ϫ -dependent partial structure factors and radial distribution functions. The oscillation patterns of all Cl Ϫ -related structure factors normalized by the concentration persist up to the high momentum transfer region ͑ϳ10 Å Ϫ1 ͒, suggesting the presence of the short-range ordering around chloride ion in the three solvent systems. The normalized radial distribution functions have revealed that methanol molecules are hydrogen bonded to a chloride ion with almost linear orientation of Cl•••D-O, as in the case of chloride hydration. The nearest-neighbor Cl-D distance and the solvation number for Cl Ϫ in the methanol solutions were determined as 2.21Ϯ0.03 Å and 3.6Ϯ0.5, respectively, compared with 2.29Ϯ0.01 Å and 5.8Ϯ0.5 for the aqueous solutions. The smaller solvation number for Cl Ϫ in the methanol solutions suggests that an Li ϩ -Cl Ϫ ion association takes place in the solutions. In the DMF solutions, the first peak was observed at a much longer distance, ϳ2.85 Å, and assigned to the distance between Cl Ϫ and the formyl H atoms of the DMF molecule due mostly to the ion-dipole interaction. The number of DMF molecules around the chloride ion was estimated as 6.8Ϯ0.5. The most likely conformations of the solvent molecules around the chloride ion are proposed and discussed on the basis of the solvent properties.