The structure of acetonitrile−water mixtures has been investigated by X-ray diffraction with an imaging plate detector and IR spectroscopy over a wide range of acetonitrile mole fractions (0.0 ≤ X AN ≤ 1.0). Reichardt E T N and Sone-Fukuda D II,I values were also measured for the mixtures. It has been found from the X-ray data that in pure acetonitrile an acetonitrile molecule interacts with two nearest neighbors by antiparallel dipole−dipole interaction together with a small shift of the two molecular centers and that two acetonitrile molecules in the second-neighbor shell interact with a central molecule through parallel dipole−dipole interaction. Thus, acetonitrile molecules are alternately aligned to form a zigzag cluster. On addition of water into pure acetonitrile, water molecules interact with acetonitrile molecules through a dipole−dipole interaction in an antiparallel orientation. The IR spectra of O−D and C⋮N stretching vibrations, observed for mixtures of acetonitrile AN and water containing 20% D2O, suggested that hydrogen bonds are also formed between acetonitrile and water molecules in the mixtures at X AN ≤ 0.8. The average numbers of the first- and second-neighbor acetonitrile molecules gradually increase with increasing water content with an almost constant first-neighbor distance and slightly decreased second-neighbor ones. Thus, acetonitrile molecules are assembled to form three-dimensionally expanded clusters; the acetonitrile clusters are surrounded by water molecules through both hydrogen bonding and dipole−dipole interaction. The X-ray radial distribution functions and IR spectra suggest that the hydrogen bond network of water is enhanced in the mixtures at X AN < 0.6. The concentration dependence of E T N and D II,I values determined reflects well the above-mentioned behavior of water molecules in the mixtures. These findings suggest that both water and acetonitrile clusters coexist in the mixtures in the range of 0.2 ≤ X AN < 0.6, i.e., “microheterogeneity” occurs in the acetonitrile−water mixtures.

The structure of water in the liquid and supercritical states has been investigated with a newly developed rapid liquid and amorphous x-ray diffractometer using an imaging plate area detector. This new method has enabled us to reduce the measuring time to only one hour for each sample, which is less by a factor of about 100 than the time usually needed with a conventional θ–θ type diffractometer, and thus to measure x-ray scatterings of water at high temperatures and pressures, including supercritical state. In this study the temperature range of 300–649 K with pressures of 0.1–98.1 MPa was covered (Tc=647.3 K, Pc=22.12 MPa, ρc=0.322 g/cm3 for water). Densities of sample water were kept constant at 1.0, 0.95, 0.9, 0.8, and 0.7 g/cm3 by controlling temperature and pressure. The radial distribution functions (RDFs) have shown that the peaks for the second and further neighbors interactions disappear over 416 K and 0.95 g/cm3, showing the breakdown of local tetrahedral icelike structure in water. The analysis of the first peak of the RDFs has revealed that with increasing temperature the coordination number of the first neighbor interaction around 2.9 Å decreases from 3.1 at 300 K and 1.0 g/cm3 to 1.6 at 649 K and 0.7 g/cm3, whereas the interaction around 3.4 Å increases from 1.3 to 2.3 at the corresponding temperatures, resulting in a constant coordination number of around four in the first shell under the nearly constant densities. These findings are discussed with the recent results of computer simulation, neutron scattering, and Raman spectroscopic studies on water at high temperatures and pressures.

Extended x-ray absorption fine structure (EXAFS) measurements were performed for concentrated aqueous rare earth perchlorate solutions (R=28; R is the moles of water per mole of salt) in the liquid state at room temperature and in the glassy state at liquid nitrogen temperature. The quantitative analysis of the EXAFS data has revealed that the hydration number changes from about nine for light rare earth ions to about eight for heavy rare earth ions through the intermediate ions of Sm3+ ∼Eu3+ in both liquid and glassy states. The average Ln3+ –OH2 distances were determined and they are in agreement with previously reported values from x-ray and neutron diffraction. The Debye–Waller factor of the average Ln3+ –OH2 bonds for the light rare earth ions was larger than that for the heavy ions, suggesting that the hydration shell of the light rare earth ions is statically disordered, consisting of different Ln3+ –OH2 bonds.

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.