Large-Angle X-ray Scattering and Small-Angle Neutron Scattering Study on Phase Separation of Acetonitrile−Water Mixtures by Addition of NaCl

Abstract

Phase separation of acetonitrile-water mixtures by addition of NaCl has been studied on the molecular level by large-angle X-ray scattering (LAXS) and small-angle neutron scattering (SANS) methods. A phase diagram of acetonitrile-water-NaCl mixtures at 298 K has shown that phase separation occurs over a wide range of acetonitrile mole fraction (x AN ) of ∼0.1 < x AN e ∼0.7, where the microheterogeneity of the mixtures occurs. The radial distribution functions obtained by the LAXS measurements have revealed that before phase separation the amounts of preferentially hydrated Na + and Clgradually increase with increasing NaCl concentration and that the number of linear hydrogen bonds among water molecules increases when the concentration of NaCl increases. After phase separation of the acetonitrile-water-NaCl mixtures the structures of the acetonitrile-rich phase are very similar to those of the acetonitrile-water binary mixtures at the corresponding acetonitrile mole fractions. The water-rich phase which contains most of the Na + and Clalso shows structures similar to those of the acetonitrile-water mixtures at the same solvent compositions, except for the hydration structures of Na + and Cl -. The SANS data have shown a change in size of aggregates formed in acetonitrile-D 2 O and acetonitrile-D 2 O-NaCl mixtures before phase separation. The Debye correlation lengths L D determined have demonstrated that aggregation or microheterogeneity in the acetonitrile-D 2 O mixtures is most enhanced with L D ∼ 20 Å between x AN ) 0.3 and 0.4. In the acetonitrile-D 2 O-NaCl mixtures the size of aggregates gradually increases with increasing NaCl concentration and reaches a plateau value L D ∼ 20 Å at x AN ) 0.2 at the salt concentration of ∼80% to a value required for phase separation. A possible mechanism for NaCl-induced phase separation is discussed from the present results.

The Effect of Tetraphenylphosphonium Chloride on Phase Behavior and Nanoscale Structures in a Mixture of D2O and 3-Methylpyridine

“…14 Various experiments have been performed to verify the existence of solvation clusters to explain the phase behavior, and the diameters of the clusters are shown to be on the order of several tens of angstroms. 2,3 Recently, some of the authors showed the opposite phase behavior in a deuterated water (D 2 O)/3-methylpyridine (3MP) binary mixture upon the addition of an antagonistic salt, sodium tetraphenylborate (NaBPh 4 ). NaBPh 4 is composed of hydrophilic cations and hydrophobic anions, 57 and they are distributed heterogeneously in the mixture because they either attract water or 3MP molecules.…”

Multilamellar Structures Induced by Hydrophilic and Hydrophobic Ions Added to a Binary Mixture ofD2Oand 3-Methylpyridine

“…However, not enough attention has yet been paid to unique ion effects in such mixtures, where preferential hydration around each ion should affect the composition fluctuations [1,2]. Salts composed of small hydrophilic ions can drastically change the phase behavior even at small concentrations [3,4,5,6,7]. More strikingly, many groups have observed long-lived heterogeneities (sometimes extending over a few micrometers) in one-phase state [8,9] and a third phase visible as a thin plate formed at a liquid-liquid interface in two-phase state [10].…”

Micro-solvent Cluster Extraction Using Aqueous Mixed Solvents of Ionic Liquid

“…The separation principle is micro-solvent cluster formation in mixed solvents [3][4][5][6] and preferential solvation of the solvent molecules to analytes in the mixed solvents. 7,8 Aqueous mixed solvents, like the mixture of acetonitrile and water, are mixed homogeneously on the macro scale, though the mixtures are heterogeneous on the micro scale.…”

“…The microheterogeneity increased upon the addition of NaCl. 6 In addition, preferential solvation occurs to chemical species dissolved in aqueous mixed solvents. This phenomenon was confirmed by measuring the change in the fluorescence spectrum.…”