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.

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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.

Equilibrium study of ion-pair extraction of a cationic water-soluble porphyrin [5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, H(2)tmpyp(4+)] and its metalloporphyrins (MP) into the acetonitrile layer, separated by addition of sodium chloride (4.00 mol dm(-)(3)) to a 1:1 (v/v) acetonitrile-water mixed solvent, was carried out to develop a new and useful method for the determination of a subnanogram amount of copper(II). M denotes Zn(2+), Cu(2+), Co(3+), Fe(3+), and Mn(3+), and P(2)(-) is porphyrinate ion. The extraction and dissociation constants of the ion-pair complexes, defined by K(ex) = [MP(ClO(4))(4)](org)[MP(4+)](aq)(-)(1)[ClO(4)(-)](aq)(-)(4), K(dis,1) = [MP(ClO(4))(3)(+)](org)[ClO(4)(-)](org)[MP(ClO(4))(4)](org)(-)(1), and K(dis,2) = [MP(ClO(4))(2)(2+)](org)[ClO(4)(-)](org)[MP(ClO(4))(3)(+)](org)(-)(1), were determined by taking into account the partition constant of sodium perchlorate (K(D) = 1.82 ± 0.01). The equilibrium constants were found to be K(ex)K(dis,1) = (7.2 ± 1.3) × 10(4), (6.4 ± 0.9) × 10(4), (1.35 ± 0.13) × 10(5), (4.8 ± 0.6) × 10(3), (1.23 ± 0.05) × 10(4), and (1.42 ± 0.07) × 10(3) at 25 °C for the free base porphyrin (H(2)tmpyp(4+)) and the metalloporphyrins of zinc(II), copper(II), cobalt(III), iron(III), and manganese(III), respectively. The K(dis,2) values were (2.9 ± 1.4) × 10(-)(2), (3.1 ± 1.1) × 10(-)(2), (8.0 ± 4.9) × 10(-)(3), and (5.1 ± 2.2) × 10(-)(2) for the free base porphyrins and the metalloporphyrins of zinc(II), copper(II), and cobalt(III), respectively. The results were developed for determination of a trace amount of copper(II) (3 × 10(-)(8)-4 × 10(-)(6) mol dm(-)(3)) in natural water samples using H(2)tmpyp(4+) with a molar absorptivity of 3.1 × 10(5) mol(-)(1) dm(3) cm(-)(1) at a precision of 1.3% (RSD). The determination of copper(II) was not interfered by the presence of 10(-)(4) mol dm(-)(3) of Mn(2+), Co(2+), Ni(2+), Hg(2+), Cd(2+), Ag(+), Cr(3+), V(5+), Al(3+), Mg(2+), Ca(2+), Br(-), I(-), SCN(-), and S(2)O(3)(2)(-) and 10(-)(5) mol dm(-)(3) of Fe(3+), Zn(2+), and Pd(2+).

Fourteen water-miscible polar solvents were investigated for the separation from their aqueous solutions by salting-out using sodium chloride (4 mol dm-3). The following solvents showed the phase separation: acetone , acetonitrile, 1,4-dioxane, tetrahydrofuran, l-propanol, and 2-propanol. The chemical properties of the separated organic solvents were determined by measuring ET(30) (1.196X105/2 (kJ mol-')) and D11 ,1 (=1.196X105 ()i'1) (kJ mol-')) values from the spectral change of 2,6-Biphenyl-4-(2,4,6-triphenylpyridinio)phenolate (DTP) and bis(1,3-propanediolato)vanadium(IV) (VO(acac)2), where 2, A,, and 2" denote the absorption maximum wavelengths (nm) of DTP and VO(acac)2. Solvent properties of acetone, acetonitrile, 1,4-dioxane, and tetrahydrofuran were dramatically altered by the saltingout. Acceptability of the phase-separated solvents increased due to the dissolution of water molecules having large acceptor numbers. The ion-pair complex of tris(1,10-phenanthroline)iron(II) chloride was easily extracted into the phase-separated acetonitrile by the salting-out. Some metal chelates of 1-(2-pyridylazo)-2-naphthol (Hpan) and 8-quinolinol (Hox), 5,10,15,20-tetraphenylporphyrin (H2tpp), and ionic species (H2ox+, ox, and H4tpp2+) were also extracted into 1,4-dioxane. The raised donor and acceptor abilities of the phase-separated solvents allowed application to solvent extraction.Keywords Salting-out, water-miscible solvents, ET(30), solvent extractionThe salting-out technique has long been used for extraction of metal-chelates, ion-pairs, or organic materials, prior to atomic absorption spectrophotometry1, high-performance liquid chromatography2'3, polarography4, and absorption spectrophotometry.5 More recently, the salting-out using a surfactant of a watersoluble polymer has been used for preconcentration of porphyrins.6'' Application of the salting-out to solvent extraction of ionic species will become easier when we understand the chemical properties of the phaseseparated solvents, because the solvents will have high polarity resulting from dissolution of water and electrolyte into the solvents by salting-out, in addition to water-miscible solvents themselves.Several mechanisms have been proposed for the effects of electrolytes on the salting-out of water-miscible solvents. For example, some electrolytes have been classified as having either salting-out or salting-in effects and ranked according to their salting strength. Most of theories concerned with the salting-out effect have used salting-out coefficient (Setschenow constant) defined as ks=1/m(log So/S), where So and S are the solubilities of the organic solvent in water and in an electrolyte solution of molality m, respectively.g-10 However, microscopic properties like ET(30)11 of the solvents phase-separated by the salting-out have not been studied.In this paper we describe the chemical properties as donor and acceptor abilities of the phase-separated solvents by salting-out, i.e. ET(30) (=1.196X105/, (kJ moL1)) and D11,1 (=1.196X105(211-1-21-1) (kJ mol...

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