Liquid Structure of Acetonitrile−Water Mixtures by X-ray Diffraction and Infrared Spectroscopy

Abstract

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.

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

“…22,52 On the other hand, in addition to electrostatic ion-dipole interactions, also hydrogen bonding between [EtNH 3 ] + and the nitrile group of AN is possible, similar to what was observed for AN + water mixtures. 53,54 Thus, it appears reasonable to attribute also the present Z ib values mainly to cation-solvent interactions. From its definition Z ib is the number of ''missing'' AN molecules per EAN ion pair, i.e.…”

Thermodynamics of Complexation between Thiourea-based Receptor and Acetate in Water/Acetonitrile Mixture

“…16,17 Water molecules exist as free water among the MeCN clusters. 17 Since the amounts of the receptor and acetate are much smaller than those of water and MeCN under our experimental conditions, the presence of receptor and acetate would not be expected to influence these solvent structures.…”

“…16,17 Water molecules exist as free water among the MeCN clusters. 17 Since the amounts of the receptor and acetate are much smaller than those of water and MeCN under our experimental conditions, the presence of receptor and acetate would not be expected to influence these solvent structures. The water molecules present on the MeCN clusters are likely to preferentially hydrate the thiourea moiety of BNPTU due to its larger acceptor number (54.8) than acetonitrile (19.3), 17 and would also preferentially hydrate the carboxylate moiety of acetate.…”

“…Complexation induces the release of a number of hydrated water molecules in the water/MeCN mixture. Since the released water molecules are present as free water among the MeCN clusters, 17 the release of the hydrated water molecules could be a contributory factor to the entropy-driven complexation reaction. Hydration is thus likely responsible for the endothermic and entropy-driven complexation of BNPTU with acetate in the water/MeCN mixture.…”

Enhanced Separation and Elution of Catechins in HPLC Using Mixed-Solvents of Water, Acetonitrile and Ethyl Acetate as the Mobile Phase

“…23 Since the volume ratios of 10 and 30% (v/v) of AN correspond to 0.037 to 0.129 in mole fraction of AN (XAN), under the conditions AN molecules do not assemble with each other. AN molecules bind to water molecules through both a dipole-dipole interaction and hydrogen bonding.…”