J E Enderby

The hydration of Dy3+ and Yb3+ in aqueous solution: A neutron scattering first order difference study

The coordination of water molecules around the metal ion in acidified 1 m DyCl3, 1.0 m Dy(ClO4)3, 0.3 m Dy(ClO4)3, and 1.0 m Yb(ClO4)3 solutions in D2O have been determined by the neutron diffraction difference technique. A coordination number of eight is found for the two ions, which depends neither on the counterion, nor on the concentration. The near metal–oxygen and metal–deuterium distances are, respectively, 2.39±0.02 and 3.03±0.02 A for dysprosium and 2.33±0.02 and 2.98±0.02 A for ytterbium. A general discussion concerning the hydration of the lanthanide ions is given.

Lithium ions in aqueous solution

The nature of hydration around Li+ in two concentrations of LiCl in D2O has been investigated by the methods of neutron diffraction and isotopic substitution. It is found that the hydration number varies with concentration, and that the angle of tilt between the Li-O axis and the plane of the water molecule, though substantial, is less than that expected if a lone pair orbital of the oxygen atom is directed towards the Li+ ion.

The structure of molten zinc chloride

The structure of molten ZnCl2 has been investigated by applying the technique of neutron diffraction to isotopically enriched samples. The three partial structure factors relating to Zn-Zn, Cl-Cl and Zn-Cl correlations have been successfully extracted from the experimental data. It is concluded that the correct structural description of the melt is in terms of Zn2+ and Cl- ions rather than the range of complex ionic species which have earlier been proposed. It is further shown that specific chemical bonding effects associated with the Zn-Cl interaction make a very minor contribution to the structure.

The structure of an aqueous solution of nickel chloride

The method of multipattern analysis based on neutron diffraction from isotopically different samples has been applied to a 4.35 molal solution of NiCl2 in D2O. Seven distinct scattering patterns were obtained at a temperature of 21 °C and the particle-particle pair functions for correlations between Cl- and D2O, Ni2+ and D2O, Cl- and Cl-, Ni2+ and Ni2+ and Ni2+ and Cl- were extracted from the data. The results are used to obtain a real-space description of the ions and their immediate environment.

The structure of Cl- in aqueous solution: an experimental determination of gClH(r) and gClO(r)

The first and second isotopic difference methods of neutron diffraction have been applied to solutions of 2 molal NiCl2 in H2O and 1.8 molal NiCl2 in D2O. The results were used to obtain the radial pair distribution functions gClH(r) and gClO(r). Analysis of these functions shows that Cl- is coordinated to 6.4(3) water molecules at near-neighbour Cl-H(1), Cl-O and Cl-H(2) distances of 2.28(3) AA, 3.1(1) AA and 3.7(1) AA respectively. The data transcend those obtained in previous studies, which were carried out only at the level of the first difference, and so provide a sharper test of theoretical results obtained from computer simulation or integral equations.

Ion hydration in aqueous CaCl2 solutions

The neutron first-order difference method has been applied to a 4.49 molal aqueous solution of CaCl2. Isotopic changes were made in both the Ca2+ ion and the Cl- ion so that a complete picture of ionic hydration could be obtained. Both ions coordinate water in a non-dipolar configuration with ion-oxygen distances close to those predicted by gas-phase quantum molecular calculations. The measured hydration numbers for cations and anions (5.5+or-0.3 and 5.8+or-0.3 respectively) imply substantial sharing of water molecules between the ions.

The structure of a highly concentrated aqueous solution of lithium chloride

The structure of a concentrated aqueous solution of LiCl has been investigated by neutron diffraction. The methods of first- and second-order differences were used. It is concluded that structural features characteristic of both the molten salt and the dilute solution coexist in concentrated solution and that each Li+ ion is surrounded, on average, by three water molecules and three chloride ions.

Comments on the structure of molten salts

The structure of molten NaCl, previously published by Edwards et al. (see ibid., vol.8, p.3483, 1975) has been reconsidered in the light of new values for the neutron scattering amplitudes of 35Cl and 37Cl. The conclusions reached by Edwards et al. about the general properties of ionic melts have been confirmed although some specific conclusions about NaCl need slight revision. The determination of experimental coordination numbers from total diffraction patterns is also discussed and it is shown that serious errors may arise when the melt contains large cations.

The structure of Cu2+ aqueous solutions

The neutron first-order difference method has been used to determine the Cu2+-D2O and Cl--D2O coordination in 4.32 molal (mol kg-1) CuCl2 and the Cu2+-D2O coordination in 2.00 molal Cu(ClO4)2. In the copper chloride solution the Cu2+ near-neighbour distances are given by rCuO=1.96+or-0.03 AA and rCuD=2.54+or-0.03 AA and the hydration number is nO=3.4+or-0.2. The Cl- coordination is described by distances rClD(1)=2.27+or-0.03 AA and rCuO=3.25+or-0.05 AA and a hydration number nD=3.3+or-0.4. In the case of the perchlorate solution a Cu2+ near-neighbour distance of rCuO=1.96+or-0.03 AA is found with nO=4.1+or-0.3. The observed structure is discussed with reference to the ion-water dynamics and is contrasted with that of Ni2+ aqueous solutions.

The dynamics of water molecules in ionic solution

High-resolution neutron quasi-elastic scattering has been applied to the problem of aqueous solutions. It is shown that for ions for which the primary hydration shell is in slow exchange with the remaining water, the frequently used two-state model is not appropriate. The experiments provide unambiguous evidence that the dynamics of water molecules other than those in the first shell are affected by the presence of ions.