Spectroscopic studies of aqueous gallium(III) and aluminum(III) citrate complexes.

Aqueous gallium(III) citrate complexes have been studied in the 10(-2) M concentration range with extended X-ray absorption fine structure (EXAFS) and FTIR techniques. From EXAFS data, one mononuclear and one oligomeric species were identified at different Ga(III) to citrate ratios. The first shell of the mononuclear complex was found to be distorted, with average Ga-O bond lengths of 1.95 and 2.06 A, in agreement with the solid-state structure of Ga(Cit)2(3-) (Cit=citrate). Also the oligomeric species was found to have a distorted first shell, with average Ga-O bond lengths of 1.95 and 2.04 A. This complex was found to contain two Ga-Ga distances at 3.03 and 3.56 A, typical for edge and corner sharing GaO6 octahedra, respectively. The gallium(III) and aluminum(III) citrate systems were compared by means of FTIR, and were found to be analogous. The IR results suggest that the bond lengths derived from EXAFS for the 1:2 gallium(III) citrate complex also provide a good estimate of the corresponding distances in the mononuclear 1:1 complex. Direct coordination of citrate to the metal ions in the oligomeric gallium(III) citrate complex was indicated from both EXAFS and IR results, and this complex is stoichiometrically analogous to the Al3(H-1Cit)3(OH)(H2O)4- complex, which has been structurally determined. However, while the formation of the aluminum trimer has been shown to be slow, the gallium trimer was significantly more labile with a rate of formation indicated to be in the order of seconds or faster.

Interactions between GaO4Al12(OH)24(H2O)12(7+) and cellulosic materials.

The adsorption qualities of GaO(4)Al(12)(OH)(24)(H(2)O)(12)(7+), a polycation with ε-Keggin structure, and its stability in contact with anionic cellulosic materials, was investigated under different concentration and ionic strength conditions. The cellulosic materials employed were two different fully bleached fibre materials, carboxyl methyl cellulose (CMC), and a spin-coated cellulose model surface. As analytical techniques, pH-measurements, potentiometric titrations, ICP-OES, QCM-D, equilibrium calculations and Extended X-ray Absorption Fine Structure (EXAFS) were used. The adsorption is substantial and the addition of GaO(4)Al(12)(OH)(24)(H(2)O)(12)(7+) to a fibre suspension results in a rapid decrease in pH, followed by a small and slow increase in pH. This behaviour can be explained as due to a rapid and strong (log ß>2) equilibrium adsorption of intact GaO(4)Al(12)(OH)(24)(H(2)O)(12)(7+) ions, followed by a slow, and minor, 3-8%, decomposition into different monomers. Alternative layer by layer adsorption of this ion, and CMC, on a spin-coated cellulose model surface constitutes further evidence for the strong interactions between the anionic cellulose materials and GaO(4)Al(12)(OH)(24)(H(2)O)(12)(7+). It is shown that the adsorption observed could not be described as due to an unspecific Donnan adsorption behaviour, neither of GaO(4)Al(12)(OH)(24)(H(2)O)(12)(7+) nor Ga and Al monomers, and specific surface complex formation is therefore discussed and applied. The (≡COO)(7)GaO(4)Al(12)(OH)(24)(H(2)O)(12) species found to explain the pH- and metal adsorption data should be considered strictly as a stoichiometric entity.

A multitechnique study of the interactions between H+, Na+, Ca2+ and Cu2+, and two types of softwood Kraft fibre materials.

Aiming for a better understanding of the interactions between water suspended cellulose fibres and metal ions, this study was focused on characterising the interactions between Ca2+, Cu2+ and two different fibre materials--a fully bleached softwood Kraft pulp, and a chemically modified fully bleached softwood Kraft fibre material. The study was conducted as a function of pH (2-7), and both in the absence and presence of an excess of Na+ ions, 0-100 mM Na(Cl). For both fibre materials, adsorption data collected in the absence of Na+ were fully explained by the unspecific Donnan ion-exchange model. However, in an excess of Na(Cl), data clearly indicated that higher amounts of divalent metal ions adsorbed, than predicted by the Donnan model. Therefore, to model these data, specific metal ion-fibre surface complexes were assumed to form, in addition to the Donnan ion-exchange. A neutral surface species involving two surface carboxylate groups and one metal ion was, for both metal ions, found to yield a good description of data at all ionic strengths. In the case of Cu2+, the existence of this complex was corroborated by Cu K-edge EXAFS data, suggesting that copper ions interacts directly with carboxyl groups present int the fibres. EXAFS data also indicate that one Cu2+ interacts with two carboxyls.