Interactions between GaO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ and cellulosic materials
2012
Sundman, Ola | Persson, Per | Banerjee, Dipanjan | Öhman, Lars-Olof
The adsorption qualities of GaO₄Al₁₂(OH)₂₄(H₂O)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 GaO₄Al₁₂(OH)₂₄(H₂O)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 β>2) equilibrium adsorption of intact GaO₄Al₁₂(OH)₂₄(H₂O)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 GaO₄Al₁₂(OH)₂₄(H₂O)₁₂ ⁷⁺. It is shown that the adsorption observed could not be described as due to an unspecific Donnan adsorption behaviour, neither of GaO₄Al₁₂(OH)₂₄(H₂O)₁₂ ⁷⁺ nor Ga and Al monomers, and specific surface complex formation is therefore discussed and applied. The (≡COO)₇GaO₄Al₁₂(OH)₂₄(H₂O)₁₂ species found to explain the pH- and metal adsorption data should be considered strictly as a stoichiometric entity.
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