Effects of structural charges on points of zero charge and intrinsic surface reaction equilibrium constants of Zn-Al and Zn-Al-Fe hydrotalcite-like compounds

A series of Zn-Al and Zn-Al-Fe hydrotalcite-like compounds (HTlc) with different n(Zn)/n(Al)/n(Fe) molar ratios are synthesized by using coprecipitation method. The samples have the following general formula [Zn1-x,Al-x(OH)(2)](x+)[(OH, Cl)(x)](x-) or [Zn1-x(Al, Fe)(x)(OH)(2)](x+)[(OH, Cl)(x)](x-). The points of zero net charge (PZNC) and the intrinsic surface reaction equilibrium constants (pK in 1-pK model and pK(a1)(int) , pK(a2)(int), in 2-pK model) of the samples were determined by applying potentiometric titration (PT). It is found that the values of the pH(PZNC), pK(a1)(int) and pK(a2)(int) decrease linearly with the increase of structural charge density (sigma(st)). The interrelations of the intrinsic equilibrium constants with the pH(PZNC) are also demonstrated. Similar to oxides without structural charges, the fairly linear dependence of the pK, pK(a1)(int) and pK(a2)(int) on pH(PZNC) for HTlc samples with structural positive charges is also observed. (c) 2006 Elsevier B.V. All rights reserved.