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 co-precipitation method. The samples have the following general formula [Zn1−xAlx(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 pKa1int, pKa2int in 2-pK model) of the samples were determined by applying potentiometric titration (PT). It is found that the values of the pHPZNC, pK  , pKa1int and pKa2int decrease linearly with the increase of structural charge density (σst). The interrelations of the intrinsic equilibrium constants with the pHPZNC are also demonstrated. Similar to oxides without structural charges, the fairly linear dependence of the pK  , pKa1int and pKa2int on pHPZNC for HTlc samples with structural positive charges is also observed.