Intrinsic surface reaction equilibrium constants of structurally charged amphoteric hydrotalcite-like compounds

The relative equations among intrinsic surface reaction equilibrium constants (K in 1-pK model, K-a1(int) and K-a2(int) in 2-pK model, and *K-Na(int) and *K-C1(int) in inert electrolyte chemical binding model), points of zero charge (PZC), and structural charge density (sigma(st)) for amphoteric solids with structural charge were established to investigate the effects of sigma(st) on intrinsic equilibrium constants and PZC. The intrinsic equilibrium constants of HTlc with general formulas [(Zn,Mg)(1-x)Al-x(OH)(2)1(Cl,OH)(x)] and [Mg1-x(Fe,Al)(x)(OH)(2)](Cl,OH)(x) were evaluated. The following main conclusions were obtained. For amphoteric solids with structural charge, a point of zero net charge (PZNC) independent of electrolyte concentration (c) exists. A common intersection point (CIP) should appear among the acid-base titration curves at different c, and the pH at the CIP is pH(PZNC). The pK, pK(al)(int), and pK(a2)(int) may be expressed as a function of pH(PZNC) and sigma(st), and these intrinsic equilibrium constants can be directly calculated from pH(PZNC) and sigma(st). The inert electrolyte chemical binding does not exist for amphoteric surfaces with structural charge. PZNC is not equal to the point of zero net proton charge (PZNPC) when sigma(st) not equal 0. pH(PZNC) > pH(PZNPC) when sigma(st) > 0; pH(PZNC) < pH(PZNPC) when sigma(st) < 0; and pH(PZNC) = pH(PZNPC) only when sigma(st) = 0. With increasing c, the different: between pH(PZNC) and pH(PZNPC) decreases; i.e., pH(PZNPC) moves forward to pH(PZNC) with increasing c. For the HTlc samples studied, with increasing x, the pH(PZNC) and the pK(a1)(int) and pK(a2)(int) decrease, and the pK increases. These results can be explained on the basis of the affinity of metal cations for H+ or OH- and the electrostatic interaction between the charging surface and H+ or OH-. (C) 2003 Published by Elsevier Inc.