Points of Zero Charge and Surface Acid-base Reaction Equilibrium Constants of Mg-Al-Ti Layered Double Hydroxides

Five Mg-Al-Ti-CO3 layered double hydroxides (LDHs) were synthesized using a coprecipitation method with a constant Mg2+/(Al3++Ti4+) molar ratio of 3.00 but varying Ti4+/(Al3++Ti4+) molar ratio (R-Ti, 0-0.40). Their structural charge density (sigma(st)), point of zem net charge (pH(PZNC)), and point of zero net proton charge (pH(PZNPC)) were determined using potcntiomctric, salt and potcntiomctric mass titrations, and their intrinsic equilibrium constants of surface acid-base reactions (pK, pK(a1)(int) and pK(a2)(int)) were estimated in turn based on the general 1-pK and 2-pK models. Special emphasis was placed on the effect of R-Ti on their crystal structure and interface electrochemical (IEC) properties. With an increase in R-Ti the cell constants (a, b and c) and d-spacing increase, arising from the strong electrostatic repulsion between Ti4+ ions. An increase tendency was also observed for pH(PZNC), pH(PZNPC), pK, pK(a1)(int) and pK(a2)(int) with the increase of R-Ti, indicating a decrease in the depmtonation tendency of surface hydroxyl groups. In addition, it was observed that pH(PZNPC) is lower than pH(PZNC) and increases with increasing the electrolyte (NaNO3) concentration, arising from the effect of structural charges. This work provides a better understanding for the IEC properties of Ti4+- containing LDHs.