Influence of synthesis pH on kaolinite "crystallinity" and surface properties

Hydrothermal syntheses were performed at various pH values and temperatures to induce variability in kaolinite defect density. Temperature of synthesi s ranged from 200 to 240 degrees C, for 21 d. Initial pH at room temperatur e ranged from 0.5 to 14. The starting material was a hydrothermally treated gel, with an atomic Si/Al ratio of 0.93, partly transformed into kaolinite . Kaolinite was obtained for a wide range of pH. Although no influence of tem perature on "crystallinity" (i.e,, defect density) was observed, the effect of pH was important. A continuous series was obtained from a low-defect ka olinite, with high thermal stability and a hexagonal morphology for the mos t acidic final pH, to a high-defect kaolinite, with low thermal stability a nd lath shape for the most basic final pH. These variations of kaolinite pr operties appear related to the pH dependence of kaolinite surface speciatio n, Increasing pH value results in increased cation adsorption on the kaolin ite external surfaces and increases in the elongation of particles.