Zero Point of Charge and Charge Density of Kaolinite

The theoretical analysis on the zero point of charge (ZPC) and charge density of colloidal particle possessing permanent charges indicates that ZPC determined directly by means of potentiometric titration (PT) should be zero point of net charge (ZPNC) and the permanent charge density ( σ P) can be obtained from the adsorption amounts of H + and OH -( Γ OH - Γ H ) at ZPNC. ZPNC does not change with the electrolyte concentration ( c ), whereas the zero point of variable charge (ZPVC) changes with c . When σ P is at zero, ZPNC equals to ZPVC, and only under this condition is ZPC measured directly by PT equal to ZPVC. The relationship among ZPNC, ZNVC, σ P, the variable charge density ( σ V), the total net surface charge density ( σ T), pH, and c is derived. ZPNC and σ P of kaolinite are determined by PT method. It is found that the ZPNC is at pH=3.12 and the σ P is -0.051mmol·g -1 , the ZPNC is lower than ZPVC, and the ZPVC decreases with the increase of the c , while negtive σ T and σ V increase with the increase of pH and the c .