Defects in structure as the sources of the surface charges of kaolinite

•The permanent charge density achieved by theoretical and experimental methods•Permanent charge density related with isomorphic substitution of kaolinite•The variable charges depend on the degree of the broken bonds at edges of kaolinite.•The distortions in structure may not directly affect the surface charges.

The sign and magnitude of the surface charge govern the particle interactions and hence determine the physical properties of dispersions. The present work is to evaluate the distortions, isomorphic and broken surface bond as the sources of the permanent or variable surface charges of kaolinite. The theoretical permanent charge density (σcc) was calculated by XRF. The experimental permanent charge density (σp) and variable charge density (σv) were obtained by the method of potentiometric titration. The degree of broken bonds (Db, b) of Al–O–Al and Si–O–Si at the side of kaolinite structure were characterized by the numbers and strength of surface hydrogen in 1H MAS NMR, absorbance of Al–OH or Si–OH in FTIR and edge fractures in FESEM. The surface area was measured by N2 adsorption–desorption. The defects such as basal z corrugation (∆ z), tetrahedral rotation angles (θ) and octahedral flattening angles (ψ) were obtained by the XRD rietveld refinement. The negative σcc or σp of three samples KA, KB and KC are 0.012, 0.007 and 0.009 C·m− 2 or 0.24, 0.19 and 0.22 C·m− 2, respectively. Db, b of three samples are Db, b-KA > Db, b-KC > Db, b-KB. The σv depend on the degree of the broken bonds at edges of kaolinite and can be affected by pH significantly. The negative variable charge density at alkaline condition decreased with the increase of the degree of broken bonds at the side of kaolinite structures. Surface charges of kaolinite come from the defects such as isomorphic substitutions and broken bonds Al–O–Al and Si–O–Si. The distortions in structure e.g. ∆ z, θ and ψ may not directly create the surface charges.

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