Acid-base properties of 2:1 clays: The role of electrostatics
Résumé
We present a theoretical and experimental investigation of the titratable charge of 2:1 clays with various structural charge ('b): pyrophyllite ('b = 0 e. nm(2), montmorillonite ('b = -0.7 e. nm(2) and illite ('b = -1.2 e. nm(2). The calculations were carried out using a Monte Carlo method in the Grand Canonical ensemble and in the framework of the primitive model. The clay particle was modeled as a hexagonal platelet, with an ‘ideal' crystal structure. The only fitting parameters used are the intrinsic equilibrium constants for the protonation/deprotonation reactions of the broken-bond sites on the lateral faces. Simulations are found to give a satisfactory description of the acid-base titration of 2:1 clays without any additional fitting parameter. In particular, combining the electrostatics from the crystal substitutions with the protonation constants, the simulations satisfactorily catch the shift in the point of zero net proton charge (PZNPC) of 2:1 clays according to the ionic strength, in good qualitative agreement with experimental observations following the sequence: pyrophyllite < montmorillonite < illite.