A triple-layer model of the surface electrochemical properties of clay minerals

• Published in: Journal of colloid and interface science 2004-02, Vol.270 (2), p.371-380
• Main Authors: Leroy, P., Revil, A.
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Porous materials
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2003.08.007

We propose an electrical triple-layer model (TLM) to describe the electrochemical properties of clay minerals. This model includes a speciation model of the active crystallographic surface sites plus a classical description of the Stern and diffuse layers. In addition to the surface charges associated with the surface groups (aluminols, silanols, and >AlOSi< sites), the model takes into account the degree of isomorphic substitution rate inside the crystalline network. The model computes both the ζ potential and the low-frequency (few kHz) surface conductivity as these two properties are equally important to interpret electrokinetic properties of colloids and clay-rich porous materials. For surface conductivity, the model comprises two contributions; one is associated with the Stern layer (dynamic Stern layer) and the second is associated with the excess of counterions located in the diffuse Gouy–Chapman layer. The parameters of the TLM model are optimized using the Simplex algorithm. Comparison between the model and the experimental data shows good agreement for a reasonable choice of parameters close to the a priori values determined from quartz and gibbsite. Surface conductivity of smectite appears rather independent of salinity, while for kaolinite, surface conductivity increases with salinity. In both cases, the Stern layer contribution to surface conductivity dominates but the contribution associated with the diffuse layer cannot be neglected.