Low pressure adsorption on kaolinite clay minerlas modified by cation exchange at the solid/liquid interface
Kaolinite is one of the most abundant aluminosilicate minerals, occurring primarily as clay-sized particles with high surface area-to-volume ratios. The surface of kaolinite particles can be considered as heterogeneous as crystallites present at least two different types of faces, basal and edge faces, with different surface chemistry and exchange proprieties. It is commonly believed that exchangeable cations of kaolinites are mainly located on the edge faces. However, the basal surfaces can also carry a constant charge due to isomorphous substitution of Si4+ by Al3+ in tetrahedra and Al3+ by Mg2+ in octahedra. This aim of this work is to study the influence of the nature of surface cations on the surface properties of kaolinite at the solid/gas interface. Argon low pressure gas adsorption coupled to the Derivative Isotherms Summation method [1-3], used for long time to derive geometrical properties of kaolinites powder was applied on monovalent kaolinite samples (Li+, Na+, K+, Cs+) [4-6]. By comparing the adsorption isotherms it appears that the nature of surface cation influences the adsorption phenomena on edge and basal faces. On edge faces, the exchanged cations generate nanorugosity and take part to the first argon monolayer. On basal faces, two families of cations are presents in small quantity: for the first family, argon adsorption energy is slightly sensitive to the nature of the cation; for the second one, argon adsorption energy depends on the nature of surface cation [4]. The influence of outgassing temperature (110°C-350°C) on surface properties of kaolinite shows that, for the first family, surface cations are strongly linked to surface and probably correspond to the compensation of local charge defects generated by tetrahedral or octahedral substitutions. For the second one, surface cations were physisorbed on the surface and correspond to neutral compounds [5]. On the quantitative point of view, it was shown that the estimation of the aspect ratio of kaolinite should be carried out with monovalent cations of small size (Li+, Na+) with surface outgassing at 200°C [4-6]. [1] F. Villiéras et al., Langmuir, 8 (1992) 1789. [3] J.M. Cases et al., C.R. Acad. Sci., Paris, 331 (2000) 763. [2] F. Villiéras et al., C.R. Géoscience, 334 (2002) 597. [4] M. Sayed Hassan et al., Langmuir, 21 (2005) 12283. [5] M. Sayed Hassan et al., J. Colloid Interface Science, 296 (2006) 614. [6] M. Sayed Hassan et al., Langmuir, Submitted