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Assessment of surface energetic heterogeneity of synthetic Na-saponites. The role of layer charge

Laboratoire Environnement et Minéralurgie, INPL-ENSG-CNRS UMR 7569, BP 40, 54501 Vanduvre Cedex, France

^* E-mail: laurent.michot{at}ensg.inpl-nancy.fr

(Received 28 September 2000; revised 6 April 2001)

High-resolution gas adsorption techniques were used to analyse the evolution of the aspect ratio and adsorption energy distribution on synthetic saponite samples with increasing layer charge. Using Ar as a gaseous probe, the aspect ratio of the saponite particles can be determined easily by decomposing the derivative adsorption isotherms and taking into account high-energy sites which can be assigned to talc-like ditrigonal cavities. Changes in the shape of the elementary particles are observed for layer charges above 1.30, i.e. when all the ditrigonal cavities contain at least one Al atom substituting for Si. When N₂ is used as a probe, high-energy sites that could be wrongly interpreted as micropores on the basis of classical t-plot treatments are observed whatever the layer charge. Using the information obtained from both Ar and N₂, schemes for describing adsorption can be proposed for all layer charges and suggest complex adsorption mechanisms for charged clay minerals.

KEYWORDS: adsorption, argon, heterogeneity, nitrogen, saponite

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