Journal of Colloid and Interface Science 223, 61–73 (2000) doi:10.1006/jcis.1999.6624, available online at http://www.idealibrary.com on Analysis of Thermally Stimulated Depolarization Currents (TSDC) Measured on Exchanged Clays V. M´ edout-Mar` ere, H. Belarbi, ∗ A. Haouzi, ∗ J. C. Giuntini, ∗ J. M. Douillard, 1 J. V. Zanchetta, ∗ and J. Vanderschueren† ,2 LAMMI, UPRESA 5072 CNRS, Universit´ e Montpellier II, Place Eug` ene Bataillon, CC 15, 34095 Montpellier Cedex 5, France; ∗ LPMC, UMR 5617 CNRS, Universit´ e Montpellier II, Place Eug` ene Bataillon, 34095 Montpellier Cedex 5, France; and †GEPIDA, Universit´ e de Li` ege, Institut de Chimie au Sart-Tilman, B4000 Li` ege, Belgium Received May 5, 1999; accepted November 9, 1999 This work shows that hydration of clays can be studied by means of a new interpretation of thermally stimulated depolar- ization currents technique. These measurements have been per- formed on four exchanged natural clays:Na–montmorillonite, Ca– montmorillonite, kaolinite, and illite. The results are analyzed using both the recently developed analysis of relaxation times distribu- tion and the electronegativity equalization method. They provide evidence of the surface heterogeneity of clays. It is established that sites, identical from a crystallographic point of view, are different when the energy of the phenomenon is considered. The main inter- est of this work is to give forthe first time a value of the hydration energy of cation onto clay surfaces. C  2000 Academic Press Key Words: clays; montmorillonite; kaolinite; illite; TSDC; ARDT; EEM; surface heterogeneity. I. INTRODUCTION In a previous paper (1), we have shown how the thermally stimulated depolarization currents (TSDC) technique appears fruitful and complementary to usual adsorption techniques as a way to study solid surfaces. This approach combined with clas- sical adsorption methods such as surface excess determination and calorimetry could be an efficient tool to determine micro- scopic arrangements on the surface of divided solids. The aim of this paper is to present recent developments of this new solid surface analysis, allowing an interpretation of the interaction of structural cations with adsorbents and the rest of clay surfaces. Usually, the adsorption of water vapor on the surface of a solid is analyzed by volumetric and calorimetric methods (2). Information obtained by these techniques allows some under- standing of the behavior of the solid surface at a macroscopic level, but these techniques do not provide information about the microscopic mechanisms of adsorption. However, it is well known that the energy of individual adsorption sites is a key factor in order to theoretically predict the adsorption properties of a solid surface, like the quantity of molecules adsorbed or 1 To whom all correspondence should be addressed. E-mail: jmd@univ- montp2.fr. 2 Research associate of the National Fund for Scientific Research (Belgium). the heat of adsorption (2). A great variety of adsorption sites can be interpreted in terms of surface heterogeneity (3), which is a very important concept in quantifying results in such do- mains as catalysis, wettability, and adsorption. For these types of problems, it is necessary to develop new spectroscopies. This work is part of a general study of the phenomenon of polarization that occurs when some aluminosilicates containing alkaline cations are submitted to an electric field. In a recent work (4), we studied the behavior of a Na-exchanged montmorillonite by measuring the dielectric losses as a function of temperature and frequency. The experimental results led to the assumption that the conductivity is due to discrete hops of charge carriers between localized sites in these materials (4–7). Applying this assumption to the analysis of the results obtained by TSDC (8), we were able to determine the distribution of relaxation times characterizing these hops. A simple description of the elemen- tary process leads to the evaluation of the energy of the charge carrier trapped in its site. By studying a series of well-known compounds, we established without ambiguity that the charge carriers are the exchanged cations moving between the surface sites of the aluminosilicates. This paper is an application of this new technique to some minerals, still studied in other papers, with a background of ore- field or oil-field engineering. In this paper, we examine a series of clays of different types, using the thermally stimulated depolar- ization currents. Thus, measurements of TSDC were performed on samples previously submitted to the same processes of ex- change and purification. They were also characterized by the usual methods such as X-ray diffraction, infrared spectroscopy, differential thermal analysis, and Mossba¨ uer spectroscopy. The samples are a montmorillonite of Mostaganem (Algeria) (4, 9), a kaolinite of Provins (France) (10–12), and an illite of the Vosges (France) (10–12). The aim of this study is the determination of the energy dis- tribution characterizing the interactions between the exchanged ions and the lattice that traps them. Thanks to a new analysis developed recently (8), we have been able to characterize by an energy a given type of atomic environment. The main objective is the experimental evaluation of the value of the energy of cation hydration. 61 0921-9797/00 $35.00 Copyright C  2000 by Academic Press All rights of reproduction in any form reserved.