DETERMINATION of the SPECIFIC SURFACE AREA in SWELLING CLAYS F. Salles 1, J.M. Douillard 1, O. Bildstein 2, M. Jullien 3 and H. Van Damme 4 (1) ICGM, Université Montpellier – France (2) CEA, DEN, LMTE – CEA Cadarache – St Paul lez Durance – France (3) ECOGEOSAFE – Europole Arbois – Aix-en-Provence (4) ESPCI – Paris –France Material and Method Introduction and Principle Aim of this study : determination of the reactive specific surface area and understanding of the hydration process upon water adsorption for samples saturated with alkaline cations the amount of adsorbed water measured in swelling clay is different for each sample (with different interlayer cation ) = hydration capacity and competition with the hydration capacity of the layer surface and the swelling the evolution of the specific surface area and therefore of the reactive surface area confirms this strong influence the value of the adsorption enthalpy gives us the controlling step as a function of the relative humidity and the nature of the interlayer cation (see also F. Salles, J.M. Douillard, R. Denoyel, O. Bildstein, M. Jullien, I. Beurroies, H. Van Damme, J. Colloid Interf. Sci., 2009, 333, ) Purified powder of montmorillonites (Mont) from the MX-80 bentonite (octahedral substitutions) saturated with a large majority of Na + and Ca 2+ as interlayer cations During the water adsorption, the structure of the swelling clays is strongly modified due to interactions between water molecules and the constitutive parts of the clay structure: the interlayer cation and the layer surface. The modification of the interaction equilibrium between cations and layer surfaces is the cause of the interlayer swelling. The understanding of the hydration process in swelling clays requires the determination of the reactive surface area and the identification of the driving-force for water adsorption. For this purpose, the simultaneous measurement of water adsorption isotherm and adsorption heat is performed for MX-80 (Wyoming) samples saturated with homoionic alkaline cations. Material Experiments Samples are fine powder heated at 150°C for 12h under vacuum Continuous volumetric adsorption + Tian-Calvet Microcalorimeter Equilibrium time imposed after adding water: minimum 4 hours Duration: 7 days Results and Interpretation Conclusion Interpretation of adsorption isotherms Theoretical isotherms according to Lecloux- Pirard or de Boer (n m = value for the mono- layer, C = BET constant, X=P/P0 and N = number of water layer) n m and C are determined using BET model and N is calculated from thermogravimetric analysis Calorimetry: differential heat and integrated heat (= sum of differential heat divided by the adsorbed water amount for a given RH) Evolution of the specific surface area Fitting of the adsorption isotherm using the theoretical isotherms  determine the evolution of the specific surface area during water adsorption Existence of plateaus on the isotherms All isotherms are of II or IV type  BET equation can be applied Maximal adsorbed water amount depends on the nature of the interlayer cation: Li > Cs > Na > K > Ca > Na/Ca Water affinity depends on the MX sample Existence of plateaus on the isotherms // XRD data All isotherms II or IV type  BET equation can be applied The specific surface area are determined using 12 Å as cross sectional area of the water molecules Sequence of the adsorbed water amount different of the one for cations in solution the driving force for hydration is not only the hydration energy of the interlayer cation Strong evolution of the specific surface area for small cations In the case of Cs-sample: no evolution For Na/Ca, less evolution than for the other cations Evolution of the adsorption enthalpy Description of the differential (black) and integrated (red) heat to discriminate the swelling and the hydration (for the cation or for the layer surface) steps Evolution for the values at low RH values: Li > Na > Na-Ca> Ca > K > Cs As a function of the RH: - RH < 40%: exothermic peaks = HYDRATION - RH > 50% : endothermic peaks = SWELLING For Na/Ca: we distinguish the hydration for both cations Exchanged powders of MX-80 bentonite saturated with alkaline or Ca 2+ cations: Li +, Na +, K +, Cs +