Thermodynamic analysis of the immersion of a smectite substituted with Na or Ca: Heat effect due to the cation

The enthalpy produced during immersion in water of a smectite, identified as a montmorillonite, substituted by Ca or Na is analyzed using three different approaches. The heat of immersion is measured using the classical calorimetric method. It is compared to an estimate using adsorption techniques. And it is calculated from a theoretical estimate of the surface energy of a crystal model of montmorillonite. The comparison of the different values allows the enthalpy linked to the swelling phenomenon to be estimated. The Ca form appears in every case to interact more strongly with water molecules than the Na form. The solid surface tensions of Na-montmorillonite and Ca-montmorillonite from an Algerian deposite are computed in order to interpret data on heat of immersion in water. The immersion heat contains adsorption, spreading, and swelling terms. Analysis of results is in agreement with a quantitative analysis of adsorption isotherms.