On the Cation Dependence of Inter-lamellar and Inter-particular Water and Swelling in Smectite Clays

The osmotic character of long-range inter-lamellar swelling in smectite clays is widely accepted and has been evidenced in the interlayer space by X-ray diffraction. Such a behavior in meso-pores was not experimentally confirmed until the determination of the meso-pore size distribution in Na-montmorillonite prepared from MX80 bentonite using thermo-poro-metry experiments. This is confirmed here for other montmorillonite samples where the interlayer cations are alkaline and Ca2+ cations. The nature of the interlayer cation is found as strongly influencing the behavior of the size and the swelling of meso-pores. These results are supported by the BJH (Barrett, Joyner and Halenda) pore radius values issued from the nitrogen adsorption-desorption isotherms at the dry state. Thermo-poro-metry results as a function of relative humidity ranging from 11% to 97% have shown an evolution of the meso-pore sizes for a purified Na-montmorillonite. New thermo-poro-metry data are presented in this article and confirm that the interparticle spaces in K-, Cs-, or Ca-montmorillonites are not strongly modified for all the range of relative humidity: the swelling is not observed or is strongly limited. It appears in contrast that only Li- and Na-montmorillonites undergo a meso-pore swelling, distinct from the interlayer swelling. More generally, our results confirm the possibility to use thermo-poro-metry or differential scanning calorimetry to study the structure and the evolution of swelling materials in wetting conditions such as natural clays or biological cells. In this paper, we describe the different key steps of the hydration of swelling clays such as montmorillonites saturated with alkaline cations. Using thermo-poro-metry results combined with X-ray diffraction data, we distinguish the evolution of the porosity at the two different scales and propose a sequence of hydration dependent on the interlayer cation. From this study, it is shown that the interlayer spaces are not completely filled when the meso-pores start to fill up. This implies that the swelling observed in the meso-pores for Li and Na samples is due to an osmotic swelling. For the other samples, it is difficult to conclude definitively. Furthermore, we determine the different proportion of water (interlayer water and meso-pore water) present in our samples by the original combination of (1) X-ray diffraction data, (2) the pore size distribution obtained by thermo-poro-metry, and (3) recent adsorption isotherm results. It is found that the interlayer space is never completely filled by water at the studied relative humidity values for all samples except for the Cs sample. (authors)