Interactions at the mineral - organic interface : the DMSO-nacrite thermal deintercalation.
Résumé
Intercalation and deintercalation of neutral organic molecules in the interlayer space of phyllosilicates represent an important interest in the development of new materials with desirable properties (catalysts, sorbents, ...) or in environmental protection and rehabilitation (waste deposits, polluted soils,...). To analyse these mechanisms several approaches are usually profitable. The small dimensions and the high dipolar momentum value of dimethylsulfoxide (DMSO) give it a high ability to be intercalated in 1:1 phyllosilicates and initialize the intercalation of other larger organic molecules. The aim of this work is to show the interest of FTIR spectroscopy and X-ray powder diffraction (XRD) to characterise the deintercalation process of the DMSO-nacrite complex. The DMSO-nacrite complex was prepared by heating nacrite in a 92/8 wt % DMSO/H2O solution at 60°C. In order to localize the interaction between DMSO molecule and the clay mineral, infrared absorption spectroscopy has been performed. Spectra of pure nacrite, freshly prepared DMSO-nacrite complex, and thermally-treated complex were recorded. The presence in the freshly prepared complex of OH bands at about 3400 cm-1, 3500 cm-1 et 3580 cm-1 is characteristic of the interaction between DMSO and nacrite. These results allow to conclude that organic molecules are fixed by H-bonding between a DMSO oxygen and a nacrite surface hydrogen. These bands disappear after heating at 360°C. The intercalated sample has been studied by high-temperature X-ray diffraction to follow the deintercalation. Samples have been heated from room temperature to 360°C, with a heating rate of 100°C/hour. The XRD patterns showed that (i) the deintercalation occurs at 120°C, (ii) the reticular distances (001) and (002) are not in the expected 2/1 ratio, (iii) from T = 200°C a strong and sharp reflection near 25° (in 2Θ Cu KΑ) and a broad one around 12° appear. In this range, the peak intensities are temperature dependent. This analysis indicates the presence of non-homogenous phases during the structural transformations. To quantify the structural evolution upon heating, a simulation method has been used. The model is based on the comparison between experimental and calculated XRD patterns. The model is obtained with a weighted sum of different contributions. Each contribution corresponds to a random interstratified phase formed with the two end-member phases (the intercalated complex and the deintercalated nacrite) in variable proportion. For example, a fair agreement between experimental and calculated intensities is obtained at 300°C with the parameters given in the table. The result of this simulation allows to conclude that 87.7% of the layers are intercalated with DMSO. Contribution Proportion of intercalated complex 100 90 80 70 60 Proportion of deintercalated nacrite 0 10 20 30 40 Proportion of the contribution 39 19 27 7 8 The case of 2:1 phyllosilicates, like smectites, with interest in applications is more complicated due to the presence of interlayer water and stacking faults. For the organic molecules-clay interaction study,13C and 1H RMN spectroscopies should provide additional informations to complete FTIR analyses. To quantify the intercalation or desintercalation process, chemical analysis techniques like CNS and HPLC could complete the XRD study.