Experimental Studies of Smectite Clays: Colloids and Nanoporous Materials
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- Institutt for fysikk 
This thesis presents experimental work on two synthetic clay minerals: fluorohectorite and Laponite RD. Both are hectorite clays belonging to the widely studied smectite group of clays. The results presented can broadly be divided into two categories: Clays as colloids By combining visual observation of birefringence, x-ray scattering, and MRI imaging, we show that in aqueous dispersions of Na-fluorohectorite, several regions develop with differing particle orientations. We demonstrate that at the boundaries of the nematic regions, the Na-fluorohectorite particles orient homeotropically to the isotropic–nematic interfaces, in the same way that they orient to solid interfaces. From x-ray diffraction, we are able to quantify the degree of nematic order, as well as the average particle thickness, and show that the particles in a liquid-like nematic region are sensitive to magnetic fields. By varying the concentration of clay and the salinity of the suspensions in a systematic way, we observe a distinct change in structure of the dispersions between ionic strength 10 and 25 mM NaCl, and discuss this qualitatively within the framework of the DLVO theory. We postulate that the structural changes are due to a transition from a repulsive interparticle potential characterized by a high barrier toward flocculation to a potential which exhibits a minimum for a given particle separation. Furthermore, we demonstrate for the first time that the colloidal structure and rheological properties of clay dispersions can be highly dependent on temperature, and that the reason for this behavior is to be found in the interlayer space. We observe from in situ x-ray scattering measurements that the regular stacking of layers in Na-fluorohectorite is gradually lost when aqueous dispersions of Na-fluorohectorite are heated from room temperature to 80 ○C. Based on conductivity measurements, we argue that this behavior is due to the entropy that is gained by counterions leaving the interlayer space, making it energetically favorable for the clay to delaminate. Clays as nanoporous materials We show that there are small but reproducible variations in d-spacing within the hydrodynamically stable hydration states ofNa-fluorohectorite. We have applied this observation to study transport of water through a quasi one-dimensional capillary filled with clay powder, concluding that the diffusive behavior is either normal or weakly anomalous. We demonstrate that intercalation of CO2 is possible in Na-fluorohectorite at conditions close to ambient (−20 ○C, 15 and 5 bar), adding to the still scarce number of experimental studies in this field. Our structural observations using x-ray diffraction is the first evidence of CO2 intercalation in hectorites, and the first at these conditions in any clay.The in situ measurements also provide a time-scale for the process, which is found to be orders of magnitude slower than the time scale for intercalation of water. We find that the rate of CO2 intercalation in fluorohectorites is dependent on the interlayer cation, with about one order of magnitude faster intercalation rate in Li-fluorohectorite, compared to Na-fluorohectorite. Finally, we show that Li-fluorohectorite is able to retain the intercalated CO2 at roomtemperature. This observation could have applications related to CO2 capture and storage.