Soft Matter Physics of Clays and Clay suspensions: structural arrest, ordering, and host-guest interactions

Abstract

This thesis contains fundamental experimental studies in soft matter physics, focusing on plate-shaped hectorite and uorohectorite clays as nanomaterials with applications in the formation of glasses, gels and liquid crystals, as naturally occurring minerals, as drug carrier systems, and as hosts for CO2 storage.

The first part presents fundamental studies on the physics of dispersions of clays in aqueous solvents. We find that gravity induces phase separation in aqueous suspensions of the synthetic clay Na- fluorohectorite, and compare our observations on the volume fractions where orientational order is observed with theoretical predictions for the isotropic-nematic transition of hard discs. In experiments performed during free fall aboard an aircraft in parabolic ights, we investigate the rotational diffusion which Na- fluorohectorite particles undergo at different salt concentrations, and relate our observations to the phase behaviour. We also present novel observations of orientational particle ordering in glasses of Laponite RD clays in deionized water. This system has been extensively studied for its ergodic to non-ergodic transitions, and the existence of an underlying isotropic-nematic phase transition has been debated. We conclude that our orientationally ordered glass has a history dependent degree of order that originates from plastic deformations and not from an underlying isotropic-nematic transition.

The second part presents studies on swelling clays and quick clays. Swelling clay minerals are challenging in construction engineering and oil well drilling, because of large swelling pressures associated with water uptake, and the shrinkage associated with drying. We demonstrate for the rst time that a clay which is non-swelling (crystalline but hydrated) at low temperatures becomes active and swells (delaminates) when temperature is increased. We present a simpli ed thermodynamical model and argue that the driving force for swelling is of entropic origin. Like swelling clays, quick clays are also challenging materials; they are solid-like when undisturbed but undergo a spectacular liquefaction when suciently perturbed. In a reply to a comment on a previously published paper, we summarize some important features of quick clays from a physicist's point of view. In particular we comment on how results from laboratory experiments on thixotropic, remoulded or rehydrated quick clays, although not trivially applicable to in situ quick clay landslides, may still shed light on the possible ow regimes of post-failure deposits of quick clays.

The third part concerns the utilization of clay minerals as drug carrier systems or as hosts for CO2 storage. We show experimentally that gaseous CO2 intercalates into the interlayer space of Na-fluorohectorite at conditions of - 20 °C and 15 bar. The dynamics of the process is observed to be dependent on the pressure. We also present some early results on the incorporation of the antibiotic cipro oxacin into the interlayers of a fluorohectorite clay. We study the release of the drug from the clay interlayers in synthetic gastric acid at temperatures ranging from below body temperature and up to 70 °C, and nd that the release profiles are strongly temperature dependent. We find that rapid release occurs in basic solutions, and use this observation to support our hypothesis that incorporation takes place via ion exchange.