Water in clay nanopores
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troduction Clay minerals derive from the weathering of minerals formed deep in the earth’s crust. They are rich in aluminum and silicon and contribute significantly to the crust mass. They are characterized by a lamellar structure and dimensions smaller than a micron, giving rise to a large specific surface area and, for some of them, to swelling and ion exchange in the presence of water.1 Dispersed in water, clay minerals display colloidal behavior, as they are subject to thermal motion because of their small size. The properties of clay minerals led to their use by humans, long before there was any understanding of the underlying processes, for a variety of applications, including components in the fabrication of bricks, tiles, terracotta, earthenware, and porcelain; texturizing agent in paints; fillers in paper and plastics; and cat litter. They are also used as catalysts in the pharmaceutical industry, as additives in drilling fluids in the oil industry, and even as a medicine in the treatment of acute gastroenteritis. Because of their abundance in soils and underground and their various properties, clay minerals are also involved in a number of processes in the environment. They can act as water “reservoirs,” filled by rain or irrigation, that provide water during droughts. Their ability to sorb traces of both inorganic and organic pollutants also plays an important role in the containment of soil pollution. Clay-rich geological
formations are usually tightly packed, exhibiting a low permeability to fluids. They are found as hard, resistant caprocks, impermeable rocks above reservoirs, above natural gas or sequestered CO2 reservoirs, preventing the rise of buoyant fluids toward the surface.2 On the contrary, overcoming this low permeability is the goal of hydraulic fracturing to extract oil or gas from shales.3 The combination of mechanical, hydraulic, and sorption performances makes them efficient barriers against the transport of fluids and solutes. Several countries are therefore considering clayrich geological formations, possibly with clays as buffer and backfill materials as well, for the disposal of high-level and long-lived radioactive waste.4 The breadth of applications listed previously is made possible by the variety of physicochemical properties of clay minerals, arising from their microscopic structure and resulting in rich behavior with respect to water, solutes, and other fluids. This article provides a brief overview of the structure, dynamics, thermodynamics, and reactivity of water in clays, highlighting the role of the various types of water–mineral interfaces in these materials. In the following sections, we first introduce the structure of clay minerals. We then discuss several features of these interfacial materials arising from the interplay between clay minerals and water, including swelling, wetting, hydrodynamics in clay nanopores, reactivity of clay edge sites, ion exchange, and sorption.
Benjamin Rotenberg, CNRS and Sorbonne Universités, Université Pierre et Marie Curie, France; benjamin.rotenb
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