Structure and Dynamics of Fluids in Microporous and Mesoporous Earth and Engineered Materials

Abstract

The behavior of liquids in confined geometries (pores, fractures) typically differs, due to the effects of large internal surfaces and geometrical confinement, from their bulk behavior in many ways. Phase transitions (i.e., freezing and capillary condensation), sorption and wetting, and dynamical properties, including diffusion and relaxation, may be modified, with the strongest changes observed for pores ranging in size from $<2$ to 50~nm---the micro- and mesoporous regimes. Important factors influencing the structure and dynamics of confined liquids include the average pore size and pore size distribution, the degree of pore interconnection, and the character of the liquid-surface interaction. While confinement of liquids in hydrophobic matrices, such as carbon nanotubes, or near the surfaces of mixed character, such as many proteins, has also been an area of rapidly growing interest, the confining matrices of interest to earth and materials sciences usually contain oxide structural units and thus are hydrophilic. The pore size distribution and the degree of porosity and inter-connection vary greatly amongst porous matrices. Vycor, xerogels, aerogels, and rocks possess irregular porous structures, whereas mesoporous silicas (e.g., SBA-15, MCM-41, MCM-48), zeolites, and layered systems, for instance clays, have high degrees of internal order. The pore type and size may be tailored by means of adjusting the synthesis regimen. In clays, the interlayer distance may depend on the level of hydration. Although studied less frequently, matrices such as artificial opals and chrysotile asbestos represent other interesting examples of ordered porous structures. The properties of neutrons make them an ideal probe for comparing the properties of bulk fluids with those in confined geometries. In this chapter, we provide a brief review of research performed on liquids confined in materials of interest to the earth and material sciences (silicas, aluminas, zeolites, clays, rocks, etc.), emphasizing those neutron scattering techniques that assess both structural modification and dynamical behavior. Quantitative understanding of the complex solid-fluid interactions under different thermodynamic situations will impact both the design of better substrates for technological applications (e.g., chromatography, fluid capture, storage and release, and heterogeneous catalysis) as well as our fundamental understanding of processes encountered in the environment (i.e., fluid and waste mitigation, carbon sequestration, etc.).