BEHAVIOR OF CARBON-BEARING FLUIDS AT NANOSCALE INTERFACES

It is becoming increasingly clear that organic molecules present as gas species, in aqueous and mixed volatile fluids - ranging from simple hydrocarbons and carboxylic acids to branched and cyclic compounds, to proteins and humic substances - play major roles in controlling geochemical processes, not just at Earth‘s surface, but also deep within the crust and mantle. The origin of life may be partly attributable to the properties of such molecules in complex fluids under extreme conditions, as they appear to play an important role in mineral reactivity and templating of mineral precipitates. The behavior of C-O-H fluids at mineral surfaces or in confined geometries (pores, fractures) typically differs from their bulk behavior in many ways due to the effects of large internal surfaces and geometrical confinement. Phase transitions (i.e., freezing and capillary condensation), sorption and wetting, and dynamical properties, including diffusion, relaxation and chemical reactivity, may be modified, with the strongest changes observed for pores ranging in size from <2 to 50 nm. Important factors influencing the structure and dynamics of confined fluids include the average pore size and pore size distribution, the degree of pore interconnection, and the character of the liquid–surface interaction. The confining matrices of interest to earth and materials sciences usually contain oxide structural units and thus are typically hydrophilic. The properties of neutrons make them an ideal probe for comparing the properties of bulk fluids with those in confined geometries. In this presentation, we provide an overview of research performed on fluids at nanoscale interfaces in materials of interest to the earth and material sciences (e.g., silica, alumina, zeolites, clays, rocks, etc.), emphasizing those techniques that assess both structural modification and dynamical behavior such as small-angle (SANS) and quasielastic neutron scattering (QENS), and neutron reflectivity (NR). Molecular dynamics (MD) simulations will be described that provide atomistic characterization of interfacial and confined fluid behavior as well as aid in the interpretation of the neutron scattering results.