Minerals in low-water environments, such as when pore fluids are non-aqueous or at nanoparticle surfaces in the atmosphere, are covered in films of H2O that are only monolayers thick. Reactions in these films are subject to dramatically different conditions than in bulk aqueous solutions. Within films H2O is nanoconfined between the surrounding fluid or gas on one side and the mineral surface on the other. Strong hydrophilic interactions with the surface disrupt the H-bond network of bulk H2O and restrict the ability of H2O to reorient to solvate ions and facilitate diffusive mass transport for processes such as dissolution or precipitation and growth. This surface imposed structuring is progressively relaxed as the H2O film thickens, but bulk-like behavior may never be obtained because of an inadequate driving force for the formation of a liquid phase in H2O-undersaturated conditions. Despite numerous examples of H2O-limited domains in nature, we currently lack a fundamental understanding of coupled mineral dissolution, nucleation, and growth reactions in confined adsorbed H2O films. Here, we present results of three case studies that use infrared (IR), 18O exchange, and impedance experiments to interrogate molecular-level controls on thin H2O film reactivity at subsurface reservoir temperatures and pressures. First, we discuss the role of H2O structure in the formation of carbonic acid in H2O films on fumed silica (SiO2) in humidified supercritical CO2 (scCO2). Self-associated “liquid-like” H2O adsorbed on SiO2 catalyzes the formation of carbonic acid, yet surface-structured “ice-like” H2O is unreactive within our measurement uncertainty. Second, we show the relationship between H2O structure and mobility in H2O films on forsterite (Mg2SiO4) in wet supercritical N2 (scN2). Concentrations of liquid-like H2O detected by IR correlate with diffusion-related parameters measured by impedance spectroscopy. Third, we discuss the carbonation of forsterite covered in thin H2O films in humid scCO2. Although IR cannot resolve structurally distinct H2O in this system, impedance spectroscopy links diffuse-layer mobility with carbonation reactivity. This research advances a predictive understanding of complex fluid systems involving interfaces and confinement and is important for CO2 utilization and storage operations, including enhanced gas recovery (EGR), geologic carbon sequestration (GCS), and enhanced geothermal systems (EGS).

Role of H2O structure and mobility in controlling reactivity in adsorbed H2O films

2019 Spring National Meeting

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