Deciphering Fluid Dynamics in Siliceous Nanoscale Confinements: A Path to Sustainable Energy Solutions
The continuum of energy systems from historical to futuristic paradigms is IN PART linked to harnessing subsurface siliceous materials. Siliceous materials are abundant in naturally occurring minerals, and rocks mainly consist of SiO2 either in its amorphous form or as quartz, which we actively harness to meet our energy and resource needs. Current research into fluid behavior at siliceous interfaces is transforming low carbon geo-energy technologies for scalable decarbonization with renewable sources and carbon storage. The burgeoning interest in utilizing CO2 emissions to extract energy-critical metals and induce mineralization of calcium and magnesium in siliceous resources—forming solid carbonates—necessitates a deeper understanding of fluid interactions with siliceous substances. Single-digit, nano-sized pores ubiquitous in such subsurface materials exhibit unique reactivity, altered thermodynamics, and distinctive transport dynamics  compared to larger pores. These phenomena are primarily driven by the pronounced fluid–wall interactions within the confines of the nanoscale realm .
Understanding these unique fluid behaviors at the nanoscale is pivotal to unlocking new findings for CO2 storage and separation in naturally occurring geological and architected materials, further offering insights into diverse phenomena. These phenomena include the thawing of permafrost in response to climate change, the subsurface behavior and transportation of light gases like H2 and CO2, and the development of innovative sensing and advanced separation technologies. Hence, research across various Energy Frontier Research Centers (EFRCs) is drawing inspiration from geological processes and examining the impact of nanoscale environments on fluid organization, thermodynamics, and transport in siliceous matrices. Studies within the Multi-scale Fluid–Solid Interactions in Architected and Natural Materials (MUSE) EFRC have illuminated the “core-shell” structuring of CO2 within silica nanopores such as SBA-15 and MCM-41 with diameters of 6.8 nm and 3.3 nm, respectively . The core–shell structures of CO2 were predicted using computational (molecular dynamics) simulations, and shell thicknesses were obtained using small angle neutron scattering (SANS) measurements. The organization of CO2 molecules in confinement based on molecular dynamics simulations is also shown in Figure 1. Interestingly, the observed denser “shell” is attributed to the adsorption of CO2 molecules on the silica surface, intensified by the strong CO2-silica interfacial interactions. This interaction eludes conventional thermodynamic models, contributing to the discrepancies between predicted and actual rates of carbon mineralization . Thermodynamically downhill carbon mineralization pathways can be coupled with energy and resource conversion pathways. Anomalous structuring of fluids at solid interfaces can be used to engineer crystallization pathways including carbon mineralization.
These insights spurred further research at the Center for Mechanistic Control of Unconventional Formations (CMC-UF), aiming to elucidate the emergent thermodynamic properties from fluid−wall interactions in nanoconfined spaces . Research at CMC-UF employs a combination of experimental and theoretical approaches to study the behavior of hydrocarbons such as methane  and ethane  in these nanoconfined spaces at higher pressures. The presence of ethane in nanoscale confinement, for instance, revealed a progressive pore-filling process, with ethane molecules forming an adsorptive layer without a clear first-order phase transition from liquid-like to gas-like states. This reorganization of energy within nanoconfined ethane results in a tightly packed, orderly arrangement of molecules aligned with the pore walls . These findings contribute toward (i) the development of novel approaches and strategies aimed at promoting low-carbon energy and resource recovery and (ii) utilizing and storing CO2 within both natural and architected materials.
The leaps made in computing power and multi-modal characterization techniques are revolutionizing our ability to understand and predict the behavior of fluids in nanoscale confinement. The pressing need for climate-forward innovations further fuels our quest to leverage the unique properties of confined fluids to engineer sustainable solutions for a thriving planet. These research endeavors demonstrate how the changes in both chemical and structural aspects of siliceous interfaces in intricate surroundings are interconnected, highlighting the consequent effects on technologies related to low-carbon geo-energy technologies. These insights also inform the development of functional materials with tunable pore structures for tuning fluid interactions for applications related to sustainable catalysis and advanced separations.
 S. Mohammed, M. Liu and G. Gadikota, Resolving the organization of CO2 molecules confined in silica nanopores using in situ small angle neutron scattering and molecular dynamics simulations, Environ. Sci.: Nano, 8, 2021, 2006.
This work was supported as part of the EFRC-MUSE, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Grant DE-SC0019285. The authors would like to thank Dr. Yun Liu and Juscelino Leao for SANS experimental support at the NIST Center for Neutron Research.
 N. Singh, F. Simeski, M. Ihme, Computing Thermodynamic Properties of Fluids Augmented by Nanoconfinement: Application to Pressurized Methane, J. Phys. Chem. B, 126, 2022, 8623−8631.
This work was supported as part of the Center for Mechanistic Control of Water–Hydrocarbon–Rock Interactions in Unconventional and Tight Oil Formations (CMC-UF), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under DOE (BES) Award DE-SC0019165. The authors acknowledge the use of computational resources from the National Energy Research Scientific Computing Center (NERSC).
 F. Simeski, J. Wu, S. Hu, T. T. Tsotsis, K. Jessen, M. Ihme, Local Rearrangement in Adsorption Layers of Nanoconfined Ethane, Phys. Chem. C, 127, 2023, 17290–17297.
This work was supported as part of the Center for Mechanistic Control of Unconventional Formations (CMC-UF), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under DOE (BES) Award DE-SC0019165. The authors are grateful for the resources at the National Energy Research Scientific Computing Center and at the Stanford Research Computing Center that were utilized for this work.