DCO Project Summary

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Project Title
C-O-H Fluid-Rock Interaction and the Role of Serpentine
Start DateEnd Date
2013-07-01 2015-06-30
NameRoleInstitutionDCO ID
Related GrantsDCO ID
11121/9902-5454-7041-1832-CC
Description
The crustal environments where abiotic synthesis of organic compounds is thought to occur are largely inaccessible to direct sampling, making laboratory simulations and theoretical models a critical component in understanding the origin of these compounds. This work will involve a series of experimental and theoretical studies to investigate the reactions responsible for abiotic organic synthesis within Earth’s crust, focusing on serpentinization processes. Specifically, the hydrothermal autoclave experiments will explore the evolution of solution and gas species associated with the alteration of olivine to serpentine minerals. The objective of this project is to quantify, using molecular simulations and complementary NMR and neutron spectroscopy, how such effects could be manifested in the thermodynamics and kinetics of the Fisher-Tropsch-type reactions.
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Reporting Year 2014 Click to expand


  • RY2014-1 - submitted on Apr 01, 2014

    Update Details:

    [2014-04-01] Cole presented an invited talk at the American Chemical Society Dallas meeting on neutron scattering and molecular dynamics modeling of hydrocarbons confined to nanopores.
  • RY2014-2 - submitted on Aug 08, 2014

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    Publications (as of August 8 2014):

     

    • Andreani, M., Daniel, I., and Pollet-Villard, M. (2013) Aluminum speeds up the hydrothermal alteration of olivine. American Mineralogist, 98, 1738–1744.
    • Cole, D. R., Ok, S., Phan, A., Rother, G., Striolo, A. and Vlcek, L. (2013) Carbon-bearing fluids at nanoscale interfaces.  Procedia Earth Planet. Sci. 7, 175-178.
    • Cole, D. R., Striolo, A., Phan, A. and Ok, S. (2013) Chapter 19. Hydrocarbon behavior at nanoscale interfaces. In: Carbon in the Earth (Eds. R. Hazen, R. Hemley, A. Jones, J. Baross) Rev. Mineral. Geochem. 75, 495-545
    • Galvez M., Beyssac O., Martinez I., Benzerara K., Chaduteau C., Malvoisin B., Malavieille J., 2013, Graphite formation by carbonate reduction during subduction, Nature Geoscience, 6(6), 473-477, DOI: 10.1038/NGEO1827.
    • Hu, Y., Devegowda, D., Striolo, A.,  Ho, T. A.,  Phan, A.,  Civan, F. and  Sigal, R. F. (2013)  A pore scale study describing the dynamics of slickwater distribution in shale gas formations following hydraulic fracturing, SPE Journal, Conference Paper, SPE-164552-MS.
    • Hu, Y., Devegowda, D.,  Civan, F., Phan, A., Ho, T. and  Striolo, A.  (2013) Microscopic dynamics of water and hydrocarbon in shale-kerogen pores of potentially mixed wettability, SPE Journal, Conference Paper, SPE-167234-MS.
    • Le, T., D.R. Cole, and A. Striolo (in press) Propane Simulated in Silica Pores: Adsorption Isotherms, Molecular Structure, and Mobility. Chem. Eng. Sci.
    • McCollom, T. M., Seewald, J. S. and German, C. R. (in review) Investigation of non-volatile organic compounds in deep-sea hydrothermal vent fluids along the Mid-Atlantic Ridge. (submitted to Geochimica et Cosmochimica Acta).
    • Phan, A., D.R. Cole, A. Striolo (2014) Aqueous Methane in Slit-Shaped Silica Nanopores: High Solubility and Traces of Hydrates. J. Phys. Chem. C 118, 4860-4868.
    • Phan, A., D.R. Cole, and A. Striolo (2014) Preferential Adsorption from Liquid Water-Ethanol Mixtures in Alumina Pores, Langmuir 30, 8066-8077.
    • Striolo, A. (in review) Understanding Interfacial Water and Its Role on Various Practical Applications Using Molecular Simulations. MRS Bulletin (Invited article).
  • RY2014-3 - submitted on Aug 08, 2014

    Update Details:

    [08 August 2014]

    University of Colorado (Dr. Tom McCollom, Lead).

    Research during the past year focused on the reduction of carbon and generation of abiotic organic compounds during serpentinization of ultramafic rocks.  As part of this work, several carbon-rich particles were identified among the products of hydrothermal serpentinization experiments that potentially formed through reduction of dissolved CO2. In the coming year, I will work with DCO colleagues at IPGP and University Claude Bernard in Lyon to characterize these particles and determine their origin.  In a related effort, the origin of methane generated during the hydrothermal serpentinization experiments was investigated using isotopic labels.  In contrast to several recent reports, we found that the methane came primarily from background sources rather than reduction of CO2 in the experiments.  Lastly, hydrothermal fluids from mafic- and ultramafic-hosted deep-sea hydrothermal systems along the Mid-Atlantic Ridge were analyzed for the presence of non-volatile organic compounds.  Non-volatile hydrocarbons with an abiotic origin were not found at any of the systems, although a number of carboxylic acids and other compounds of clear biological origin were observed at the moderate-temperature, serpentine-hosted Lost City system.

    University of Lyon (Prof. Isabelle Daniel, Lead). We have measured the kinetics of serpentinization of olivine and orthopyroxene in the diamond anvil cell at the European Synchrotron Radiation Facility (high-pressure beamline ID27). This has allowed assessing partly the role of aluminum on the fast kinetics of serpentinization. One aspect is that the nucleation of aluminous serpentine is instantaneous. The role of carbonate and pH was also investigated. This represents hundreds of X-ray patterns that are being subjected to Rietveld refinement by a PhD student (Maria Pens, Venezuela). Complementary SEM and Raman measurements have been done after quenching. The same quenched samples were also investigated by X-ray absorption spectroscopy at the Fe K-edge at the French synchrotron facility SOLEIL (LUCIA beamline) to evaluate the degree of oxidation in mineral phases, which should be correlated to the amount of H2 or C-reduced produced during the experiments. Data are currently being processed.

    At this stage, the analysis of mineral phases is almost completed.

    In the meantime, long-term serpentinization experiments have been started at 80°C. They will be soon analyzed using the micro GC-MS that was delivered at springtime.

    We have also searched for a DCO postdoctoral fellow: Dr. Steve Peuble, who has defended his PhD thesis (University of Montpellier, June 2014) on the experimental characterization of hydration and carbonation of basic and ultrabasic rocks. Steve has already started in the lab, as an assistant during summer 2014 (salary university Lyon1). He will start officially on the DCO grant Sept. 1st 2014 for one year.

         Future Work. This second year will be dedicated to the analysis of the gas, fluid phase and carbon phase. We’ll take advantage of the micro GC-MS, which is now operational, and we will use vibrational spectroscopic techniques to analyze the carbon phase. A variety of thermodynamic conditions will be investigated, from 50°C to 350°C under the relevant pressure.

    Institut de Physique du Globe de Paris, IPGP (Dr. Benedicte Menez, Lead).

    Experimental assessment of reactions and carbon transfers

    • C. Vacquand started the first round of experiments involving synthetic analogs of oceanic crust and catalysts. She first developed protocols to synthesize fayalite (Fe2SiO4) and olivine (FeMgSiO4) at nanometric scale as well as magnetite and chromite, which are supposed to be good candidates for the catalysis of the Fischer-Tropsch-type reactions. During the last few months, she also developed in our lab the protocol to collect and analyze the gas phase from the capsule by GC. The first set of experiments was performed during June 2014 at 200 bars, 200°C, with 7 gold cells containing the silicate phase and a solution (pure water or water with 30g.l-1 NaHCO3). The analyses of the run products is in progress.

    • A second set of experiments, involving a catalyst (magnetite and chromite) is being prepared. Using the same protocol and to increase their reactivity, mixtures of silicate and catalysts were ground at a nanometric scale and stored under argon atmosphere.

      Associated collaboration and networking: Collaboration has been established with the LRCS for nanosynthesis (N. Recham, Laboratoire de Réactivité et Chimie des Solides, UPJV Amiens). Part of the nano-fayalite was also sintered at the ISTerre (Grenoble, France) to be used as starting material (coll.: F. Brunet). We started a collaboration with Tom McCollom (Boulder, Colorado) invited for one month at IPGP to start the characterization of the high-molecular-weight reaction products of some of his abiotic organic synthesis experiments, using some of the techniques we have been developing at IPGP for natural samples (SEM, TEM, Raman, and IR). Same work could be done also on a suite of samples from drill cores into an active serpentinite in California where we could also look for organic matter. A new collaboration on the abiotic production of hydrocarbons starts with the IFP (French Petroleum Agency) with a PhD thesis now open. This experimental work deals with percolation experiments on oceanic crust samples, in collaboration with Geosciences Montpellier (M. Godard, France).

    University College London (Prof. Alberto Striolo, Lead).

    We have continued the investigation of mixed fluids under confinement. We have considered methane, propane, octane, CO2, ethanol, and water in narrow slit-shaped pores. Results for the systems containing CO2 are not yet mature for publication. These investigations are based on equilibrium molecular dynamics, with attempts to reproduce experimental observations in collaboration with Cole and coworkers.

    DCO funds have been instrumental in establishing a new web of collaborators for Striolo in Europe. Together with Adrian Jones, Striolo is developing a close collaboration with Rob Hull of Halliburton and with Duncan Nicholson of ARUP. The collaboration is focused on shale gas. A large number of academic collaborators have been contacted thanks to DCO.

         Future Work. The main goal for the next period is to simulate chemical reactions in pores (the DCO project involves investigating the thermodynamics of the Fischer-Tropsch reaction under confinement). Now that the group is completely moved to our new location (University College London), we can implement the new algorithm. In the meantime, we will continue the investigation of mixed fluids containing alkanes and CO2.

    Ohio State University (Prof. Dave Cole, Lead).  The overarching objective of this effort, which leverages complementary support from DoE, is to obtain a fundamental atomic- to macro-scale understanding of the sorptivity, structure, and dynamics of simple and complex C-H-O fluids at mineral surfaces or within nanoporous matrices over temperatures, pressures, and compositions encountered in near-surface and shallow crustal environments.  To achieve this goal we (a) assess the adsorption-desorption behavior of methane, related hydrocarbons, and CO2 on a variety of mineral substrates and in nanoporous matrices, (b) characterize the microstructure and dynamical behavior of methane and related HC volatiles at mineral surfaces and within nanopores with and without H2O present at relevant P-T-x subsurface conditions, and (c) utilize molecular-level modeling to provide critically important insights into the interfacial properties of these mineral-volatile systems, assist in the interpretation of experimental data, and predict fluid behavior beyond the limits of current experimental capability. A scientifically diverse, multi-institutional team (Ohio State University, University College London (with A. Striolo), Oak Ridge National Lab, Pacific Northwest National Lab, Hunter College) is utilizing novel experimental and analytical techniques in concert with state-of-the-art theory, modeling, and simulation approaches to address these issues. There is a special emphasis on building synergistic links between results obtained from various neutron scattering and NMR studies which are integrated into our research portfolio with molecular dynamics modeling, to provide new phenomenological insights.

    Thus far, we have conducted a number of different kinds of experiments focusing on the behavior of propane, ethane, methane, carbon dioxide, or their mixtures with or without water present interacting with different types of mesoporous matrices (e.g., SiO2, Al2O3, TiO2, ZrO2). High temperature-high pressure gravimetric measurements on these fluids have revealed profound fluid densification in nanopores as the density (pressure) approaches that of the bulk critical density followed by a dramatic density decrease (fluid depletion). Densification of propane in model silica pores has been observed in MD simulations, in qualitative agreement with experiments. The CO2 adsorption on TiO2 has been modeled with density functional theory (DFT) that employs a new version of the dispersion correction. Quasielastic neutron scattering (QENS) experiments have been conducted at Oak Ridge National Laboratory on the system propane-mesoporous silica with and without CO2 present. Results from these experiments are interpreted in terms of translational “diffusive” motion of propane, residence times between diffusion jumps, and the jump distances. Interestingly, the presence of CO2 seems to enhance the mobility of propane in the mesopores. Mixed water-methane and water-ethanol systems have also been investigated using MD simulations, suggesting that preferential adsorption might be responsible for the results observed experimentally in mixed systems. We used a recently developed high-pressure Magic Angle Spinning (MAS) NMR system (EMSL) to investigate methane interacting with mesoporous silica having 200 nm particle size and 4 nm pores, a high surface area non-porous silica and montmorillonite. The proton decoupled 13C NMR spectra were acquired with high-pressure MAS probe at 30, 60 and 120 bars.  At each pressure, the temperature was varied from 34oC to 73oC.  The experiments revealed that pressure induces shifts in the methane peak position: ~0.25 ppm going from 30b to 60b, and ~0.50 ppm shift going from 30b to 120b. The magnitudes of these spectral chemical shifts have been emulated by Amity Andersen at EMSL using DFT. The simulations indicate that the chemical shifts reflect the effects of “molecular crowding” and adsorption on magnetic shielding.

Reporting Year 2015 Click to expand


  • Final Report Second DE project - submitted on Sep 15, 2015

    Update Details:

    Because the environments within Earth’s crust where abiotic synthesis of organic compounds is thought to occur are largely inaccessible to direct sampling, laboratory simulations and theoretical models are a critical component in understanding the origin of these compounds. Accordingly, a suite of experimental and theoretical studies were conducted to investigate the reactions responsible for abiotic organic synthesis within Earth’s crust.

    Serpentinization

    A primary focus of the effort by Tom McCollom (University of Colorado) over the past two years has been investigation of abiotic reduction of carbon during laboratory simulations of serpentinization of ultramafic rocks.  One aspect of this work has involved characterization of solid carbonaceous particles found in several laboratory experiments to evaluate whether they formed by abiotic reduction of carbon during the experiments.  This work involved a new collaboration with several groups in France facilitated through the Deep Energy directorate, including researchers at the University of Lyon (Daniel, Andreani), IPGP (Martinez), and the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie in Paris (Bernard).  Solid carbonaceous products were characterized by Raman spectroscopy, synchrotron measurements including STXM and carbon-XANES, and SEM.  While these analyses were initially promising, subsequent evaluation of carbon isotopic composition of the carbonaceous particles by nano-SIMS suggests that they formed from trace organic contaminants in the reactants rather than from reduction of inorganic carbon.  During the next phase of the Deep Energy project, the McCollom effort will continue to explore for evidence of the formation of high-molecular-weight carbon products in laboratory experiments.

    A second aspect of this work has been evaluation of pathways for abiotic formation of molecular hydrogen (H2) and methane (CH4) during serpentinization.  Several recent reports have claimed abiotic production of methane from reduction of CO2 during experimental serpentinization of ultramafic rocks and minerals.  However, our results using isotopically labeled compounds to track the source of carbon demonstrated that the methane formed in serpentinization experiments at 200-300°C was derived predominantly from background sources present in the reactants rather than from reduction of dissolved CO2.  In addition, we performed a series of water-rock reaction experiments at 90°C using olivine and harzburgite as reactants to investigate formation of H2 and CH4 at low temperatures.   Although these experiments produced H2 and CH4 at levels comparable to those in several other recent studies performed at 100°C or lower, comparison with blank experiments showed that these compounds were derived predominantly from the rubber stoppers that are commonly used as part of the reaction vessel in low temperature experiments.  Overall, our results demonstrate that the production of methane during experimental serpentinization is much more limited than other recent studies have suggested.  This result is consistent with recent results indicating that the methane found in deep-sea hydrothermal vent fluids is derived from high-temperature re-equilibration of magmatic gases rather than fluid-rock interactions (McDermott et al., PNAS, 2015)

    Lastly, the University of Colorado team completed an analysis of extractable organic compounds in hydrothermal fluids from mafic- and ultramafic-hosted deep-sea hydrothermal systems along the Mid-Atlantic Ridge.  In contrast to other reports, McCollom did not find any evidence for non-volatile hydrocarbons with an abiotic origin in any of the hydrothermal fluid samples studied.  However, he did observe trace amounts of polycyclic aromatic hydrocarbons in the highest temperature fluids that may contribute to the recalcitrant dissolved organic matter in the deep ocean, and a number of carboxylic acids and other compounds of clear biological origin were observed at the moderate temperature, serpentine-hosted Lost City system. 

    In a related study, the group at the University of Lyon led by Prof. Isabelle Daniel investigated the role of aluminum on the serpentinization rate at 200° and 300°C, 2kbars in low-pressure diamond anvil cell show the following results and are the core of the PhD work of Maria Pens (graduate student from Venezuela, supported by the FUNDAYACUCHO in Venezuela). This work was allocated 2 weeks of beamtine at the ESRF, the European Synchrotron radiation facility and one week at SOLEIL, the French synchrotron facility. They confirmed the fact that Al strongly increases the serpentinization rate of olivine, as previously observed, and this time they determined the kinetic parameters of the reactions. They assessed this effect over a wide range of pH and propose that this is due to Al-Si complexes formation on the surface of olivine, possible from pH 4 to 10. It was shown that Al plays the opposite role on orthopyroxene, the other main mineral of ultramafic rocks, at least at 350°C, possibly because the charge surface of pyroxene differs from that of olivine during the first stages of dissolution. This was particularly surprising since available data on olivine and pyroxene serpentinization under similar P-T conditions in pure water report faster rate for orthopyroxene serpentinization than for olivine. The other surprising result was the fact that their data demonstrated a general increase of olivine serpentinization rate with pH in presence of Al, while all available data on olivine dissolution display the opposite, i.e. a decreasing rate with pH. This effect of pH is also observed in the series of experiments that were run using HCO3- -enriched solution that displayed fast reaction rates, the fastest observed being when both Al and alkaline conditions are gathered. These results are currently being written in two papers that will be submitted within the next months, while Maria will be defending her thesis within the next year.

    They are currently investigating the effect of Al ± HCO3- at lower pressure conditions using large volume reactors that also allow fluid sampling for liquid and gas analyses. This work is being conducted by Steve Peuble, a DCO post-doctoral fellow and has benefitted from the acquisition of a dedicated and optimized micro-GC supported by the labex LIO. Steve will continue the work for another year under the auspices of the ANR grant deepOASES to B. Menez. A first series of experiments has been realized using pure olivine powder reacting at 200-250°C and 200-250 bars with Al- and HCO3-- enriched saline water of initial neutral pH. Blank experiments (with same solution ± inert solids) were realized at the same time to test for possible carbon contamination. After a month, 4-5% of serpentinization has been observed, which is much lower than expected. These preliminary results already suggest that the Al effect is lower at lower P. However, after 4 days, first detections of H2 and CH4 are observed, and ethane is detected after 20 days. Their concentrations are much higher than the background ones measured in blanks. The H2 content tends to decrease rapidly while CH4 one increases. Solid product characterization confirms the formation of serpentine and magnetite plus a carbon-rich material locally that is under identification. These first results are extremely encouraging since they validate the possible reduction bicarbonate ions to both solid and gas carbon compounds during serpentinization.

    Concerning related projects, M. Andreani recently organized the ECORD-DCO “Rainbow workshop” in Lyon, 10th-12th June 2015 (http://rainbow2015.univ-lyon1.fr) that aimed at preparing a possible IODP drilling proposal on the Rainbow massif that hosts the ultramafic hosted hydrothermal field Rainbow. A summary of the workshop will be send soon to the DCO and ECORD, and it will be also highlighted in the ECORD Newsletter.

    Catalytic Reactions

    A second major experimental activity in the second Deep Energy project was to explore the constraints on redox reactions and carbon transfer. The group at IPGP investigated the catalytic properties of a single phase. A first set of experiments was conducted on magnetite (Fe3O4) following the established protocol typically by the chemistry group (Poitiers). In these experiments, 1 g of finely divided magnetite and 20 g of ultrapure water were charged into a stainless steel reactor. The experiment was run at 80 bar (by means of 40 bar CO2 and 40 bar H2) and heated at 180 ° C for 72 hours under continuous magnetic stirring. A sampling valve allowed to monitor the composition of the gas mixture, through gas chromatography. The presence of methanol (CH3OH) was detected after 72 h. A blank experiment (without magnetite) showed that methanol was not produced by simple reactivity of CO2-H2 mixture at high pressure and temperature. Several unidentified products were formed and their identification is currently underway. TEM observation on the solid products did not show any C phases produced during the experiment. 

    In parallel, two sets of experiments were performed at IPGP by mixing silicates synthesized in finely divided form (fayalite (Fe2SiO4) and olivine (FeMgSiO4)), with catalytic phases present in natural systems. In this case, the reactants were loaded into gold capsules together with a sodium bicarbonate solution. The capsules are then reacted at 200 °C and 200 bar for 3 weeks. After the experiment the capsule is first pierced in a helium flushed syringe to recover the gas products, which are then injected in a GC for analysis. Finally the capsule is opened and the recovered solid studied by conventional microscopy techniques. They did not detect any gas phase in the capsule, probably because there was not enough gas for GC analysis. Next experiment will be run at 300°C which should enhance the production of hydrogen and methane. The solid phases showed interesting reactions: run products are mainly phyllosilicate (talc still to be confirmed by DRX), magnesium carbonates (derived from carbonation of olivine), and magnetite. Furthermore, in experiments where sulfides are present (Cu2S and FeS2), they observed large carbon-rich phases (several microns in size), sometimes associated with magnesite which then show smaller grain sizes. These phases were not detected in the reaction products where the magnetite was present.   While it remains to be examined in more details, it seems that sulfides are more effective in the CO2 reduction reaction than magnetite and chromite.

    A complementary set of experiments were conducted by Jeff Seewald at WHOI to determine how potential catalytic phases might influence the isotopic exchange of reduced species. In the first, a laboratory experiment was designed to elucidate mechanisms of hydrogen exchange between aqueous methane and water at elevated temperatures and pressures with the goal of understanding factors that regulate the clumping of carbon and hydrogen isotopes in methane. Clumped isotopes represent a potentially powerful tool to examine the formation temperature for methane in subsurface environments, provided thermodynamic isotopic equilibrium is attained. The experiment he conducted was designed to test the hypothesis that sulfur and aromatic hydrocarbon radicals will catalyze the exchange of hydrogen between methane and water and facilitate attainment of isotopic equilibrium. A U.S. Gulf Coast crude oil with added methane was heated at 325°C and 350 bar in the presence of D2O and a pyrite-pyrrhotite-magnetite redox buffer that also served as a source of sulfur. Mass spectrometric analysis of the methane during the course of the experiment revealed that deuterium was not significantly incorporated into methane, suggesting that hydrogen/deuterium exchange with D2O was not facilitated by the presence of aromatic radicals from the oil and/or sulfur radicals form the redox buffer. He will further test our hypothesis in a second experiment of this type conducted under more reducing conditions with greater oil and sulfur abundances.

    A second experiment currently underway at WHOI is being conducted to assess the extent to which pH and temperature influence the catalytic activity of Fe- and Cr-bearing minerals at near critical hydrothermal conditions. Abiotic formation of low molecular weight hydrocarbons has been shown to occur in the presence Fe- and Cr-bearing minerals, but the extent of their catalytic activity is presently unclear due to the absence of mineral-free control experiments. The experiment he is conducting examines the polymerization of methane under hydrothermal conditions in the absence of added minerals and constrain the potential for abiotic synthesis in a pure aqueous system.

    In addition to the experiments described above, Jeff has been working with John Eiler at Caltech and Shuhei Ono at MIT to provide hydrothermal fluid samples for measurement of the rare isotopologues of methane. Seewald carefully assessed our large collection of methane-rich hydrothermal fluids from peridotite influenced hydrothermal systems and distributed selected samples that are optimally suited for clumped isotopic analysis. These data will provide some of the first clumped methane isotopic data for methane at a sediment-deficient mid-ocean ridge system and constrain temperatures of formation. Results will also allow comparison of two mass spectrometric and spectroscopic approaches for the measurement of clumped methane isotopes.

    Behavior of Reduced Carbon Species in Porous Matrices

    The goal of this portion of the project was to employ molecular dynamics approaches to understand the structure and dynamics of fluids containing hydrocarbons, water, and oxygenated compounds confined within narrow pores that could be found in sub-surface formations. The theoretical results obtained by Prof. Alberto Striolo and his students are compared systematically to experimental data obtained by collaborator Prof. David Cole of the Ohio State University, and his team. The distinctive feature of this project is the inclusion of both organic and aqueous fluids. Additionally, access to the DCO Cluster greatly facilitated the molecular dynamics simulations for some of the systems investigated.

    Building on the detailed summary of the behavior of hydrocarbons in nanoporous systems summarized in Chapter 19 in the 2013 Carbon in Earth MSA volume, the Cole-Striolo team have systematically explored the effects of nanoconfinement on the structure and dynamics of assorted alkanes as pure fluids as mixtures with either water or CO2. Thus far, the team has conducted a number of different kinds of experiments focusing on the behavior of propane, ethane, methane, carbon dioxide or their mixtures with or without water present interacting with different types of mesoporous matrices (e.g., SiO2, Al2O3, TiO2, ZrO2). High temperature-high pressure gravimetric measurements on these fluids have revealed profound fluid densification in nanopores as the density (pressure) approaches that of the bulk critical density followed by a dramatic density decrease (fluid depletion). Densification of propane in model silica pores has been observed in MD simulations, in qualitative agreement with experiments. Quasielastic neutron scattering (QENS) experiments have been conducted at Oak Ridge National Laboratory on the system propane-mesoporous silica with or without CO2 present. Results from these experiments are interpreted in terms of translational “diffusive” motion of propane, residence times between diffusion jumps and the jump distances. Interestingly, the presence of CO2 seems to enhance the mobility of propane in the mesopores.

    To establish a close connection with the experimental data obtained by Cole and coworkers, Striolo’s team simulated the adsorption of propane in slit-shaped silica pores. The results, in qualitative agreement with the experiments, show a maximum in the density of the confined fluid that occurs at conditions approaching the gas-to-liquid transition of the bulk fluid. Mixed water-methane and water-ethanol systems have also been investigated using MD simulations, suggesting that preferential adsorption might be responsible for the results observed experimentally in mixed systems. Using MD the team interrogated mixtures of water and methane confined within narrow pores carved out of silica, considered a proxy for cristobalite and other rocks. The pores were slit-shaped and of width ~ 1nm. The results suggested that the solubility of methane in water within the confined space can be up to 1 order of magnitude larger than that in the bulk. The results show that the structure of confined water, template by the solid surface, is responsible for this enhanced methane solubility. The structure of water molecules surrounding one methane molecule is reminiscent of that of hydrates. These results have been recently confirmed by high pressure-high temperature Magic Angle Spinning (MAS) NMR experiments conducted at the Environmental Molecular Science Laboratory.

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