Carbon dioxide in supercritical water is more reactive in nanoconfinement than in the bulk

Nanoconfined water has different properties to bulk water. We show that at extreme pressures and temperatures, carbon dioxide dissolved in water is more reactive when confined to the nanoscale than in bulk solutions. This has implications for the global carbon cycle and carbon sequestration efforts.
Carbon dioxide in supercritical water is more reactive in nanoconfinement than in the bulk
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In our previous work1, we looked at the reactions of CO2 in water at the pressures and temperatures that are relevant in the upper mantle of Earth, which extends down from Earth’s crust to ~410 km depth. The assumption made in traditional geochemical models that seek to predict the reactions between water, minerals and solutes is that aqueous CO2 does not react with water. Instead, we found that CO2 reacts with water quite readily at pressures greater than ~6 GPa at 1000 K.

However, in this previous work, we studied a bulk mixture of CO2 and H2O. In reality, the chances of finding macroscale pockets of water in Earth’s upper mantle, where the pressure is extremely high, are small. Instead, aqueous fluids tend to locate at grain boundaries in minerals, which have sizes on the nanometre scale2. This means that to understand the reactions between CO2 and H2O at the conditions of Earth’s interior, we must understand how nanoconfinement affects those reactions.

In our new paper3, we use ab initio molecular dynamics simulations to study the effects of 2-dimensional nanoconfinement on aqueous CO2 at Earth's upper mantle pressure–temperature conditions (10 GPa, 1000–1400 K). Firstly, we confine the solution between two graphene sheets placed ~0.9 nm apart. The fluid does not react with graphene sheets, so graphene confinement allows us to study the effects of confinement without reactions taking place at the interface between the fluid and the confining material. Secondly, we confine CO2(aq) between stishovite (SiO2) surfaces placed ~0.7 nm apart. Stishovite is hydrophilic and reactions occur between the fluid and the confining surface, so that we can study the concurrent impact of confinement and surface chemistry. Stishovite as well as other silicate minerals are found in Earth’s upper mantle, meaning that we are studying a realistic situation in our simulations.

In the bulk at 10 GPa and 1000 K, CO2(aq) makes up 15.2 ­± 2.0% of all carbon-containing molecules. We find that between graphene sheets placed ~0.9 nm apart, the amount of dissolved CO2(aq) is reduced to 1.3 ± 0.9%. The main reaction products of the reaction between carbon dioxide and water are bicarbonate ions (HCO3-) and carbonic acid (H2CO3). The same trend is found at 10 GPa and 1400 K, where the percentage of CO2(aq) is reduced from 58.8 ± 2.0 % to 14.5 ± 3.2 % when going from bulk to nanoconfined solutions.

In the 2-dimensional stishovite pore with width ~0.7 nm, CO2(aq) makes up 10.5 ± 2.3% of all carbon species at 10 GPa and 1000 K. At 10 GPa and 1400 K, 39.8 ± 3.6% of all carbon is CO2(aq). What is more, a large percentage (48.5 ± 2.1% at 1000 K and 19.1 ± 3.1% at 1400 K) of all carbon species are bonded to one of the two confining surfaces.

Thus, we show that CO2(aq) is more reactive in nanoconfined water than in the bulk. This can be attributed to the enhanced dielectric constant of water under nanoconfinement4, which stabilizes aqueous ions with respect to neutral solutes. The effect is at play both in graphene- and stishovite-confined solutions. However, the dielectric constant of water is not increased as much near hydrophilic interfaces5, so CO2(aq) will be more stable in stishovite confinement than in graphene confinement. Additionally, the reactions between stishovite and water lead to a more acidic solution in stishovite confinement. Consequently, the chemical equilibrium

CO2 + 2H2O  HCO3- + H3O+

is shifted to the left in stishovite confinement compared to the graphene-confined case. Still, CO2(aq) is more reactive when confined in a silicate mineral than in bulk solution.

Our findings in this work are important to our understanding of the global carbon cycle. For example, reactions of CO2 in water in Earth’s interior can affect the connectivity of aqueous fluids along mineral grain boundaries. Additionally, in light of these results, it seems likely that CO2 mineralization efforts that aim to sequester atmospheric CO2 will be more successful in localities where CO2(aq) can penetrate nanoscale cracks and pores in the host rock.

https://www.nature.com/articles/s41467-022-33696-w

References

  1. Stolte, N. & Pan, D. Large presence of carbonic acid in CO2-rich aqueous fluids under Earth’s mantle conditions. Phys. Chem. Lett. 10, 5135–5141 (2019).
  2. Marquardt, K. & Faul, U. H. The structure and composition of olivine grain boundaries: 40 years of studies, status and current developments.  Phys. Chem. Miner. 45, 139–172 (2018).
  3. Stolte, N., Hou, R. & Pan, D. Nanoconfinement facilitates reactions of carbon dioxide in supercritical water. Nat. Commun. 13, 5932 (2022).
  4. Muñoz-Santiburcio, D. & Marx, D. Nanoconfinement in slit pores enhances water self-dissociation. Phys. Rev. Lett. 119, 056002 (2017).
  5. Bonthuis, D. J., Gekle, S. & Netz, R. R. Dielectric profile of interfacial water and its effect on double-layer capacitance. Phys. Rev. Lett. 107, 166102 (2011).

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