Molecular Dynamics Study of CO₂ and H₂O Intercalation in Smectite Clays: Effect of Temperature and Pressure on Interlayer Structure and Dynamics in Hectorite

Abstract

International audienceGrand Canonical Molecular Dynamics (GCMD) simulations were performed to investigate the intercalation of CO2 and H2O molecules in the interlayers of the smectite clay, Na-hectorite, at temperatures and pressures relevant to petroleum reservoir and geological carbon sequestration conditions and in equilibrium with H2O-saturated CO2. The computed adsorption isotherms indicate that CO2 molecules enter the interlayer space of Na-hectorite only when it is hydrated with approximately three H2O molecules per unit cell. The computed immersion energies show that the bilayer hydrate structure (2WL) contains less CO2 than the monolayer structure (1WL) but that the 2WL hydrate is the most thermodynamically stable state, consistent with experimental results for a similar Na-montmorillonite smectite. Under all T and P conditions examined (323–368 K and 90–150 bar), the CO2 molecules are adsorbed at the midplane of clay interlayers for the 1WL structure and closer to one of the basal surfaces for the 2WL structure. Interlayer CO2 molecules are dynamically less restricted in the 2WL structures. The CO2 molecules are preferentially located near basal surface oxygen atoms and H2O molecules rather than in coordination with Na+ ions. Accounting for the orientation and flexibility of the structural −OH groups of the clay layer has a significant effect on the details of the computed structure and dynamics of H2O and CO2 molecules but does not affect the overall trends with changing basal spacing or the principal structural and dynamical conclusions. Temperature and pressure in the ranges examined have little effect on the principal structural and energetic conclusions, but the rates of dynamical processes increase with increasing temperature, as expected

Competitive Adsorption of H2O and CO2 in 2-Dimensional Nanoconfinement: GCMD Simulations of Cs- and Ca-Hectorites

“…Importantly, for the 1L structures, which for each cation have the greatest interlayer CO2 contents, both Cs-and Ca-hectorite have more interlayer CO2 than Na-hectorite, as also observed in the experimental high T and P IR results. 17,40 Recent GCMC simulation studies by Kadoura et al 34 show similar behavior, with Ca-and Mg-montmorillonite having higher CO2 mole fractions than Na-montmorillonite at a given R.H. The interlayer CO2 mole fractions from our studies for Ca-hectorite are similar to the reported values by Kadoura et al 34 for Camontmorillonite, despite our simulations having a higher layer charge.…”

“…The presence of tetrahedral charge can greatly change the interlayer adsorption, structure, and dynamics of CO2 and H2O molecules, as discussed by Makaremi et al 36 It should be noted that the interlayer arrangements of CO2 molecules in Ca-hectorite in both the 1L and 2L structures are similar to the comparable systems with Na + , although the H2O distributions are different because the larger H2O content in the present study leads to a more ordered interlayer H2O structure than in Na-hectorite. 40 In the 1L structure of Cs-hectorite and the 1L and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 motion around an axis perpendicular to their O-C-O axis. The values 0 and ±1 for P (cos θ)CO2 correspond to parallel and perpendicular orientation of CO2 with respect to the plane of the basal surface, respectively.…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 9 This similarity emphasizes the critical role played by the H2O molecules in propping open the interlayer region to allow CO2 intercalation when the interlayer cations have relatively high hydration energies. 40,58 In contrast, for Cs-hectorite neither H2O nor CO2 intercalate at basal spacings <11.1 Å.…”

“…Computational molecular modeling studies for Na-smectites are in good agreement with the experimental results showing that CO2 intercalation requires the presence of at least a sub-1WL H2O layer. [33][34][35][36][37]40 Grand Canonical Monte Carlo (GCMC) simulations by Botan et al. 33 also suggest a stable bilayer (2L) hydrate structure (basal spacing ~15.0Å) for Namontmorillonite at 348 K and 125 bar in contact with H2O-saturated scCO2.…”

“…41,42 We address this gap here by examining the intercalation behavior of H2O and CO2 in hectorite containing a cation with a low hydration energy (Cs + ) and one with a high hydration energy (Ca 2+ ) at 323 K and 90 bar in equilibrium with H2O-saturated scCO2 using GCMD simulations. [40][41][42] Cs + and Ca 2+ are representative of cations with greatly different hydration energies and charge/radius ratios, 48,53,58 and they have greatly different CO2/H2O intercalation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 behavior. 17 The results here are in excellent agreement with experimental data of Bowers et al 17 and provide a basis for their detailed molecular scale structural, dynamic, and energetic interpretation and understanding.…”