Preferential adsorption in ethane/CO2 fluidmixtures confined within silica nanopores

M. Dolores Elola & Javier RodriguezDepartamento de Fısica de la Materia Condensada,

Centro Atomico Constituyentes, Comision Nacional de Energıa Atomica (CAC-CNEA), Bs As

1. Introduction & Methodology

Structural and dynamical properties of fluid ethane confined within silicananopores have been investigated by Molecular Dynamics simulations.The pure ethane phase and equimolar mixtures of ethane and CO2 wereconsidered; at densities in the range ρ/ρcrit = 0.05− 2.18.

x

z

reservoir reservoirLzp

Dp

x

yLx

Ly

• Molecular Dynamics simulations, times ∼ 60 ns).• Microcanonical Ensemble NVE , at T ∼ 320 K.• Inter-molecular interactions: Coulomb & Lennard-Jones;Intra-molecular interactions: stretching, bending & torsion.

2. Structural Properties: Local Densities

Densities along z:

2 4 6

−50 0 50

ρb/ρc= 0.25

z (Å)

2 4 6 ρb/ρc=

0.46

2 4 6 ρb/ρc=

0.70

ρ(z)

/ ρ b

2 4 6 ρb/ρc=

1.35

2 4 6 ρb/ρc=

2.28Ethane

CO2

Figure 1: Normalized local densitiesalong the axial z-axis for ethane/CO2equimolar mixtures.

Excess density:Difference between the averagedensity inside the pore and thebulk density,∆ρexc = 〈ρpore〉 − ρb

Density profiles reveal the forma-tion of a dense adsorption layer,rich in CO2.

Densities along r :

0

2

4

Ethane

ρ C2H

6(r)

/ ρc

0

4

8

12

0 5 10 15 20

CO2

ρ CO

2(r)

/ ρc

r (Å)

ρb / ρc = 0.25ρb / ρc = 0.46ρb / ρc = 0.70ρb / ρc = 1.35ρb / ρc = 2.28

Figure 2: Normalized local densities ofethane and CO2 species along the radialdirection, for ethane/CO2 equimolar mix-tures.

Excess sorption densities:

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5

∆ρexc / ρ

c

ρb / ρc

Ethane (This work)

Ethane (Expt.)

CO2 (This work)

CO2 (Expt.)

Figure 3: Excess densities of confinedethane and CO2 in the pure fluids. Exper-imental data measured on 11.1 nm CPG-75 silica glass pores.

3. Structural Properties: Orientations

Orientation wrt the silica wall

0.5

1

0 0.5 1

Ethane

0 0.5 1

CO2

rx

yqû

-0.4

-0.2

0

0.2

2 4 6 8

a rij

ûi

ûj

P(c

osθ)

cos θ

interfacialintermediate

bulk

cos θ

G2(

r)r(A)

eth-ethCO2-CO2

eth-CO2

Figure 4: Top: Probability distributionP(cos θ) for ethane and CO2 confinedspecies; bottom: Legendre polynomials ofthe intermolecular cosine angle vs. center-of-mass separation. Curves correspond toequimolar mixture with ρb/ρc = 0.46.

• Interfacial Ethane → parallel tosurface.• Interfacial CO2 → bimodal dis-tribution, parallel and perpendicu-lar to surface. Due to direc-tional hydrogen-bond like interac-tions [O=C=O · · · H-O-Si] with thesilica walls• Bulk region → no preferential ori-entations.

Orientational functionG2(r) = 〈P2(ui · uj) δ(rij − r)〉,whereP2(x) = (3x2 − 1)/2is the 2nd-rank Legendre polyno-mial. T-shaped parallel

• G2 < 0 at small r → T-shapedconfigurations;• G2 ∼ 0.2 at r ∼ 3− 4 A→ inter-mediate between T-shaped and par-allel arrangements.

4. Dynamical Properties

Diffusion: Self-diffusion coefficients were computed from the classicalEinstein relation for the mean squared displacements.

Table 1: Diffusion coefficients (in 10−4cm2s−1).

pure Ethane mixture 50% CO2P(bar) Dblk

eth Dconfeth Dblk

eth Dconfeth Dblk

CO2Dconf

CO2

20 20.8 2.71 24.2 2.88 24.6 2.0640 7.44 2.25 8.85 2.64 8.88 1.8357 6.99 1.85 8.86 2.38 8.96 1.5970 4.91 1.88 7.67 2.02 7.74 1.43100 3.15 1.43 3.79 1.80 3.84 1.22400 1.60 1.15 1.51 1.98 1.55 0.84

• Diffusion is reduced under confine-ment and increasing Pressure.• Incorporation of CO2 into the fluidenhances the diffusion of ethanespecies.• Coexistence of fast and slowtranslational modes within the cavity.• Orientational correlations of inter-facial molecules decay more slowlythan bulk-like ones; CO2 correlationsbeing the slowest.

Local diffusion along r

0

2

4

6

8

10

0 5 10 15 20

Det

h(r)

[1

0−4 c

m2 s

−1 ]

r (Å)

20 bar40 bar70 bar

100 bar400 bar

Figure 5: Local diffusion coefficients of con-fined ethane species. Filled and emptysymbols correspond to pure ethane andequimolar mixture, respectively.

5. Conclusions

Preferential adsorption of CO2 over ethane within the adsorbed layerinside the pores led to significant increments in ethane mobility, dueto displacements of interfacial ethane molecules towards more internal,bulk-like, locations. These effects were found to be more pronounced atlow densities and under strong confinement.

3rd Workshop on Structure and Dynamics of Glassy, Supercooled and Nanoconfined Fluids, BsAs, 14-16 July 2019 doloreselola@gmail.com