Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture

Abstract

Molecular dynamics simulations are carried out to get insights into the interfacial behavior of the decane + brine + surfactant + CH₄ + CO₂ system at reservoir conditions. Our results show that the addition of CH₄, CO₂, and sodium dodecyl sulfate (SDS) surfactant at the interface reduces the IFTs of the decane + water and decane + brine (NaCl) systems. Here the influence of methane was found to be less pronounced than that of carbon dioxide. As expected, the addition of salt increases the IFTs of the decane + water + surfactant and decane + water + surfactant + CH₄/CO₂ systems. The IFTs of these surfactant-containing systems decrease with temperature and the influence of pressure is found to be less pronounced. The atomic density profiles show that the sulfate head groups of the SDS molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The sodium counterions of the surfactant molecules are located very close to their head groups. Furthermore, the density profiles of water and salt ions are hardly affected by the presence of the SDS molecules. However, the interfacial thickness between water and decane/CH₄/CO₂ molecules increases with increasing surfactant concentration. An important result is that the enrichment of CH₄ and/or CO₂ in the interfacial region decreases with increasing surfactant concentration. These results may be useful in the context of the water-alternating-gas approach that has been utilized during CO₂-enhanced oil recovery operations.

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1 Introduction

The emission of anthropogenic CO₂ is one of the major causes of global climatic changes.^1–4 Carbon capture and storage technology might be beneficial for mitigating these emissions. Various adsorbents (e.g., carbon nanotubes and clays)^5–14 have been extensively utilized for carbon dioxide capture. In enhanced oil recovery (EOR) operations, the oil recovery could also be combined with the carbon dioxide storage.^15–21 The water-alternating-gas (WAG) approach has been utilized for mobility control during CO₂-EOR operations.^16,18,19,21 The WAG cycles consist of injecting water (or surfactant) and CO₂ alternatively into the reservoirs. Lowering the interfacial tension (IFT) of the oil + water system leads to an increase in the capillary number,^17,20 which may help to recover more oil. In general, the presence of surfactant/CO₂ decreased this IFT. In addition, the captured CO₂ contains impurities (e.g., CH₄)^7,22–24 that may have an important influence on the EOR operations.

Experiments,^25–37 theory,^{32–34,36–39} and simulations^38–45 have been successfully employed to understand the bulk and interfacial properties of alkane + water + CH₄/CO₂ systems. These studies have reported the occurrence of, for example, a two-phase region at high pressures. The IFT of the alkane + water + CH₄/CO₂ two-phase systems was more similar to that of the corresponding alkane + water system alone.^{25,35,36,38–43,45} The IFTs of the alkane + water + CH₄/CO₂ and alkane + brine + CH₄/CO₂ systems increased with decreasing x_CH4/x_CO2 (x_CH4 and x_CO2 are the mole fractions of CH₄ and CO₂ in the alkane-rich phase, respectively). This can be attributed to the fact that the interface was enriched with CH₄ and CO₂ molecules.^38–43 The IFTs of the alkane + brine and alkane + brine + CH₄/CO₂ systems were reported to increase with increasing salt concentration.^{26,27,29,39,44,45} It is also known that the addition of surfactants such as sodium dodecyl sulfate (SDS) generally decreases the IFT.^46–58 Molecular simulations showed that, in the water + surfactant system, water molecules and sodium counterions are relatively near the SDS headgroups.^49,51–58 Bruce et al. found distortions in the water–water hydrogen bonding network because of the SDS–water hydrogen bond formations.⁴⁹ Lin et al. found that, in the water + surfactant + CH₄ system, the solubility of CH₄ in water is not affected by the presence of SDS molecules at the interface.⁵⁵ da Rocha et al. observed favorable CO₂-fluorinated surfactant tail interactions at the interface in the water + surfactant + CO₂ system.⁵⁰ The high density of CO₂ in the bulk enabled strong interaction between CO₂ and the hydrocarbon tail of SDS at the interface.^56,57 However, the interfacial behavior of the decane + brine + surfactant + CH₄ + CO₂ system has not been studied yet.

Molecular simulations have emerged as important tools for accurately predicting the bulk and interfacial properties.^{9,12–14,38,59–62} Here, we perform molecular dynamics (MD) simulations to get insights into the interfacial behavior of the decane + brine + surfactant system in the presence of CH₄ and CO₂ at reservoir conditions.

2 Simulation details

MD simulations of decane + water + surfactant and decane + brine + surfactant two-phase systems in the presence of CH₄ and CO₂ at 323 and 443 K, and pressure up to 100 MPa were carried out using the GROMACS package.⁶³ The salt (NaCl) concentration is 2.7 mol kg⁻¹ and the amounts of surfactant adsorbed at the interface are 0.008 and 0.016 SDS per Ų. The method is similar to that used previously by us.^39,64,65 In short, the TraPPE force field was used to model normal decane, methane, and carbon dioxide.^66–68 Water is represented by the TIP4P/2005 model⁶⁹ and the Na⁺ and Cl⁻ ions are described using the Smith and Dang⁷⁰ parameters. As in the case of, e.g., decane, the hydrocarbon tail of SDS (C12) was also modeled using the TraPPE united atom force field. The sulfate head group of SDS was modeled using the all-atom CHARMM36 forcefield.⁷¹ The Lennard-Jones energy (ε) and distance (σ) parameters, and the charges (q) of the SDS molecules are given in Table 1. The number of each species employed in our simulations is given in Table 2. For example, all the systems had 2048 water and 200 decane molecules. Also, all the systems had the dimensions of 36 × 36 Å parallel to the interfaces (Fig. 1). The cell size in the z-direction (perpendicular to the interfaces) L_z was about three times this value. The system sizes used here ensure that the finite-size effects are negligible.^{38,40–43,72–74} Each system was equilibrated for 5 ns in the NPT ensemble (only L_z varied) and we ran a 5 ns production under NVE conditions. The temperature was controlled using the Nosé–Hoover thermostat and pressure using the Parrinello–Rahman barostat.

Table 1 Force field parameters of SDS

Site	σ (Å)	ε (kJ mol^−1)	q (e)	Ref.
O	3.029	0.502	−0.650	71
S	3.741	1.966	1.330	71
O (in O–C bond)	2.939	0.418	−0.280	71
CH2 (in O–C bond)	3.950	0.382	−0.100	66
CH2	3.950	0.382	0	66
CH3	3.750	0.815	0	66

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Table 2 Number of molecules used in MD simulations

System	Water no.	Decane no.	CH4 no.	CO2 no.	Na^+/Cl^− no.	SDS no.
Water + decane + SDS	2048	200				20–40
Brine + decane + SDS	2048	200			100	20–40
Water + 50%decane + 50%CH4 + SDS	2048	200	200			20–40
Brine + 50%decane + 50%CH4 + SDS	2048	200	200		100	20–40
Water + 50%decane + 50%CO2 + SDS	2048	200		200		20–40
Brine + 50%decane + 50%CO2 + SDS	2048	200		200	100	20–40
Water + 50%decane + 25%CH4 + 25%CO2 + SDS	2048	200	100	100		20–40
Brine + 50%decane + 25%CH4 + 25%CO2 + SDS	2048	200	100	100	100	20–40

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Fig. 1 Equilibrium snapshot of the decane + brine + surfactant + CH₄ + CO₂ (x_CH4 = x_CO2 = 0.25) system at 443 K and 20 MPa. The NaCl concentration is 2.7 mol kg⁻¹ and the amount of surfactant adsorbed at the interface is 0.016 SDS per Ų.

The IFT was estimated from the below equation:^{38,39,64,65,72–75}

where the diagonal components of the pressure tensor are represented by P_xx, P_yy, and P_zz. The radial distribution function (RDF) was determined as described previously.¹⁴ The validation of models and a detailed analysis of the interfacial behavior of decane + H₂O and decane + brine (NaCl) systems in the presence of CH₄, CO₂, and their mixture can be found in our previous studies.^38,39 The amounts of surfactant chosen in our simulations (0.008 and 0.016 SDS per Ų) seem to be well below the critical micelle concentration (CMC). The overlap of the simulated IFTs of the water + SDS and water + SDS + CH₄ systems with the corresponding experimental data^46,47 is shown in Fig. 2. To achieve the overlap, the bulk concentration of surfactant reported in the experiments was multiplied by a constant (same value used in both systems). It is important to mention that these estimates could be further improved by using the Gibbs adsorption isotherm.^55,58 For these systems, the CMC is about 8 mM (about 0.025 SDS per Å^2 76,77). Note, however, that higher surfactant concentrations might be considered by using coarse-grained models.⁷⁸

Fig. 2 IFT dependence on the surface concentration of surfactant for the (a) water + surfactant system at 298 K and (b) water + surfactant + CH₄ system at 273.2 K and 7.2 MPa.

3 Results

3.1 Interfacial tension

The simulated IFTs of the decane + water + surfactant and decane + brine + surfactant systems in the presence of CH₄ (x_CH4 = 0.5), CO₂ (x_CO2 = 0.5), and their equimolar mixture (x_CH4 = x_CO2 = 0.25) at 443 K are provided in Fig. 3. The salt (NaCl) concentration was 2.7 mol kg⁻¹ and the amounts of surfactant adsorbed at the interface were 0.008 and 0.016 SDS per Ų. The corresponding simulation results at 323 K are provided in Fig. S1 (ESI†). Note that our previous results of the simulated IFTs of the decane + water³⁸ and decane + brine (NaCl)³⁹ systems in the presence of CH₄, CO₂, and their mixture compared well with the corresponding experimental^25,35 and density gradient theory (DGT)^38,39 results. Past studies have shown that the IFT of the alkane + water + CH₄/CO₂ systems is more similar to that of the corresponding alkane + water system alone.^{25,35,36,38–43,45} Also, the IFT of the decane + brine + CH₄/CO₂ systems was more similar to that of the corresponding decane + brine system alone.^{26,29,39,44,45} The IFTs of all these systems decreased with temperature. For instance, the simulation value of the IFT of the decane + water system (20 MPa) decreased from about 47.8 mN m⁻¹ at 323 K to about 36.5 mN m⁻¹ at 443 K.³⁹ It was found that the IFTs of these systems generally increased with pressure. The influence of pressure was, however, found to be less pronounced at lower temperatures and higher values of x_CH4/x_CO2. Note that the IFT of the water + CH₄/CO₂ and brine + CH₄/CO₂ systems depends nonmonotonically on pressure.^{72–75,79–88} Furthermore, the IFTs of the alkane + water + CH₄/CO₂ and alkane + brine + CH₄/CO₂ systems increased with decreasing x_CH4/x_CO2. Here, the influence of methane was found to be less pronounced than that of carbon dioxide. This can be attributed to the fact that the interface was highly enriched with CO₂ molecules than with methane molecules. It was found that the IFTs of the alkane + brine and alkane + brine + CH₄/CO₂ systems increase with salt concentration.^{26,27,29,39,44,45} Here, the linear slope in the IFT versus salt (NaCl) concentration plot was about 2 mN (m mol kg⁻¹)⁻¹ under all conditions. This value was similar to that reported for the brine + CH₄/CO₂ systems.^72,73,79 However, higher slopes were reported for the corresponding systems containing divalent ions.^73,79

Fig. 3 IFTs of (a) decane + brine + surfactant, (b) decane + brine + surfactant + CH₄ (x_CH4 = 0.5), (c) decane + brine + surfactant + CO₂ (x_CO2 = 0.5), and (d) decane + brine + surfactant + CH₄ + CO₂ (x_CH4 = x_CO2 = 0.25) systems at 443 K. Error bars are smaller than the symbol size.

Previous studies^47,55 have also shown that the IFTs decreased with increasing SDS concentration, e.g., at the methane/water interface (see Fig. 2). Our current results show that the addition of the SDS surfactant reduces the IFTs of the decane + water and decane + water + CH₄/CO₂ systems. Similar behavior is also observed for the decane + brine and decane + brine + CH₄/CO₂ systems. For example, the IFT of the decane + water + surfactant (0.016 SDS per Ų) system is about 16.5 mN m⁻¹ at 20 MPa and 443 K. As in the case of, for example, the alkane + water system, the IFTs of these surfactant containing systems decrease with temperature and the influence of pressure is found to be less pronounced. We see that the IFTs of the decane + water + surfactant + CH₄/CO₂ and decane + brine + surfactant + CH₄/CO₂ systems decrease with increasing x_CH4/x_CO2. Again the influence of methane is found to be less pronounced than that of carbon dioxide. For example, the IFTs of the decane + water + surfactant (0.016 SDS per Ų) system in the presence of CH₄ (x_CH4 = 0.5) and CO₂ (x_CO2 = 0.5) were about 13.7 and 11.5 mN m⁻¹, respectively, at 20 MPa and 443 K. It can be seen that the IFT of, for instance, the decane + water + surfactant + CH₄ + CO₂ system is more similar to that of the corresponding decane + water + surfactant + CO₂ system. Furthermore, the addition of salt increased the IFTs of the decane + water + surfactant and decane + water + surfactant + CH₄/CO₂ systems. For example, the IFT of the decane + brine + surfactant (0.016 SDS per Ų) system is about 25.2 mN m⁻¹ at 20 MPa and 443 K.

3.2 Atomic density profiles

The atomic density profiles may provide insights into the bulk and interfacial properties of the studied systems. The simulated density profiles for the decane + brine + surfactant + CH₄ + CO₂ (x_CH4 = x_CO2 = 0.25) system at 443 K and 20 MPa are provided in Fig. 4. These profiles at other conditions are provided in Fig. S2–S4 (ESI†). Note that our simulation results of the atomic density profiles for the decane + water³⁸ and decane + brine (NaCl)³⁹ systems in the presence of CH₄, CO₂, and their mixture compared well with the corresponding DGT^38,39 results. Previous studies of the alkane + water + CH₄/CO₂ and alkane + brine + CH₄/CO₂ systems^{36,38–43,45} have shown that the presence of CH₄ and CO₂ hardly affects the density profiles of water, alkane, and salt. It was found that, in general, the density profiles of water and alkane vary monotonically across the interfacial region. These profiles might be approximated by a hyperbolic tangent function.⁸⁹ The salt ions were excluded from the interfacial region and distributed homogenously within the H₂O-rich phase. However, water is enriched at the interfacial region with the addition of salt.^39,73,86,90 This can be explained by the enhanced ionic desorption from the interfacial region at high salt concentrations. Also, at high x_CH4, the interfacial region was enriched with decane in the alkane + water + CH₄ system.³⁸ It is worth noting that the simulated distributions of alkane exhibited artificial oscillations in the interfacial region due to finite-size effects.^89,91 The interfacial region was enriched with CH₄ and CO₂ molecules for the alkane + water + CH₄/CO₂ and alkane + brine + CH₄/CO₂ systems.^{36,38–43,45} This enrichment was found to decrease with temperature and increase with pressure. Also, in general, this enrichment increased with increasing x_CH4/x_CO2. Here, the enrichment of methane was found to be less pronounced than that of carbon dioxide. The presence of salt had no significant effect on the CH₄/CO₂ enrichment.³⁹ At low pressures, however, the enrichment of the interfacial region with CO₂ (CH₄) depends nonmonotonically on x_CO2 (x_CH4).^38,40 Details of the interfacial behavior of the water + CH₄/CO₂ and brine + CH₄/CO₂ systems have been described by us and others.^{72–75,80–84,86–88} Here the enrichment of the interfacial region with CH₄ and CO₂ depends nonmonotonically on pressure.

Fig. 4 Atomic density profiles for the decane + brine + surfactant + CH₄ + CO₂ (x_CH4 = x_CO2= 0.25) system at 443 K and 20 MPa: (a) 0.008 SDS per Ų and (b) 0.016 SDS per Ų. The NaCl concentration is 2.7 mol kg⁻¹.

Note that the behavior of the IFT can be further understood by means of the Gibbs adsorption equation:

where Γ_i and μ_i are the surface excess and the chemical potential of component i, respectively. The surface excesses could be calculated using the density profiles.^{38,39,64,65,73,74,84,85} A detailed description of the surface excess of different species in the decane + water + CH₄/CO₂ and decane + brine + CH₄/CO₂ systems is provided in our previous reports.^38,39 For these systems, it was found that the IFTs increase with pressure (salt concentration) due to the negative surface excesses of alkanes (salt).^38,39 For the water + CH₄/CO₂ and brine + CH₄/CO₂ systems, a minimum was found in the IFT versus pressure plot when the surface excess of CH₄/CO₂ changes sign from positive to negative.^73,75,84,85 Furthermore, for the alkane + water and alkane + water + CH₄/CO₂ systems, the solubilities of decane/CH₄/CO₂ in the water-rich phase and water in the decane-rich phase were very low.^38,92,93 The solubility of CH₄/CO₂ in the water-rich phase was found to decrease with the addition of salt (salting-out effect).³⁹

Our current results show that the sulfate head groups (see, e.g., sulfur atoms) of the SDS surfactant molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The Na⁺ counterions of the SDS surfactant molecules are located very close to their head groups (see, e.g., Fig. S2, ESI†). Furthermore, the distributions of water and Cl⁻ ions are independent of the surfactant concentration for the studied systems (see also Fig. S5, ESI†). However, the interfacial thickness between water and decane/CH₄/CO₂ molecules increases with increasing surfactant concentration. We estimated the interfacial thickness between water and decane by applying the “90–90” interfacial thickness criterion (distance between positions where densities of decane and H₂O were 90% of their own bulk densities).⁵¹ The results are provided in Fig. S6–S9 (ESI†). The interfacial thickness between water and decane is in the range of about 1.1–1.7 nm for the studied systems. This interfacial thickness shows an opposite trend to that seen for the IFT. For example, the interfacial thickness decreases with the addition of salt and increases with the addition of CH₄/CO₂. Interestingly, the enrichment of CH₄ and/or CO₂ in the interfacial region decreases with increasing surfactant concentration. It seems that the enrichment is followed by a minimum in the density profile of CO₂ at high surfactant concentrations. This minimum is found near the location of the surfactant tails.

We calculated the surface excess^{38,39,64,65,73,74,84,85} by using these density profiles. Our results show that the surface excess of CH₄, CO₂, and decane decreases with increasing surfactant concentration (Fig. 5). The surface excess of CH₄/CO₂ changes sign from positive to negative as surfactant concentration increases. The addition of SDS surfactants has a more pronounced effect on the surface excess of decane. For example, at 443K and 20 MPa, the surface excess of decane in the decane + brine + CH₄ + CO₂³⁹ and decane + brine + surfactant + CH₄ + CO₂ (0.016 SDS per Ų) systems is about −0.47 × 10⁻⁶ mol m⁻² and −2.7 × 10⁻⁶ mol m⁻², respectively. The corresponding surface excess of CH₄ is about 0.19 × 10⁻⁶ mol m⁻² and − 0.73 × 10⁻⁶ mol m⁻², respectively, and that of CO₂ is about 0.57 × 10⁻⁶ mol m⁻² and −0.03 × 10⁻⁶ mol m⁻², respectively. Here the effects of pressure, temperature, and mole fraction on the surface excess are similar to those observed for the decane + water + CH₄/CO₂ and decane + brine + CH₄/CO₂ systems.^38,39 For example in all cases, we see that the surface excess of decane decreases with pressure, whereas the surface excess of CH₄ and CO₂ increases with pressure.

Fig. 5 Simulated surface excess (symbols) of (a) CH₄, (b) CO₂, and (c) decane for the decane + brine + surfactant + CH₄ + CO₂ (x_CH4 = x_CO2 = 0.25) system at 323 K and NaCl concentration of 2.7 mol kg⁻¹. The corresponding surface excess at 443 K is shown in (d), (e), and (f), respectively. The lines denote the DGT results.³⁹

3.3 Radial distribution functions

The RDFs may give further insights into the interfacial properties of the surfactant-containing systems. The simulated RDFs for the decane + brine + surfactant + CH₄ + CO₂ system at 443 K and 20 MPa are provided in Fig. 6. We see that water molecules and counterions are nearest to the SDS headgroups. The first peak in the RDF of S–H (water), S–Na⁺, and S–O (water) is around 0.29, 0.36, and 0.38 nm, respectively. These peak positions obtained here are consistent with the previous simulation results.^52–54,58 The presence of water molecules near the SDS headgroups is possibly due to the hydrogen bonding between SDS headgroups and water molecules.⁴⁹ Whereas, CO₂, CH₄, decane, and Cl⁻ are further away from the SDS headgroups. The first peak in the RDF of S–O (CO₂), S–CH₄ site, and S–Cl⁻ is around 0.40, 0.46, and 0.49 nm, respectively. The RDFs show that the interactions of the surfactant headgroups with methane are weaker than those with CO₂. This is expected due to the quadrupole moment of CO₂ molecule.⁹⁴ Similar positions of these peaks were found in all studied systems. It is worth mentioning that here the RDFs may not go to unity at large distances due to the inhomogeneous nature of the system.^52–54,58 In comparison, the alkyl tails of SDS interact strongly with decane molecules leading to peaks, for example, at 0.55 nm (Fig. S10, ESI†). Furthermore, it is found that the alkyl tails of SDS interact similarly with CH₄ and CO₂. For both these interactions the first peak in the RDF plot is around 0.47 nm and its magnitude is about 1.7. A similar result is obtained for the interactions between alkane and CH₄/CO₂.^64,65

Fig. 6 RDFs between the S-atom of SDS and different species in the decane + brine + surfactant + CH₄ + CO₂ (x_CH4 = x_CO2= 0.25) system at 443 K and 20 MPa. The NaCl concentration is 2.7 mol kg⁻¹ and the amount of surfactant adsorbed at the interface is 0.016 SDS per Ų.

We calculated the end-to-end distance and radius of gyration^62,64,65 of the alkyl tails of SDS molecules for the decane + water + surfactant system (Table 3). Here the end-to-end distance is in the range of about 1.12 to 1.18 nm. This end-to-end distance decreases with temperature and increases with the amount of surfactant. We found that pressure has no effect on the end-to-end distance. Similar trends were observed for the radius of gyration. Furthermore, these sizes were not affected by the presence of salt/CH₄/CO₂ under the studied conditions.

Table 3 Chain size of the alkyl tail (C12) of SDS for the decane + water + surfactant system at 20 MPa

No. of SDS per Å^2	End to end distance/radius of gyration (nm)
	323 K	443 K
0.008	1.16/0.394	1.12/0.383
0.016	1.18/0.396	1.14/0.385

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Regarding the SDS force field (see Table 1), it was shown that the difference among the force fields does not have much effect on the overall structure of small aggregates of surfactant.⁵⁴ Among different water models, the surface tension simulated using TIP4P/2005 model was close to the experimental data.⁷² Moreover, our simulated RDF peaks for SDS–water spatial correlations are consistent with previous studies.^52–54,58 It will be challenging to obtain the relevant activity coefficients of, e.g., salt from molecular simulations for these multi-component systems.⁹⁵ However, for decane + brine + CH₄/CO₂ systems, theoretical analysis showed that changing salt concentration does not have much effect on the chemical potential of decane, CH₄, and CO₂.^38,39 This explains the fact that the IFTs increased with salt concentration because of the negative surface excess of salt ions (see eqn (2)).^38,39 Note that we did not consider any SDS in the bulk and the salting-out of SDS to the interface.⁴⁸ Therefore, further studies are necessary to fully understand the effects of surfactants on the decane + brine + CH₄/CO₂ systems.

4 Conclusions

The interfacial behavior of the decane + brine + surfactant + CH₄ + CO₂ two-phase system was studied using MD simulations at 323 and 443 K, and pressure up to 100 MPa. Note that our previous results^38,39 of the simulated IFTs of decane + brine + CH₄/CO₂ systems compared well with the corresponding experimental^25,35 and DGT^38,39 results. Our current results show that the addition of CH₄, CO₂, and the presence of SDS surfactant at the interface reduced the IFTs of the decane + water and decane + brine (NaCl) systems. Notably, the influence of methane was observed to be less pronounced than that of carbon dioxide. For example, the IFT of the decane + water + SDS + CH₄ + CO₂ (x_CH4 = x_CO2= 0.25) system is more similar to that of the corresponding decane + water + SDS + CO₂ (x_CO2= 0.5) system. This may be attributed to the fact that the interface was highly enriched with CO₂ molecules than with CH₄ molecules. As expected, at a fixed surface concentration of SDS, the addition of salt increases the IFTs of the decane + water + SDS and decane + water + SDS + CH₄/CO₂ systems. The IFTs of these surfactant-containing systems decreased with temperature and the influence of pressure was found to be less pronounced.

The atomic density profiles show that the sulfate head groups of the SDS molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The Na⁺ counterions of the surfactant molecules are positioned very close to their head groups. Furthermore, the density profiles of water and salt ions are independent of the surfactant concentration. However, the interfacial thickness between water and decane/CH₄/CO₂ molecules increases with increasing surfactant concentration. For instance, the interfacial thickness between water and decane is in the range of about 1.1–1.7 nm for the studied systems. This interfacial thickness decreased with the addition of salt and increased with the addition of CH₄/CO₂. An important finding is that the enrichment of CH₄ and/or CO₂ in the interfacial region decreases with increasing surfactant concentration. The calculated RDFs show that the interactions of the surfactant headgroups with methane are weaker than those with CO₂.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the support of this work by the King Abdullah University of Science and Technology (KAUST) under Award No. OSR-2019-CRG8-4074. We also thank the computational support from KAUST.

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Footnote

† Electronic supplementary information (ESI) available: Additional details of simulation analysis. See DOI: 10.1039/d1sm01267c

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