Molecular Dynamics Study on CO₂ Foam Films with Sodium Dodecyl Sulfate: Effects of Surfactant Concentration, Temperature, and Pressure on the Interfacial Tension

Abstract

CO2 foam flooding technology can be used for geological storage of greenhouse gas CO2 while enhancing oil recovery. In this paper, we performed molecular dynamics simulations for the CO2 foam film systems stabilized with an anionic surfactant of sodium dodecyl sulfate (SDS). Compared to other factors of a foam system, interfacial tension was used as the primary indicator to evaluate the stability of the SDS foam film. The effects of the concentration of the SDS surfactant, temperature, and pressure on the interfacial tension were studied. Based on the calculated results, the stability of the CO2 foam film was discussed. It is found that under the high-concentration condition, SDS molecules can form a dense and thick molecular layer at the interface, blocking the contact of CO2 molecules and water molecules and reducing the interfacial tension. Consequently, the stability of the foam liquid film can be improved. Low temperature and high pressure lead to high density of the CO2 phase, which enables strong interactions between CO2 and the hydrophobic tails of SDS molecules. The interfacial structure thus formed can reduce the contact probability between CO2 and water molecules, generating lower interfacial tension. Therefore, high SDS concentration, low temperature, and high pressure are beneficial to the stability of the CO2 foam. We show that the adsorption of CO2 molecules at the interface, the interfacial thickness, and solvent accessible surface area of the surfactant to the CO2 phase are affected by the density of CO2 bulk phase, which are important interfacial properties affecting the CO2/water interfacial tension and the stability of the CO2 foam.

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

“…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][47][48][49][50][51][52][53][54][55][56][57][58] Molecular simulations showed that, in the water + surfactant system, water molecules and sodium counterions are relatively near the SDS headgroups. 49,[51][52][53][54][55][56][57][58] Bruce et al found distortions in the water-water hydrogen bonding network because of the SDS-water hydrogen bond formations.…”

“…[46][47][48][49][50][51][52][53][54][55][56][57][58] Molecular simulations showed that, in the water + surfactant system, water molecules and sodium counterions are relatively near the SDS headgroups. 49,[51][52][53][54][55][56][57][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 + CH 4 system, the solubility of CH 4 in water is not affected by the presence of SDS molecules at the interface.…”

“…50 The high density of CO 2 in the bulk enabled strong interaction between CO 2 and the hydrocarbon tail of SDS at the interface. 56,57 However, the interfacial behavior of the decane + brine + surfactant + CH 4 + CO 2 system has not been studied yet.…”

Structure and Shape of Surface-Mediated Assembly of Surfactants

“…The ability of amphiphilic molecules, also known as surfactants, to tune the surface tension of fluids is essential for stabilizing foams of CO 2 bubbles dispersed in water at elevated temperature and pressure, which enables effective utilization of pore space for geologic storage of CO 2 . − Furthermore, surfactant chemistries can be tuned for developing pharmaceutical therapies to enhance lung capacity and respiration, − delivering medicines that have low solubility in water, , advanced separations for water purification, and the synthesis of novel materials with tunable porous architectures for catalysis, gas separations, and storage. − These wide-ranging applications are attributed to the structures of surfactants, which are formed due to the aggregation or assembly of their molecules. A surfactant molecule has a bulk hydrophilic head, which is sometimes charged, and a relatively short hydrophobic tail .…”

“…In this process, CTAB is preferentially adsorbed on the rock surfaces, , resulting in a reduction of interfacial tension between the oil and rock. With increasing interest in harnessing CO 2 as a working fluid in subsurface environments (e.g., CO 2 foams − ), a fundamental understanding of the assembly of surfactants at solid interfaces is crucial to inform advances in the use of CO 2 or alternative fluids that require the use of CTAB for subsurface energy applications.…”