Understanding the hydrophobic oil/polar water interfacial phenomena dominates many important physical, chemical, and biological processes, including oil extraction, interfacial polymerization, surfactant implications, ion transport across the membrane, protein folding, chemical separation, and nanoparticle formation. Although a wide range of classical experiments has been conducted and reported, the phenomenological arguments have remained on the interactions in such a system. To derive a molecular-level picture of this interface, recent interest has focused on fundamental simulations like molecular dynamics at the molecular/atomic level to solve the contradicted results in the classical experiments of the oil-water interface.
The well-organized structure of water molecules next to any hydrophobic surface due to the weak interactions between these hydrophilic and hydrophobic molecules has been established by researchers in terms of experimental and simulation studies. However, the natural environments are more complex than a simple hydrophobic/hydrophilic interface. Generally, the aqueous phase might have ions; gases like CO2 and organic medium might include different acidic/basic components. Therefore, we need to investigate more complicated liquid/liquid interfaces to approach a real system. One of the complex biphasic hydrophobic/aqueous interfaces is related to the oil reservoirs. To extract more oil and simultaneously store carbon dioxide in the underground reservoirs, the carbonated water has been designed to inject into the oil pools. However, there is no concrete conclusion on which screening parameters should be considered to gain higher oil recovery and higher carbon footprint storage.
Herein, we investigated the effect of the most abundant polar acidic components (Benzoic acid, Decanoic acid, and phenol) in the hydrophilic phase on the interfacial phenomena of hydrophilic phase (Decane) and carbonated water. Hence, molecular dynamics simulation was used to analyze these systems. The structure, mass and charge density, hydrogen bonds, and Interfacial Tension (IFT) have been analyzed for different scenarios.
To the authors’ best knowledge, the effect of combining acidic components and carbon dioxide in the hydrophilic/hydrophobic interface has been investigated for the first time. The results indicated that carbon dioxide molecules tend to accumulate at the interface of decane/water and then diffuse to the oleic phase. When the polar molecules are introduced to the hydrophobic phase, there is a competition between carbon dioxide and acidic components to accumulate at the interface. Since there is a Columbic interaction between polar water molecules and polar acidic components compared to non-polar carbon dioxide molecules, the carbon dioxide molecules diffuse more evenly into the oleic phase and fill the voids in the oleic phase existing due to the entanglement of the alkane chains. Therefore, the acidic components replace the carbon dioxide at the interface and serve as a bridge between the hydrophobic and hydrophilic phases. Indeed, the predominant interaction corresponds to the water and polar molecules. The density peaks of polar molecules occur at the interface where the hydrophilic part of the polar component penetrates to the aqueous phase, and the lipophilic part of polar molecules is in the oleic phase, which is a surfactant-like action.
The anchoring effect of acidic components at the water/decane interface decreases the IFT between decane and water. Although there is a significant reduction in the IFT of systems involving acidic components compared to the simple decane/carbonated water system, there is no significant difference between these three types of acids in decreasing the IFT in the presence of carbon dioxide in the biphasic system. The charge distribution of water molecules in the systems involving polar components changes. In the simple decane/water case, water molecules show a recognized charge distribution at the interface with two well-order H-bonds networks. While in the biphasic systems, including polar molecules, the well-organized charge distribution at the interface (two water surface layer structure) change, and the aqueous phase is disturbed inside itself. This is proof of destroying the shield of the well-organized water layer at the interface to the non-aqueous phase, which leads to IFT reduction as the surface forces between the two phases reach zero.