Adsorption of surfactants on the solid-oil interface continues to play an important role in many technological applications, ranging from Enhanced Oil Recovery (EOR) to wear-and-tear protection. Undoubtedly, the properties of... [ view full abstract ]
Adsorption of surfactants on the solid-oil interface continues to play an important role in many technological applications, ranging from Enhanced Oil Recovery (EOR) to wear-and-tear protection. Undoubtedly, the properties of the adsorbed surfactants are complex as they depend on many factors such as surface chemistry and roughness, surfactant nature and oil composition. Consequently, it is important to examine these factors separately under controlled environments to understand their contribution to the system’s behavior. In this work, we present a combined computer simulation and experimental study to understand the adsorption of a model surfactant, similar to those reported in the literature as oil additives, at the solid-oil interface and the effects of concentration and solvent quality are assessed.
Molecular Dynamics (MD) simulations of surfactant adsorption on iron oxide from a solvent containing iso-octane and ethanol were performed over a range of conditions. Simulations were carried out using LAMMPS with the OPLS-AA force-field to represent the interactions of molecules in the fluid phase, while the force field for iron oxide was taken from literature. The structure of iron oxide was taken as the {001} face of hematite.
Simulations show that the in systems with pure iso-octane as a solvent, the surfactant molecules lie flat on the surface at low concentrations. However, the surfactant’s hydrophobic tails extends into the oil layer at monolayer coverage. The results are consistent with experimentally measured adsorbed layer thicknesses.
It is widely known that the presence of co-solvents can affect the surfactant’s behavior. Although much research has been done in aqueous systems to understand how co-solvents affect self-assembly, their effect in oil-based systems is far from understood. Experiments show that the adsorption of surfactant on iron oxide surface is suppressed by the presence of ethanol at 5% vol concentration. The computer simulation results are supported by experimental adsorption isotherm data on related model systems. Our simulations show that surface active components such as ethanol may competitively adsorb on the solid-oil interface and hinder the surfactant adsorption. At mild temperature condition of 20 – 50 °C, we observe that as the concentration of ethanol increases, surfactant molecules are displaced from the surface which is progressively covered by ethanol. From visualisations, a clear layer of adsorbed ethanol is clearly identified. Furthermore, ethanol molecules present in the bulk interact with the polar head groups in the oil layer, stabilizing the surfactant in solution and hindering its ability to adsorb on the surface.
This work shows that the effect of surfactant concentration in the adsorbed phase structure is similar to what is expected in aqueous systems. Significantly, the competitive adsorption of ethanol, identified in the simulations, is also observed experimentally. More importantly, we show for the first time the effect of co-solvents on surfactant adsorption on oil-based systems, which is different from what is known from aqueous based systems. The microscopic phase segregation and formation of an adsorbed ethanol layer may have important implications in the formulation of surfactants in oil-based applications.
Interfacial and confined phenomena , Challenges and advances in fluid phase equilibria
