A molecular study on the behavior of polyethoxylated alkyl ethers surfactants in a water/n-alkane interface

Advanced oil recovery in oil/water and water/oil emulsions is a scientific and technological challenge, and efficient technologies depend on the water/oil interface properties. Due to their amphiphilic nature, surfactants are used to modify these interfacial properties to increase oil recovery and stabilize droplets in emulsification processes. Poly-(oxyethylene) alkyl ethers are a class of non-ionic surfactants often used as reference detergents due to their relatively simple structure compared to typical commercial surfactants. This tunable structure allows a design to tailor the substance’s properties to the desired application. Here, we conduct molecular dynamics simulations to obtain both macroscopic and microscopic properties of water/n-heptane interfacial systems in the presence of these surfactants. We evaluated the effect of both temperature and modifications to the structure of the surfactant model, varying the number of ethylene oxide units in the head group. We verify that their structural properties are specific to maximize stabilizing interactions such as hydrogen bonds with water molecules, adopting a folded structure and parallel orientation in regards to the interface. We analyze the Potential of Mean Force (PMF) that shows the affinity of the surfactant to the interface is enhanced as the number of ethoxylated units increases, which is related to the decrease in interfacial tension.