(9h) Molecular Dynamics of Ethoxylated Surfactants in Water/N-Heptane Interface

Surfactants are substances widely used in several industrial segments. Mainly, their amphiphilic nature enables them to act as emulsifiers, drug carriers, detergents, and components for chemical flooding in enhanced oil recovery. Poly-(oxyethylene) alkyl ethers are a class of non-ionic surfactants often used as reference detergents due to their relatively simples structure compared to typical commercial surfactants. From the molecular point of view, their structure, referred to as C_n(EO)_m, is composed of a hydrophobic tail with n carbon units and a hydrophilic head consisting of m ethylene oxide units. This tunable structure allows a design to tailor the substance’s properties to the desired application. To investigate the influence of structural changes, such as in the length of the hydrophobic tail and the number of ethoxylated units, we can conduct molecular dynamics simulations. With this technique, we can assess both macroscopic, such as interfacial tension and partition coefficients, and microscopic properties, such as density distributions, orientation profiles, and structural parameters, for water/oil interfaces in the presence of surfactants. Herein, we evaluate the interfacial properties of water/oil systems in the presence of poly-(oxyethylene) alkyl ethers surfactants through atomistic molecular dynamics simulations. We use the SPC/E model for water, and pure n-heptane represents the oil phase. The chosen surfactant structure was an hydrophobic tail composed of eight carbon atoms and an ethoxylated chain with a varying number of units. The GAFF force field was used for the oil phase and the surfactants molecules. We verify that the surfactant molecules exhibit tensoactive behavior, staying in the interfacial region and causing a reduction to the interfacial tension. As the number of ethoxylated units increases, so does the decrease in the tension. In terms of structure, we observe a folding behavior of the chains along with a parallel orientation in regards to the interface, maximizing their exposure to the interfacial region. Potential of Mean Force (PMF) calculations also show the relative solubility of these structures in both water and n-heptane phases. We observe that these surfactants are more soluble in n-heptane, while their solubilities in water increase with the size of the head group. We also verify that the free energy of adsorption to the interface in infinite dilution increases with the head group, as shown in the deeper free energy well calculated in the interfacial region.