The effects of surfactant tail branching on oil-water interfacial tension reduction

Because surfactants are used in a variety of applications to control interfacial properties, new interfacially active molecules, including biosurfactants, designed for a tailored set of properties, are always needed. However, the molecular mechanisms that determine the effectiveness of surfactants, e.g., lowering interfacial tension, are not fully understood. It is hypothesized that the molecular structure of the surfactants controls the mechanism of interfacial tension decrease.

Molecular dynamics simulations are employed here to quantify the relationship between tailgroup branching and interfacial tension reduction at the water/n-heptane interface. Using sodium n-hexadecylsulfate as a reference molecule, a series of isomeric surfactant analogues are employed here, with systematic variations in chain branching while maintaining a constant number of sixteen carbon atoms in the tails. The surfactants are studied at the water/n-heptane interface, and at equal surface density. To identify the mechanisms responsible for interfacial tension reduction, key interfacial properties such as entropy changes, interfacial widths, coordination lifetimes, and adsorption free energies are computed.

The results show that increasing surfactant chain branching leads to more globular and better packed surfactant molecules at the water/n-heptane interface, which effectively separates water from n-heptane. This structure also yields an increased entropy in the interfacial oil and effectively separates water from n-heptane. Combined, these two mechanisms lead to substantial reductions in interfacial tension, although branching slightly weakens the thermodynamic driving force for surfactant adsorption at the interface. These molecular insights could be useful for designing new high-performance surfactants and perhaps also biosurfactants.