Effect of surfactant structure on interfacial properties

– We study surfactants at the oil/water interface using Dissipative Particle Dynamics simulations at constant μsurfPT . The interfacial tension depends on the surfactant branching in a subtle way. For a given interfacial concentration, a double-tail surfactant is more efficient than its single-tail isomer only if the oil-head repulsion is sufficiently strong. For a given concentration in the bulk water phase, the single-tail surfactants are more efficient in both cases. We interpret these results in light of the molecular packing at the interface and free-energy considerations. Introduction. – Surfactants are molecules that consist of hydrophobic and hydrophilic parts. Their amphiphilic nature makes them surface active and, adsorbed at the oil/water interface, they can reduce the bare oil-water interfacial tension to very low values. Because of this property, surfactants are used in many practical applications ranging from crude oil recovery to state-of-the-art drug delivery [1] and are also of scientific interest. From a practical viewpoint, it is important to understand how the efficiency in reducing the interfacial tension is related to the structure of a surfactant. This question was already posed by Traube in 1899 [2] and he discovered that increasing the hydrophobic tail length results in surfactants that are more efficient (Traube’s rule). At present, the effect of branching of the hydrophobic tail is not yet fully understood, despite the fact that many of the surfactants used in industrial applications are prepared with branched hydrocarbon tails. The effect of branching on the interfacial tension was investigated by self-consistent field calculations [3] and molecular-dynamics simulations on model surfactants [3, 4]. These studies agree on the fact that surfactants with two hydrophobic chains are less efficient in reducing the interfacial tension compared to their single-tail isomers. Experimentally, however, either more, equal, or less efficient branched surfactants are reported, depending on the details of the experimental setup [3, 5–8]. (∗) On leave at Department of Chemical Engineering, University of Amsterdam, Amsterdam, The Netherlands. E-mail: [email protected]