Interfacial Properties and Monolayer Collapse of Alkyl Benzenesulfonate Surfactant Monolayers at the Decane-Water Interface from Molecular Dynamics Simulations.

The molecular structure of a surfactant molecule is known to have a great effect on the interfacial properties and the type of nanostructures formed. In this work, we have performed molecular dynamics simulations on six isomers of an alkyl benzenesulfonate surfactant to investigate the effect of the degree and position of aromatic substitution on the interfacial properties and on the collapse of the surfactant monolayer at a decane-water interface. The surface pressure of the monolayers was shown to increase with increasing surface coverage, until some of the monolayers become mechanically unstable and form large undulations. Shifting the primary alkyl chain of the surfactant from the para to the meta position was found to significantly affect the orientation of the surfactant head groups, while the attachment position of the benzene ring along the primary alkyl chain plays a greater role in the orientation of the surfactant tails. In general, to the extent considered in this work, our results suggest that additional alkyl substitution and meta substitution of the primary alkyl chains increase both the effectiveness and efficiency of the surfactants, and accelerate the onset of monolayer collapse. The interface was found to consist of an inner Helmholtz layer of partially dehydrated counterions in contact with the surfactant head groups, an outer Helmholtz layer of hydrated counterions, and a diffuse layer. The di- and trisubstituted surfactants formed nearly spherical swollen micelles encapsulating pure decane, which effectively solubilizes decane in water as a microemulsion. The monosubstituted surfactants formed elongated buds that protrude from the interface, but did not detach from the monolayer. To our knowledge, the role of aromatic substitution on interfacial properties has not been investigated by molecular simulations previously. The results from this work could provide insights to design improved surfactants by exploiting aromatic substitution to encapsulate material for drug delivery and other applications.