Opposite effect of cyclic and chain-like hydrocarbons on the trend of self-assembly transition in catanionic surfactant systems

Hydrocarbons are well-known insoluble oils which are rarely considered as stimuli to trigger molecular selfassembly transition. We report in this work that hydrocarbons are a class of distinct stimuli for the selfassembly transition in the catanionic surfactant systems. Particularly, the cyclic hydrocarbons and chain-like ones has exactly opposite impact. By solubilizing in the palisade region of surfactant bilayers, the cyclic hydrocarbons would increase the volume of the hydrophobic part of the surfactant, thus increasing the packing parameter and leading to structures with smaller curvature. On the contrary, the chain-like hydrocarbons will be insolubilized in the interior of the hydrophobic domains formed by surfactant, which therefore swells the surfactant bilayers and increases the curvature of the outer layer. As a result, the self-assembly would change into structures with larger curvature. For this reason, addition of cyclic alkanes, such as ethylcyclohexane and ethylbenzene, leads to the transition from vesicles to lamellae, whereas addition of chain-like ones, such as octane, results in the transition from vesicles or lamellae to nanoemulsions. It is noticed that the saturation state of the hydrocarbons is not relevant to the self-assembly transition, and whether the alkanes are cyclic or chain-like is the determinative factor that decides which direction the self-assembly would transform to. We expect the current study is very promising in guiding enrichment and separation of organic matters.

Automated summary

Hydrocarbons are found to be a distinct stimulus for the selfassembly transition in catanionic surfactant systems, with cyclic and chain-like hydrocarbons having opposite effects. The saturation state of hydrocarbons is not relevant to the self-assembly transition; the determining factor is whether the alkanes are cyclic or chain-like.