Alkane solubilization by surfactants: Aggregate view and size analysis based on cryo-TEM

In this study the aggregation behavior for solubilization of alkanes by surfactants (rhamnolipid, sodium dodecyl benzene sulfonate (SDBS), Triton X-100) were investigated with a focus on the morphology and particle size distribution of the aggregates. The powerful tool of cryo-transmission electron microscopy was used to view the aggregates and obtain the size, which was compared to the size data obtained by the method of dynamic light scattering. The aggregation behavior at both sub-CMC and hyper-CMC concentrations was investigated. The results show that the morphology of the aggregates at both sub-CMC and hyper-CMC concentrations is spherical or spheroidal with a size smaller than 20 nm. No layered structure and no difference in transparency are observed in the aggregates, indicating micelle-type structure. The particle size distribution curve shows only one peak and it is well depicted by log-normal distribution. For all of the surfactants, the size of the aggregates at surfactant concentrations below CMC is larger than that at the concentrations above CMC, which is caused by the difference in excess of the surfactants at the surface of the aggregates. The size of the aggregates formed by anionic surfactants (rhamnolipid and SDBS) and hexadecane is smaller than that formed by nonionic Triton-X-100 and hexadecane and the effect of surfactant concentration on aggregate particle size is more significant for the anionic surfactants. The size of the rhamnolipid-dodecane aggregates is larger than that of the rhamnolipid-hexadecane aggregates at rhamnolipid concentrations above CMC, indicating that the chain length of the alkane also has an effect on the aggregate size.