Interplay of Dynamical Properties between Ionic Liquids and Ionic Surfactants: Mechanism and Aggregation

The dynamical and aggregation behaviors of sodium dodecyl sulfate (SDS) in 1-ethyl-3-methylimidazolium ethylsulfate [EMIM+][EtSO4-] are characterized experimentally and computationally. A retardation of the ionic liquid (IL) and SDS diffusion coefficients with a concentration increase of SDS is observed. In agreement with experiments, aggregation is detected for concentrations higher than the experimental critical micelle concentration (CMC), which is mostly driven by alkyl tail aggregation. Solvent-exposed hydrophobic patches are observed on the micelle's surfaces. The hydrophobic tails of the IL molecules are found to fill those micelle's hydrophobic patches. Also, penetration of the IL is found in the SDS micelles, indicating that the IL acts as a cosurfactant, allowing the formation of "mixed" micelles. A higher level of Na+ counterion dissociation compared to previous studies of SDS micelles in aqueous solutions is also observed. A multilayering effect of alternating IL anions and cations is detected at the surface of the formed aggregates. The observed increase in system ordering with SDS concentration is what hinders the mobility of each chemical species.