Molecular Dynamics Shows That Ion Pairing and Counterion Anchoring Control the Properties of Triflate Micelles: A Comparison with Triflate at the Air/Water Interface

Micellar properties of dodecyltrimethylammonium triflate (DTA-triflate, DTATf) are very different from those of DTA-bromide (DTAB). DTATf aggregates show high aggregation numbers (N-agg), low degree of counterion dissociation (alpha), disk-like shape, high packing, ordering, and low hydration. These micellar properties and the low surface tension of NaTf aqueous solutions point to a high affinity of Tf- to the micellar and air/water interfaces. Although the micellar properties of DTATf are well defined, the source of the Tf- effect upon the DTA aggregates is unclear. Molecular dynamics (MD) simulations of Tf- (and Br-) at the air/water interface and as counterion of a DTA aggregate were performed to clarify the nature of Tf- preferences for these interfaces. The effect of NaTf or NaBr on surface tension calculated from MD simulations agreed with the reported experimental values. From the MD simulations a high affinity of Tf- toward the interface, which occurred in a specific orientation, was calculated. The micellar properties calculated from the MD simulations for DTATf and DTAB were consistent with experimental data: in MD simulations, the DTATf aggregate was more ordered, packed, and dehydrated than the DTAB aggregate. The Tf-/alkyltrimethylammonium interaction energies, calculated from the MD simulations, suggested ion pair formation at the micellar interface, stabilized by the preferential orientation of the adsorbed Tf- at the micellar interface.