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 (Nagg), low degree of counterion dissociation (α), 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.