Hiding the Headgroup? Remarkable Similarity in Alkyl Coverage of the Surfaces of Pyrrolidinium- and Imidazolium-Based Ionic Liquids

The liquid–vacuum interfaces of a series of ionic liquids (ILs) containing 1-alkyl-1-methylpyrrolidinium ([Cnmpyrr]+) cations of different alkyl chain lengths have been studied by reactive-atom scattering with laser-induced fluorescence detection (RAS-LIF) and molecular dynamics (MD) simulations. A direct, quantitative comparison has been performed between [Cnmpyrr]+ and the previously better-characterized 1-alkyl-3-methylimidazolium ([Cnmim]+) ILs with the same chain lengths, n, and common anion, bis(trifluoromethylsulfonyl)imide ([Tf2N]−). Both RAS-LIF experiments, using O(3P) as the projectile and monitoring OH yield, and MD simulations indicate that the coverage of the surface by alkyl chains is almost independent of the identity of the cation headgroup. Moreover, the potentially abstractable H atoms of the saturated pyrrolidinium ring do not contribute appreciably to the experimental OH yield. In both these senses, therefore, the headgroup is “hidden” from the probe particles approaching from vacuum. More predictably, the alkyl coverage depends strongly and nonstoichiometrically on the length of the alkyl chain, n, for either cation. These results imply the presence of an alkyl-rich layer on the surface formed by preferential orientation of the cations to expose their chains to the vacuum phase. We suggest that the lack of dependence of the packing density of this layer on cation type results from compensating effects of charge density and steric blocking.