Aiming at providing a comprehensive study of the influence of the cation symmetry and alkyl side chain length on the surface tension and surface organization of ionic liquids (ILs), this work addresses the experimental measurements of the surface tension of two extended series of ILs, namely R,R′-dialkylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C_nC_nim][NTf₂]) and R-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C_nC₁im][NTf₂]), and their dependence with temperature (from 298 to 343 K). For both series of ILs the surface tension decreases with an increase in the cation side alkyl chain length up to aliphatic chains no longer than hexyl, here labeled as critical alkyl chain length (CACL). For ILs with aliphatic moieties longer than CACL the surface tension displays an almost constant value up to [C₁₂C₁₂im][NTf₂] or [C₁₆C₁im][NTf₂]. These constant values further converge to the surface tension of long chain n-alkanes, indicating that, for sufficiently long alkyl side chains, the surface ordering is strongly dominated by the aliphatic tails present in the IL. The enthalpies and entropies of surface were also derived and the critical temperatures were estimated from the experimental data. The trend of the derived thermodynamic properties highlights the effect of the structural organization of the IL at the surface with visible trend shifts occurring at a well-defined CACL in both symmetric and asymmetric series of ILs. Finally, the structure of a long-alkyl side chain IL at the vacuum-liquid interface was also explored using Molecular Dynamics simulations. In general, it was found that for the symmetric series of ILs, at the outermost polar layers, more cations point one of their aliphatic tails outward and the other inward, relative to the surface, than cations pointing both tails outward. The number of the former, while being the preferred conformation, exceeds the latter by around 75%.

Significance: Semifluorinated alkanes surfactants spontaneously form nanostructured Langmuir films at the surface of water, consisting of highly organized arrays of hemimicelles. These supramolecular nanostructures are potential candidates for numerous applications, from smart materials to medical devices. Despite all efforts, the principles governing the formation and structure of these hemimicelles are not understood. Atomistic molecular-dynamics (MD) simulations show the spontaneous formation of such aggregates strikingly resembling those experimentally observed, supporting a rational explanation for the formation, size and shape of the hemimicelles. The simulated grazing incidence X-ray diffraction spectrum matches that obtained experimentally, fully validating the MD results. Given the simplicity of these “primitive” amphiphiles compared with the common hydrophilic/hydrophobic surfactants, these results can contribute to understand self-assembly processes in general.