Surface crystallization of ionic liquid crystals

The evidence for surface crystallization in ionic liquids is scarce. The existing reports seem to be contradictory as for its driving forces, since in the two compounds investigated in the literature, the contribution of coloumbic and van der Waals forces is very different. In this work 1-dodecyl-3-methylimidazolium tetrafluoroborate was studied and its surface crystallization characterized by surface tension, ellipsometry and optical microscopy. The results obtained seem to reconcile previous observations, and it was further shown, using the same techniques, that this phenomenon is prevalent in other ionic liquids. MD simulation results illustrate the different possibilities of organization, providing reasonable models to rationalize the experimental observations. The [C 12 mim][BF 4 ] surface was characterized showing an ordered layer and solid-like behavior, supporting the hypothesis of a surface freezing phenomenon.