On the nature of nm-scale heterogeneities in ionic liquids

The issue of nanoscale structural heterogeneities has gained a great relevance in the last few years. Due to the intrinsic amphiphilic nature of conventional room temperature ionic liquids, such as alkyl-imidazolium-based salts, a self-assemblying scenario characterises the morphology of this important class of materials. This phenomenon has huge implications and several applicative features as relevant as solvation performances, catalysis and separation are expected to be affected by this occurrence. In agreement with the early findings by MD simulation groups, we highlighted the existence of X-ray as well as neutron diffraction features that represent the fingerprint of the existence of structural correlations over a spatial scale as large as few nm in the bulk liquid state of a large class of ionic liquids. We will present new results on novel classes of materials including alkyl- morpholinium and DABCO based ionic liquids, that further support the view of nm scale structural organization in ionic liquids. These results will be discussed in view of the recent arguments raised by other experimental groups. Furthermore the consequences of this structural organization onto the relaxation processes occurring over the nsec temporal scale will be discussed, in view of the new Neutron Spin Echo data that we collected on deuterated salts as a function of the alkyl chain length. Indications of the existence of a complex dynamic behaviour as a consequence of structural segregation will be presented. This experimental study provides information that validate recent MD simulations from a number of computational groups. We will also stress the importance of a joined experimental-computational approach in the investigation of these complex material systems.