  One of the many open questions concerning Ionic Liquids (ILs) is the
existence of nanoscale supramolecular domains which characterize the bulk. The
hypothesis of their existence does not meet a general consensus since their
definition seems to be based on ad hoc arbitrary criteria rather than on
general and solid first principles of physics. In this work, we propose a
suitable definition of supramolecular domains based on first principles of
statistical mechanics. Such principles can be realized through the application
of a recently developed computational tool which employs adaptive molecular
resolution. The method can identify the smallest region of a liquid for which
the atomistic details are strictly required, while the exterior plays the role
of a generic structureless thermodynamic reservoir. We consider four different
imidazolium-based ILs and show that indeed one can quantitatively represent the
liquid as a collection of atomistically self-contained nanodroplets embedded in
a generic thermodynamic bath. Such nanodroplets express a characteristic length
scale for heterogeneity in ILs.
