Generating Ionic Liquids from Ionic Solids: An Investigation of the Melting Behavior of Binary Mixtures of Ionic Liquids

Mixtures of ionic liquids (ILs) allow
enlarging the plethora of
the physical and chemical properties of these materials in addition
to the well-known tunable character associated with pure compounds.
It is shown here that mixtures also induce a significant decrease
of the melting points of the mixture to values well below those of
the original compounds allowing the tuning of the melting point of
an ionic liquid mixture and the generation of novel ionic liquids
from mesotherm salts. This work evaluates the melting behavior of
mixtures of seven hexafluorophosphate-based compounds combined with
imidazolium-, pyridinium-, pyrrolidinium-, piperidinium- ammonium-,
or phosphonium-based cations. The solid–liquid equilibrium
phase diagrams of nine of their binary mixtures were measured using
optical microscopy and differential scanning calorimetry. The pure
ILs’ melting profile reveals the presence of polymorphs with
highly energetic solid–solid transitions that are relevant
for the evaluation of these systems. The phase diagrams reported
here also allow an investigation on the nonideality of the mixtures
of ionic liquids. A classical thermodynamic approach shows that while
most of the mixtures investigated present an ideal liquid behavior,
others show slight or even marked nonideal profiles. One particular
system, [C3mpyr]­[PF6] (1-methyl-1-propylpyrrolidinium
hexafluorophosphate) + [C3mpip]­[PF6] (1-methyl-1-propylpiperidinium
hexafluorophosphate), displays a continuous solid solution as established
by differential scanning calorimetry, powder X-ray diffraction, and
crystallographic data being one of the few ionic liquid alloys ever
reported.