A series
of 1-alkyl-3-methylimidazolium dicyanamide ionic liquids
([C
n
mim][N(CN)2], n = 2, 3, 4, 5, and 6) were synthesized and characterized in this
work. The density, viscosity, and electrical conductivity of these
ionic liquids were measured from 293.15 to 353.15 K at 101.3 kPa using
a high-precision vibrating tube density meter, an automated falling
ball viscometer, and a conductivity meter, respectively. On this basis,
the corresponding molar volume and molar conductivity were also obtained.
All experimental values were well-fitted by the empirical equations.
Remarkably, these ionic liquids exhibit much lower density, lower
viscosity, and higher electrical conductivity than many other conventional
ionic liquids. According to the theoretical calculations, the weaker
cation–anion interaction and V-shaped and nonspherical structure
of the dicyanamide anion are considered as major reasons for these
properties. Meanwhile, the extension of the alkyl side chain length
can result in lower density, higher molar volume, higher viscosity,
and lower conductivity. This phenomenon is probably ascribed to the
decreasing molecular symmetry, enhanced hydrogen bonding, and increasing
steric hindrance of ionic liquids. The relationship between viscosity
and molar conductivity has been also investigated using the Walden
rule, which confirms the high ionicity of [C
n
mim][N(CN)2]. The enhanced dissociation of ions
may favor the ionic fluidity and conductivity. It is hoped that these
experimental results and theoretical findings may be useful for the
future development of dicyanamide-based ionic liquids.