  Atomistic simulations have been performed to investigate the effect of
aliphatic chain length in tetraalkylphosphonium cations on liquid morphologies,
microscopic ionic structures and dynamical properties of tetraalkylphosphonium
chloride ionic liquids. The liquid morphologies are characterized by
sponge-like interpenetrating polar and apolar networks in ionic liquids
consisting of tetraalkylphosphonium cations with short aliphatic chains. The
lengthening aliphatic chains in tetraalkylphosphonium cations leads to polar
domains consisting of chloride anions and central polar groups in cations being
partially or totally segregated in ionic liquid matrices due to a progressive
expansion of apolar domains in between. Prominent polarity alternation peaks
and adjacency correlation peaks are observed at low and high $q$ range in total
X-ray scattering structural functions, respectively, and their peak positions
gradually shift to lower q values with lengthening aliphatic chains in
tetraalkylphosphonium cations. The charge alternation peaks registered in
intermediate q range exhibit complicated tendencies due to the complete
cancellations of peaks and anti-peaks in partial structural functions for ionic
subcomponents. The particular microstructures and liquid morphologies in
tetraalkylphosphonium chloride ionic liquids intrinsically contribute to
distinct dynamics characterized by translational diffusion coefficients, van
Hove correlation functions, and non-Gaussian parameters for ionic species in
heterogeneous ionic environment. The increase of aliphatic chain length in
tetraalkylphosphonium cations leads to concomitant shift of van Hove
correlation functions and non-Gaussian parameters to larger radial distances
and longer timescales, respectively, indicating the enhanced translational
dynamical heterogeneities of tetraalkylphosphonium cations and the
corresponding chloride anions.
