Structural investigation of room-temperature ionic liquids and high- temperature ionic melts using triplet correlation functions
D Dhabal and A Gupta and HK Kashyap, JOURNAL OF CHEMICAL PHYSICS, 146, 094503 (2017).
We use means of molecular dynamics simulation to understand the local structural arrangements in three trihexyltetradecylphosphonium (P-6,6,6,14(+))based room-temperature ionic liquids (RTILs) by using triplet correlation functions (TCFs) along with pair correlation functions (PCFs) and X-ray scattering structure functions (S(q) s). The anions in these RTILs are either spherically symmetric but with different effective sizes (bromide (Br (-)) and tetrafluoroborate (BF- (4))) or angular such as dicyanamide (DCA (-)). The simulated PCFs, S(q) s, and TCFs of the three RTILs have been compared with three high- temperature ionic melts (HTIMs); NaBr, NaCl, and NaF. In general, the pair correlation function gives angle-averaged probability as a function of inter-particle distance whereas the TCFs associated with equilateral and isosceles triangle configurations can be used to delineate angle- resolved information of liquids structure within nearest solvation shells. For the three ionic liquids studied, a very careful examination of co-ionic and counter-ionic TCFs associated with the equilateral triangular configuration within the nearest solvation shells of the ions reveals that co-ions (cat-cat-cat and an-anan) favor close-packed local arrangement, but with lower probability for theDCA (-) RTIL. Co-ionic and counter-ionic TCFs obtained for HTIMs are found to be similar to that of RTILs with spherical anions. The TCFs associated with the isosceles triangle configuration signify that the ionic liquid with Br (-) anions tends to exhibit larger anion-cation-anion angle than that in the other two RTILs. Moreover, diffused counter-ionic angular correlations are observed in the DCA (-) system. We also observed angle- dependent charge ordering in all the three RTILs although its extent is enhanced for RTILs with spherical anions, very similar to what we find for NaBr, NaCl, and NaF melts. This study suggests that the presence of charge ordering is a generic feature of both the RTILs and HTIMs. Published by AIP Publishing.
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