Isolated BMI+ Cations are More than Isolated PF6− Anions in the Room Temperature 1-Butyl-3-Methylimidazolium Hexafluorophosphate (BMI-PF6) Ionic Liquid

Assuming various ionic states in ionic liquids (ILs) are in equilibrium with exchange rates too high to be distinguished by NMR experiments and the overall response of measured diffusivity is viewed as the sum of weighted responses of diffusivity of all possible components, the ratio of cation diffusivity to anion diffusivity, D+/D−, in a specified IL affords the physical meaning: relative association degrees observed by anion‐containing components to cation‐containing components. These values decrease with increasing temperature showing the equilibrium between ionic states shifting to smaller components. In the neat 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMI‐PF6), (BMI‐PF6)nPF6− anions are found preferred to (BMI‐PF6)nBMI+ cations and this phenomenon is termed as hyper anion preference (HAP). The counterpart statement, “isolated BMI+Cations Are More than Isolated PF6− Anions in the Room Temperature in the BMI‐PF6 Ionic Liquid” is employed as the research title. The HAP approach can be employed to explain the temperature‐dependent values of D+/D− obtained for BMI‐PF6/2,2,2‐trifluoroethane (TFE) mixtures at two different compositions (χTFE = 0.65 and 0.80). More significantly, this argument can rationalize numerous physical properties published for this IL: (1) higher sensitive of anionic diffusivity towards temperatures than cationic diffusivity, (2) temperature‐dependent cationic transference number, (3) low anionic donicity and high ionicity and (4) high viscosity. This “Hyper anion preference” can rationalize nearly all characteristic physical properties of BMI‐based ILs, e.g. the higher diffusivity of larger BMI cation than smaller PF6 anion.