Wide range of applications of Ionic Liquids has motivated the researchers to investigate the behavior of ionic liquids (ILs) under extreme environmental conditions such as very high pressure and temperature. The increased pressure might lead to some significant changes in the chemical structure of the constituent ions and also in the interactions between ions. To investigate such changes in the structure and behavior of ILs under very high pressure, we performed molecular dynamics studies and for this purpose we have used ionic liquids based on phosphonium and pyrrolidinium cations in combination with different anions. Special attention was paid on the study of how charge and polarity orderings, which are present in the microscopic structure of these ILs at ambient conditions, respond to very high applied pressure (i.e. 1-5000 bar). The simulated X-ray scattering structure functions, S(q)s, of the phosphonium cation based ILs reveal that both of these orderings show appreciable responsiveness towards the increasing pressure. However, a slight difference between the polarity ordering (PO), charge ordering (CO), and adjacency correlations (AC) for phosphonium ILs represent the different microscopic structure of the two systems due to change in anion. Beyond a certain pressure, we observe development of a new peak in the low q region of the S(q)s for two phosphonium IL systems and this new peak is associated with formation of crystalline order in these ILs at higher pressures and the real space length-scale corresponding to the crystalline order lies in between those of polarity- and charge-ordering. Beyond the transition pressure, the crystallinity of both the systems increases with increasing pressure and the corresponding length-scale shifts towards smaller values upon increasing pressure. We observe that the extent of the usual polarity ordering decreases upon increasing pressure for phosphonium ILs. We also demonstrate that the correlations between polar and apolar groups in the systems under study show different behavior below and above transition pressures. In addition, the shift in the charge ordering peak showed different behavior when phosphonium cation is paired with two different anions. Similar studies were performed on the pyrrolidinium cation based ILs in combination with conformationally rich NTF2- anion.

Molecular dynamics investigation of behavior of ionic liquids under high applied pressure

256th National Meeting (2018)

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