How the Structure of Pyrrolidinium Ionic Liquids Is Susceptible to High Pressure

The structural landscape of room-temperature ionic liquids (RTILs) with longer cationic alkyl tail(s) exhibits polarity ordering (PO) along with charge ordering (CO). In polarity ordering, which is also referred to as intermediate-range ordering, polar groups are separated by segregated domains of apolar groups and vice versa. Charge ordering resembles alternation of positive-negative charge groups. In this work, how these two characteristic orderings respond to applied external pressure has been investigated via molecular dynamics simulations. The present study complements the recent experimental studies of Yoshimura et al. (J. Phys. Chem. B 2015, 119, 8146-8153) and computational studies of Russina et al. (Phys. Chem. Chem. Phys. 2015, 17, 29496-29500) wherein the authors described in detail the effects of pressure on the structural and conformational changes in imidazolium based ionic liquids. Our simulations predict that for 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, Pyrr(1,n)(+)/NTf2- with n = 8 and 10, the PO and CO fade when the external pressure increases from ambient pressure to 10000 bar. We observe that the apolar tail group as well as the polar group correlations are susceptible to the applied pressure. The decrease of polar-polar and apolar-apolar correlations at higher pressure is accompanied by the enhancement in the polar-apolar correlations and increased stability/probability of gauche conformations along the cationic tails.