Nanoscale heterogeneity, hydrogen bonding and their temperature dependence in cholinium phenylalaninate bio-ionic liquid

Abstract

Abstract In this article, by employing atomistic molecular dynamics (MD) simulations, we examine the origin of nanoscale structural organization in cholinium phenylalaninate ([Ch][Phe]) biocompatible ionic liquid (IL), in which the [Phe]− anion possesses aromatic ring as its side chain. We attempt to deduce the bulk structure of [Ch][Phe] through simulated X-ray scattering structure function (S(q)), its partial components, real-space correlation functions and hydrogen bond analysis. While the simulated S(q) successfully captures the experimentally observed prepeak, resembling nanoscale heterogeneity, in the S(q) of [Ch][Phe] around q = 0.4 A−1 at ambient conditions, the temperature dependence of the prepeak in the S(q) shows an unusual behavior. The intensity of the prepeak increases with increasing the temperature, contrary to many imidazolium cation-based ILs, but similar to several ammonium and phosphonium cation-based ILs. Multiple judicial decompositions of the total S(q)s clearly reveal that while [Phe]tail− − [Phe]tail− and [Ch]+ − [Phe]head− correlations are most prominent contributors and indicators of the prepeak, a balance of three major interactions lays down the observation of the nanoscale heterogeneity in this IL. Spatial distribution function analysis also corroborates with these observations, where the segregations of polar groups in polar domains and apolar groups in apolar domains are clearly observed. Our analysis reveals that the increase in the prepeak intensity is because of the increase in all the correlations responsible for the origin of prepeak on increasing the temperature. Hydrogen bond analysis performed through angle-resolved radial distribution functions reveals that only one type of hydrogen bond is present in [Ch][Phe] in which [Ch]+ acts as hydrogen bond donor via its hydroxyl hydrogen and [Phe]− as acceptor via its oxygen atoms. Weakening of this hydrogen bond and enhancement in [Phe]tail− − [Phe]tail− correlation with increasing the temperature are also observed.