Molecular Simulations Of The Structures,Dynamics And Hydrogen Bonds Of EAN Ionic Liquid At The Liquid–vacuum Interface

In recent years,protic ionic liquids(PILs),as a new type of green solvent,have attracted more and more research interests in the fields of extraction and separation,chemical synthesis,and electrocatalysis.This is mainly due to that the unique hydrogen bond(HB)donor and acceptor sites of PILs can form three-dimensional HB networks.However,most of the current studies mainly focus on the behavior of aprotic ionic liquids in the bulk phase and/or at the interface,and there is a lack of studies for PILs in this aspect,which seriously hinders the applications of PILs in various fields.In this thesis,we explored the structure and dynamic properties of ethylammonium nitrate(EAN)at the gas-liquid interface,and special research interest was paid to the intrinsic relationship between dynamic properties and HBs,which provides theoretical guidance for the experimental scientists to understand the unique behavior of ionic liquids at the gas-liquid interface.In this paper,a series of molecular dynamics(MD)simulations have been used to systematically explore the structures,dynamics and HBs of EAN ionic liquids and their mutual relationship at the liquid-vacuum interface.At the same time,the influence of temperature effect on the interface behavior was also investigated.The simulation results clearly demonstrate that there exists a sandwich structure at the interface,with the double-layer of the EA⁺cations on both sides and one intercalated layer of the NO₃^–anions in the middle.Wherein,the outermost cation layer prefers the orientation with the CH₃groups pointing to the vacuum phase due to the hydrophobic interactions,while the CH₃groups in the second layer direct to the bulk liquid phase owing to the HB formation between their NH₃⁺groups and the intercalated NO₃^–anions in the middle layer.On the other hand,the continuous HB strength of the cations in the outermost layer(denoted as Cation-1)is found to be almost identical with the counterpart of the cations in the second layer(denoted as Cation-2),whereas the intermittent HB strength of Cation-1 is much larger than that of Cation-2 at all temperatures.Furthermore,the rotational motion of Cation-1 with the normal vector of the C–C–N plane in the cation is faster than that of Cation-2 with the same vector,resulting from more free space in the outermost layer.On the contrary,the rotational motion of Cation-1 with the vector from the mass center of the cation to its N atom is much slower than that of Cation-2 with the same vector,which can be attributed to the combined effects of the stronger intermittent HBs of Cation-1and the hydrophobic interactions of its CH₃group in the outermost layer.