Elucidating the nanoscale structure of ionic liquids via molecular dynamics simulations

摘要

In this dissertation, we present several structural studies carried out on some ionic liquids using molecular dynamics simulations. In fact, the cations found in ionic liquids are bulky and amphiphilic. Therefore, they basically govern the structure of these ionic liquids. However, the role of anions in ionic liquids in terms of X-ray scattering experiments was ambiguous. Therefore, one of the goals of this study was to determine the role of anions in ionic liquids in terms of X-ray scattering experiments. Through the molecular dynamics simulations carried out on three ammonium ionic liquids, we demonstrate that the anions play a fundamental role as reporters of structure in X-ray experiments. Moreover, we could identify a mathematical tool, the cation head - anion partial structure function that does not suffer from cancellations between peaks and anti peaks albeit can identify both polarity and charge alternations present in a system. A closer dissection of the total structure functions of a series of pyrrolidinium ionic liquids, into partial structure functions such as polar-apolar and cation head - anion reveal the types of alternations present in these ionic liquids. This study was also an attempt to reveal the association between peaks, antipeaks and different types of alternations. The enhancement of similar type group densities at a particular frequency, gives rise to u27peaksu27 in the structure function while the depletion of different type group densities (that has a phase offset) gives rise to u27antipeaksu27. These peaks and antipeaks that appear at a particular frequency, occur due to some type of alternation,such as charge and polarity alternation. Hence, SAXS peaks and anti peaks unravel the larger length scale ordering present in an ionic liquid. A temperature dependent simulation study was performed on a phosphonium, namely tetradecyltrihexylphosphonium bis(tri fluoromethylsulfonyl)amide ionic liquid. The temperature dependent structural study carried out on this ionic liquid revealed that the intensity of the prepeak (or the first sharp diffraction peak) observed in the total structure function is largely enhanced at higher temperature. It was found that at lower temperature, the apolar alkyl tails are more organized at the expense of polar groups and at higher temperature, the polar groups are more organized at the expense of apolar alkyl tails. The development of the prepeak intensity at higher temperature is merely a consequence of the better organization of polar groups at higher temperature. Such phenomenon could be observed in many other phosphonium ionic liquids, while an opposite phenomenon was observed for the 1-methyl-3-octylimidazolium tetra fluoroborate ionic liquid. Ionic liquids that can form hydrogen bonds and have polar, apolar and fluoro domains have drawn a greater attention during the past decade. In our structural study, we show that both in the fluoroprotic and non- fluoroprotic ionic liquids of our interest namely, butylammonium pentadeca fluoro-octanoate and butylammonium octanoate, the separation between two filaments that comprises the continuous, polar, hydrogen bonded network gives rise to a prepeak, while these filaments are percolating the whole ionic liquid in both cases. Finally, the continuous phase that comprises the hydrogen bonded charge alternation and the phase that contains the alkyl and fluorocarbon tails make these systems bicontinuous.