Investigation On The Structure And Phase Behavior Of Ionic Liquids Confined In Silica Nanoconfinement

Room temperature ionic liquids (ILs), solely composed of organiccations and inorganic/organic anions of varying sizes, have receivedconsiderable attention in past years due to a wide range of applicationsand scientific research interests. The tunable cations and anions lead totheir peculiar chemical and physical properties, for example, negligiblevolatility, thermal stability, nonflammability, and high ionic conductivityand so forth. In recent years, many researchers focused on developing ILsfor certain application and synthesizing a series of promising hybridionogel material, in which the ILs exist under the confinementenvironment. As a matter of fact, confined effect and interface effect alsolead confined ionic liquids to perform anomalous behavior which iscompletely different from the bulk. This thesis is directed to investigatethe properties and behavior of ionic liquids confined within silicananospace by experiment and simulation. The first part will focus on thephase behavior and structure of ionic liquids confined silica nanopores.The second part will investigate on the influence of confinement effectand interface effect on the behavior of ionic liquids confined within silicaslits. Firstly, anomalous phase behaviors of room temperature ionic liquidtributylhexadecylphosphonium bromide (P44416Br) confined in orderedmesoporous silica nanoparticles with average pore size3.7nm andadsorbed on outer surface of the same silica nanoparticles were reported.It was revealed that the melting points (Tm) of confined and adsorbed ILsdepressed significantly in comparison with the bulk one. The Tmdepressions for confined and adsorbed ILs are8℃and14℃，respectively. For comparison with the phase behavior of confined P44416Br,1-butyl-3-methylimidazolium bromide (BmimBr) was entrapped withinsilica nanopores, we observed an enhancement of50℃in Tmunderotherwise similar conditions. The XRD analysis indicates the formationof crystalline-like phase under confinement, in contrast to the amorphousphase in adsorbed IL. It was confirmed that the behavior of IL has cleardifference. Moreover, the complex π-π stacking and H-bonding do notexist in the newly proposed phosphonium-based IL in comparison withthe widely studied imidazolium-based IL. The opposite change in meltingpoint of P44416Br@SiO2and BmimBr@SiO2indicates that the cationicspecies plays an important role in the variation of melting point.Secondly, molecular dynamics simulations were performed to studythe heating process of1-ethyl-3-methylimidazolium bromide confinedwithin different silica slits. The “separation phenomena of confined ionicliquids” appeared when silt width is more than2.30nm, the separation temperature is600K for silt width2.83nm,500K for3.05nm,400K for3.45nm. It was confirmed that the “separation phenomena” was the resultof silica interface effect during heating process. The confined effectwould be more dominant than the interface effect within2.30nm slit, sointerface effect is insufficient to make ionic liquids separation.This thesis is helpful to further understand the properties andbehavior of confined ionic liquids and understand the influence ofconfinement effect and interface effect on ionic liquids.