Abstract
The phase behaviors of ionic liquids (ILs) confined in nanospace and adsorbed on outer surface of nanoparticles are expected to be different from those of the bulk. Anomalous phase behaviors of room temperature ionic liquid tributylhexadecylphosphonium bromide (P44416Br) confined in ordered mesoporous silica nanoparticles with average pore size 3.7 nm and adsorbed on outer surface of the same silica nanoparticles were reported. It was revealed that the melting points (Tm) of confined and adsorbed ILs depressed significantly in comparison with the bulk one. The Tm depressions for confined and adsorbed ILs are 8 °C and 14 °C, respectively. For comparison with the phase behavior of confined P44416Br, 1-butyl-3-methylimidazolium bromide (BmimBr) was entrapped within silica nanopores, we observed an enhancement of 50 °C in Tm under otherwise similar conditions. The XRD analysis indicates the formation of crystalline-like phase under confinement, in contrast to the amorphous phase in adsorbed IL. It was confirmed that the behavior of IL has clear difference. Moreover, the complex π-π stacking and H-bonding do not exist in the newly proposed phosphonium-based IL in comparison with the widely studied imidazolium-based IL. The opposite change in melting point of P44416Br@SiO2 and BmimBr@SiO2 indicates that the cationic species plays an important role in the variation of melting point.
Highlights
Room temperature ionic liquids (ILs), solely composed of organic cations and inorganic/organic anions of varying sizes, have received considerable attention in past years due to a wide range of applications and scientific research interests [1,2,3]
Many researchers focused on developing ILs for certain application and synthesizing a series of promising hybrid ionogel material [4,5,6,7], in which the ILs exist under the confinement or immobilization environment
X-ray diffraction (XRD) measurement patterns were recorded with a Philips X-ray diffractometer (PW-1710) using Cu Kα radiation ranging from 5° to 50°
Summary
Room temperature ionic liquids (ILs), solely composed of organic cations and inorganic/organic anions of varying sizes, have received considerable attention in past years due to a wide range of applications and scientific research interests [1,2,3]. With the combination of our continuous efforts to investigate the phase behavior of imidazolium-based ILs on the surface of mica [15], polystyrene submicrospheres [16], inside of multiwalled carbon nanotubes [17] and SiO2 nanopores [18], in this study, new proposed phosphonium-based IL P44416Br was filled into SiO2 nanopore under conditions of ultrahigh vacuum and adsorbed onto the outer surface of SiO2 by stirring the IL and SiO2 in a vessel, respectively. The difference between the newly proposed IL P44416Br and the widely used imidazolium-based IL is the distinct cationic species: the former consists of an atom phosphorus center and a hydrocarbon skeleton. The complex π-π stacking and H–bonding do not exist in P44416Br. For comparison, we entrapped imidazolium-based IL BmimBr within the same type of SiO2 nanopores. X-ray diffraction analysis was performed to investigate the property variation of ILs existing in two independent circumstances
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