Preparation And Properties Of Supramolecular Imidazolium Hexafluorophosphate Ionic Liquid Gels

In recent years, supramolecular gels formed by the self-assembly of gelators throughintermolecular hydrogen bonding,Ï€-Ï€stacking and other non-covalent bond interationswere applied on catalyzer, electrolyte and other fields because of its properties between theliquids and solids. Ionic liquids composed of anions and cations have useful properties suchas extremely low volatility, high thermal stability, wide liquid phase temperature range,nonflammab- ility, high chemical stability, high ionic conductivity, and a wideelectrochemical window. These advantages implied that ionic liquids would play animportant role in such technological applications as electrolyte materials.Three kinds of gelators named 4,4'-bis(N'-octanamino)diphenyl ether (BSDE-n),4,4'-bis (N'-octanamino)diphenyl methane (BSDM-n) and 2,4'-bis(N'-octanureido)toluene(BSUT-n) were synthesized by using functional groups such as amide group, benzenegroup and alkyl chains in molecular designing way. The gelation abilities of supramoleculargels could be obtained by controlling the length of alkyl chains, and then the mechanism ofself-assembly and gelation abilities in organic solvents were investigated. Gelators weretried to gel ionic liquids, and then ionic liquids supramolecular gels were formed. Threekinds of gelators named 4,4'-bis(octanamino)diphenyl ether (BSDE-8). 4,4'-bis(octanamino)diphenyl methane (BSDM-8) and 2,4'-bis(octanureido)toluene (BSUT-8) andthe ionic liquid [bmim]PF₆ were chosen as they formed the better ionic liquidsupramolecular gels. The electrochemical properties and rheological behavior of gels wereinvestigated by using those ionic liquids supramolecular gels.This paper is consisted of several sections as the following:1. Three kinds of gelators named 4,4'-bis(N'-octanamino)diphenyl ether (BSDE-n),4,4'-bis (N'-octanamino)diphenyl methane (BSDM-n) and 2,4'-bis(N'-octanureido)toluene(BSUT-n) which were synthesized by using functional groups such as amide group,benzene group and alkyl chains by molecular designing could be self-assemblied in morethan 20 kinds of organic solvents.The influrence of gelator types, different gelatorconcentrations, different solvents to gel-sol phase dissociation temperature(T_GS) and minimum gelation concentrations were discussed. The morphology and structures ofsupramolecular gels in organic solvents were characterized by POM and FE-SEM.2. Ionic iquids supramolecular gels were formed by using three kinds of gelators and theionic liquid [bmim]PF₆. The gelation abilities of ionic iquids supramolecular gels werediscussed. Three kinds of gelators named 4,4'-bis(octanamino)diphenyl ether (BSDE-8),4,4'-bis(octanamino)diphenyl methane (BSDM-8) and 2,4'-bis(octanureido)toluene (BSUT-8)had the best gelation abilities. The results showed that the mean minimum gelconcentrations (MGC) rose up as follows: BSDE-8, BSDM-8 and BSUT-8. The T_GS felldown as follows: BSDE-8, BSDM-8 and BSUT-8. The polarized optical microscopy imageof [bmim]PF₆ gels revealed the formation of spherical crystallites resulting from thefibrillar aggregates of the gelators.3. Linera sweep,cyclic voltammograms and impedance spectra were used to get theelectrochemical abilities of the three ionic liquids supramolecular gels. The gels hadelectrochemical stability and a wide electrochemical window. The cyclic voltammogramsof the [bmim]PF₆ gels indicate a good electrochemical stability over the range from -2.6 to2.6 V. The impedance spectra of the [bmim]PF₆ gels show good linear relationshipsbetween ion conductivity and temperature, indicating that the temperature dependence ofthe conductivity follows the classical Arrhenius equation. [bmim]PF₆ gels formed byBSDE-8 limited oxidation currents were detected in particular when potentials larger than1.5 V were applied. These oxidation currents can be ascribed to the ether group (-O-) inthe molecular structure of BSDE-8, which is easily oxidated in case of high potentials. Theion conductivities of the [bmim]PF₆ gels were found to be similar to those of the pure[bmim]PF₆, indicating that the supramolecular structures in the [bmim]PF₆ gels haveapparently no effect on the mobility of the ions. The proposed ionic liquid gels possess apotential to provide solid-like materials for the applications of electrolytes.4. The macroproperties of the three ionic liquids supramolecular gels with differentconcentrations were characterized by rheological measurements of the scillatory shear. Allthe gel samples showed similar rheological behavior. Over the range from 0.1 to 10 Hz,both the storage modulus (G′) and loss modulus (G″) were nearly frequency-independent,and G′was higher than G″. The complex viscosityη^* were decrease when the frequency increased. These results indicated that the gels were highly viscoelastic. The storagemodulus G′characterized the strength of gels, which estimated the degree of resistanceagainst mechanical stress.The tanδwere less than 1 in the whole sweep rangs, tanδdecreased first and then increased when the frequence increased. The storage modulus (G′)and loss modulus (G″) showed the same changes when the temperature increased. Thecomplex viscosityη^* were increased when the temperature increased. The tanδwere lessthan 1 in the whole temperature rangs, tanδincreased first and then decreased when thetemperature increased. With an increase of the gelators concentrations from lower to higher,the G′increased gradually, implying that three-dimensional networks became much denserwith an increase of the gelator concentrations, so both the stability and strength of the ionicliquid gel networks increased. The structures of gelators had an important effect on therheology abilities.