Properties Of Hydrophobic Ionic Liquids

Ionic liquids (ILs) are salts while can exist as liquid at room temperature or near room temperature, which are composed completely of ions. Compared with traditional organic solvents, ILs have exhibited outstanding properties, such as negligible vapor pressures, nonflammable, wide electrochemical window, high electrical conductivity, adjustable acidity, high dissolving capacity for inorganic and organic compounds or polymers and can be recycled, etc. Moreover, ILs can be designed through the introduction of functional groups on anion or cation to modify their physico-chemical properties. As the new designed and functional solvents, ILs have been applied in fields of synthesis, extraction, catalysis, electrochemistry, and etc. The basic physico-chemical properties of ILs are of great importance for their design and application, however, related data are very deficient. So, IL’s properties and related theoretical studies have received increasing attention. In this work, air and water-stable hydrophobic ILs are studied. Their basic physic-chemical properties, like density, dynamic viscosity, electrical conductivity, and molar heat capacity were measured. The other properties, like molecular volume, standard molar entropy and lattice energy were estimated in terms of empirical and semi-empirical equations based on the experimental data. The main works are listed bellow:Basic physico-chemical properties were measured at different temperatures for fourteen hydrophobic ILs which containing imidazolium, pyridinium, piperidinium and tributyl-hexylphosphonium as cations, separately. The ILs include 1-acetonitrile-3-ethylimimdazolium bis(trifluoromethylsulfonyl)imide ([MCNMIM] [NTf2]),1-ethanol-3-ethylimimdazolium bis(trifluoromethylsulfonyl)imide ([EOHMIM][NTf2]), 1-butylamide-3-ethylimimdazolium bis(trifluoromethylsulfonyl)-imide ([CH2CONHBuEIM][NTf2]), N-alkyl-3-methylpyridinium bis(trifluoromethyl-sulfonyl)imide{[Cn3Mpy][NTf2] (n= 3,6)}, N-alkyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide{[Cn4Mpy][NTf2] (n= 2,4,6)}, N-alkyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide{[PIln][NTf2] (n= 3, 4,5,6)}, tributyl-hexylphosphonium bis(trifluoromethylsulfonyl)imide ([P4446][NTf2]) and tributyl-hexylphosphonium tetrafluoroborate ([P4446][BF4]). The Westphal balance, Ubbelohde viscometer, MP522 conductivity meter, and adiabatic calorimeter (or Anton Paar SVW3000 type density viscometer) were used to measure the density, dynamic viscosity, and electrical conductivity for ILs. The molar heat capacities were measured by an automated adiabatic calorimeter with high precision for three hydrophobic ILs, N-alkylpyridinium hexafluorophosphate{[Cnpy][PFe] (n= 2,3,5)}. The molar volume, standard molar entropy, and lattice energy were estimated by the empirical and semi-empirical equations. The dependences of density, dynamic viscosity and electrical conductivity on emperature are discussed in the measured temperature range. It is found that along with the temperature increasing, the density and dynamic viscosity decreased, while the electrical conductivity increases. The influences of microstructures of ILs, like the introduction of the methylene and methyl groups on cations, on their basic physico-chemical properties are discussed. For pyridinium and piperdinium type ILs, Their density and electrical conductivity decrease while their dynamic viscosity increases as the length of the alkyl side chain of the cation increases It is also found that the introduction of methyl groups on different positions on cations of pyridinium type ILs has different influences on their density, dynamic viscosity and electrical conductivity. The density decreases by the introduction of methyl group on all of the positions. However, the density decreases more when introducing methyl group on position 4 than on position 3. The electrical conductivity increases with the introduction of the methyl group on position 4, while the value decreases when the group introduced on position 3. The dynamic viscosity decreases after introducting methyl group on position 4, while the value increases when the group introduced on position 3. The molar electrical conductivities are calculated based the measured density and electrical conductivity values. The Vogel-Fulcher-Tamman (VFT) equation and Arrhenius equation are used to discuss the dependence of dynamic viscosity and electrical conductivity on temperature. The dependence of the dynamic viscosity and electrical conductivity on temperature can be satisfactorily fitted according to VFT equation but not Arrhenius equation. The dynamic viscosity and electrical conductivity activation energies are calculated through introducing activation energy in VFT equation. The relationship of the density, electrical conductivity and dynamic viscosity are established by the Walden rule, the influences of IL’s microstructure to their degree are discussed. It is found that the molar heat capacities of N-alkylpyridinium ILs{[Cnpy][PF6] (n= 2,3, 5)} increase along with temperature increasing. Their melting point decreases while the molar heat capacity increases when more methylene groups appear on the cation side chain. Polynomial equations are established based on the measured molar heat capacities. The thermodynamic functions{HT-H298.15) and (ST-S298.15) are also derived from the molar heat capacity data relative to 298.15 K. These results are important for academic studies as well as industrial applications.