Density, Viscosity, and Conductivity for the Binary Systems of Water plus Dual Amino-Functionalized Ionic Liquids

The density, viscosity, and conductivity were investigated for binary mixtures of water and three ionic liquids (ILs), respectively: (3-aminopropyl) tributylphosphonium L-alpha-aminopropionic acid salt ([aP(4443)][Ala]), (3-aminopropyl) tributylphosphonium L-alpha-aminoisovaleric acid salt ([aP(4443)][Val]), and (3-aminopropyl) tributylphosphonium L-alpha-amino-4-methylvaleric acid salt ([aP(4443)][Leu]). The mole fractions of water increased from 0.00 to 0.75, and the temperature varied from (298.15 to 343.15) K. It was found that the viscosity decreased while conductivity increased with the increase of temperature and water content. As the alkyl side-chains of the amino acid anions which might play a role of steric hindrance were lengthened, the density (rho), viscosity (eta), and conductivity (sigma) follow the order of rho([aP4443][Ala]) > rho([aP4443][Val]) > rho([aP4443][Leu]), eta([aP4443][Ala]) < eta([aP4443][Val]) < eta([aP4443][Leu]), and sigma([aP4443][Ala]) > sigma([aP4443][Val]) > sigma([aP4443][Leu]), respectively. The temperature-dependent density is fit for the quadratic equation. The temperature dependence of viscosity and conductivity can be described by the Vogel-Tammann-Fulcher (VTF) equation.