Physical properties of phosphonium ionic liquid and its mixtures with dodecane and water

• Addition of the dodecane and water to phosphonium ionic liquid decreased its density and viscosity significantly.
• Nonlinear concentration dependences of density and viscosity are explained in terms of structural changes in the solvents.
• Two types of aggregates were identified in the dry and water saturated solvents of size above (300 and 2) nm, respectively.

Addition of molecular solvents, dodecane and water, to the hydrophobic phosphonium ionic liquid (IL) tetradecyltrihexylphosphonium bis-(2,4,4-trimethylpentyl)phosphinate decreases its density and viscosity significantly. These changes have been studied in the temperature interval between 15 and 60 °C and explained in terms of structural changes. Dependences of density and viscosity on the composition of binary and ternary systems are strongly nonlinear. Excess molar volumes and thermal expansion coefficients have been estimated for dry and water saturated solvents with IL. Strong H-bonding in water saturated solvents has been concluded from large negative excess molar volumes and strong temperature dependence of the density in water saturated solvents. VFT, Litovitz and Arrhenius models were tested for the correlation of temperature dependence of pure IL viscosity, showing that IL is a typical strong liquid. Deviations of the viscosity of the solvents to the ideal mixture viscosity based on the Eyring’s theory of absolute rates were evaluated. In dynamic light scattering measurements, two different types of aggregates have been identified in the dry and water saturated solvents, with the mean size above 300 and 2 nm, respectively. High content of water of up to 8 molecules per ion pair of IL points to the existence of reverse micelles in the water containing solvents with hydrophobic IL.