Synthesis and Physiochemical Properties of Monocationic and Dicationic Phosphonium Ionic Liquids Incorporating Dioctylsulfosuccinate Anion

A new series of monocationic (MILs) and symmetrical dicationic (DILs) phosphonium-based room temperature ionic liquids (RTILs) incorporating dioctylsulfosuccinate (docusate) anion namely, tetradecyltrioctylphosphonium docusate; [P8,8,8,14]docusate, 1,10-bis(trioctylphosphonium)decane dioctylsulfosuccinate; [P8,8,8 C10 P8,8,8]docusate, 1,6-bis(trioctylphosphonium)hexane dioctylsulfosuccinate; [P8,8,8 C6 P8,8,8]docusate have been prepared. The present ionic liquids (ILs) are characterised using NMR and elemental analysis and their physical properties such as density, viscosity and refractive index have been performed at a broad range of temperature (293.15 to 353.15) K and at atmospheric pressure. The experimental values of density for the present ILs decrease linearly with increasing temperature as consequence of the large temperature difference between their working temperature range and their critical temperatures. The results show that the present MILs possesses lower densities and higher viscosities comparable to the MILs with short alkyl chain while DILs possesses higher viscosities comparable to the MILs. The higher viscosity of the present ILs is due to the increased Van der Waals interactions results from the long alkyl chains of the cation and anion along with the large volume of the docusate anion which results in low ion mobility due to the electrostatic interaction between the cation and anion. The coefficient of thermal expansion, crystal energy and molar refraction were calculated from the experimental values of density and refractive index. These ILs show a weak temperature dependency on the thermal expansion coefficient and low crystal energy.