(191a) Surface Tension and Density of Pure Ionic Liquids EMIAc and Binary Mixtures with Water

The densities and surface tension of a new style ionic liquid EMIAc (1-ethyl-3-methylimidazolium acetate) were measured at the temperature range of 288.15-323.15K(±0.1 K) under dry argon atmosphere, To obtain precise data, we used the pendent drop method and the standard addition method (SAM) in the experiment. Since a small amount of water solubility of ILs in the physicochemical propertis have great change, so we measured the denity and surface tension in the same conditions of different the moisture content of the ionic liquid. The pendant drop method requires accurate density data measurements of the mixtures, so densities of the substances at different temperatures were also measured. The densities of all pure ionic liquids and the corresponding mixtures  were measured using a vibrating tube densimeter (Anton Paar DMA 4500 ), The values of surface tension lie between (30 and 48) mN · m^-1 and decrease with temperature. Surface tension σ of the mixtures of 1-ethyl-3-methylimidazolium actate + H₂O have also been determined as a function of the quality fraction at 298.15K.

For pure IL a new theoretic model is put forward on the basis of the following assumptions: (1) Because of the large size and the asymmetric shape, the ions may not be closely packed and lots of interstices between ions come into existence. (2) To calculate the volume easily, the interstice is regarded as a bubble. (3) There are 2N interstices for 1 mol 1-1 IL, where N is Avogadro’s constant. (4) The interstice in EMIAc can move about like an ion or another particle; In the movement the interstice does not vanish but can be compressed and expanded, which has an extra feature of motion of an interstice called the breathing motion. These assumptions called the interstice model.

The properties of volume and surface tension for pure ionic liquid EMIAc were discussed and estimated by extrapolation in terms of the interstice model and Glassor’s theory, respectively, the standard molar entropy of the IL were estimated. The thermal expansion coefficient, α, calculated by interstice model and in comparision with experimental value, It proved its value is to coincide with the literature. The results show that the interstice model of ionic liquid is reasonable, the interstice model can estimate thermal expansion coefficient of ILs.