Density of Ionic Liquids

Densities are perhaps the most straightforwardly determined and unambiguous physical property of ionic liquids. Given a quality analyhcal balance and good volumetric glassware the density of an ionic liquid can be measured gravimetrically (i.e., the sample can be weighed). 

The viscosity of a fluid arises from the internal friction of the fluid, and it manifests itself externally as the resistance of the fluid to flow. With respect to viscosity there are two broad classes of fluids, Newtonian and non-Newtonian. Newtonian fluids have a constant viscosity regardless of strain rate. Low molecular weight pure liquids are examples of Newtonian fluids. Non-Newtonian fluids do not have a constant viscosity and will either thicken or thin when strain is applied. Polymers, colloidal suspensions, and emulsions are examples of non-Newtonian fluids [1]. To date, researchers have treated ionic liquids as Newtonian fluids, and no data have so far been published to indicate that there are ionic liquids that are non-Newtonian. 

Experimentally determined viscosities are generally reported as either absolute viscosity rj) or kinematic viscosity (v). Kinematic viscosity is simply the absolute viscosity normalized by the density of the fluid. The relationship between absolute viscosity ( ), density (p), and kinematic viscosity (v) is given by  

The unit of absolute viscosity is the Poise (P, g cm s ) while the unit for kinematic viscosity is the Stoke (St, cm s ). Because of the large size of these viscosity units, absolute viscosities for ionic liquids are usually reported in centipoises (cP) and kinematic viscosities in centistokes (cSt). 

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The reported densities of ionic liquids vary between 1.12 g cm for [(n-QHi7)(C4H9)3N][(CF3S02)2N] and 2.4 g cm for a 34-66 mol% [(CH3)3S]Br/AlBr3 ionic liquid [21, 23]. The densities of ionic liquid appear to be the physical property least sensitive to variations in temperature. For example, a 5 degree change in temperature from 298 to 303 K results in only a 0.3 % decrease in the density for a 50.0 50.0 mol % [EMIM]C1/A1C13 [17]. In addition, the impact of impurities appears to be far less dramatic than in the case of viscosity. Recent work indicates that the densities of ionic liquids vary linearly with wt. % of impurities. For example, 20 wt. % water (75 mol %) in [BMIM][BF4] results in only a 4 % decrease in density [33]. 

Trohalaki S, Pachter R, Drake GW et al. (2005) Quantitative structure-property relationships for melting points and densities of ionic liquids. Energ Fuels 19 279-284... 

Overall the density of ionic liquids is somewhat easier to model than the viscosity. In general the change of density with temperature has been fitted to linear equations of the form... 

The densities of ionic liquids are also affected by the choice of the cation and anion. For imidazolium ionic liquids the density decreases slightly as the alkyl chain of the cation increases in length. In addition, the absoption of water by ionic liquids by increasing nitric acid concentration can also cause a decrease in the density (Giridhar et al. 2004). Regarding imidazolium-based ionic liquids, by increasing the alkyl chain it was found that the surface tension decreased due to the orientation of hydrocarbon tails on the surface (Kilaru et al. 2007). The vapour pressure of an ionic liquid is usually unmeasurable at room temperature. Studies on the thermal stability of ionic liquids showed that the type of the associated anion has the primary effect on the thermal stability of the ionic liquids (Holbrey and Seddon... 

Mantz RA, Trulove PC (2008) Viscosity and density of ionic liquids, hr Wasserscheid P, Welton T (eds) Ionic liquids in synthesis, vol 1,2nd edn. Wiley-VCH, Weinheim, pp 72-88... 

Density. In general ionic liquids have a larger density value than the density of water. For example, [BMIM][PF6] has a density of 1.37 g/cnP at 2CFC. The density of ionic liquids decreases when the N-alkyl chain length increases. 

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