3.5 T HERMOPHYSICAL PROPERTIES OF RTIL S
3.5.1 Density and viscosity
There are serious disagreements in the published literature for a number of the properties of ILs, especially viscosity and density. It was reported that these disagreements resulted from a number of factors, including the purities of the ILs, with the main impurities being water and halide ions, as well as the use of inappropriate measurement methods. To avoid the disagreements in thermophysical data, [C6mim]NTF2 was recommended as reference IL. The reference-quality measurements on selected thermophysical properties of this IL were reported by IUPAC team. Moreover, recommended values for the properties measured were established accordingly with recommendations on measurement methods [183].
Density is defined as the concentration of matter, measured by the mass per unit volume. The molar volume, VM, is defined as the volume occupied by 1 mol of a substance [88]. Information on solvent density values is important. It is particularly used in fluid flow calculations and for the design of liquid/liquid two phase mixer settler units. Density can be considered as a fundamental data for developing equations of state, which are the main tool used for thermophysical properties
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prediction for process design purposes, and solution theories for ILs. It is also required for many relevant industrial problems such as liquid metering applications or for the design of different types of equipment such as condensers, separation trains, or even storage vessels [124]. Uses of density data include conversion of kinematic into dynamic viscosity and vice versa, calculation of molar refraction with the Lorentz- Lorenz equation, derive cohesion parameters, estimation of liquid viscosity and to decide if an immiscible compound floats in water or sinks to the bottom [88].
The viscosity of a fluid arises from the internal friction of the fluid. It might be described as an internal resistance of a gas or a liquid to flow [37, 88]. 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 (Recently, new data have been published to indicate that there are ILs that are non-Newtonian [37, 184]).
Viscosity data are reported as dynamic viscosity, , or as kinematic viscosities, , which are related through density, ρ, by the following equation:
3-1
Viscosity is an important physical property for a number of processes. For instance, it determines the force and energy required to transfer and mix the IL with other substances. It appears in many dimensionless groups used in mass- and heat- transfer correlations. Applications that occur at high temperatures and/or pressures require reliable and accurate experimental data and mathematical models. This is especially pertinent for engineering applications as hydraulic fluids. ILs cover a wide range of viscosity [108].
The viscosities of ILs have normally been measured using one of three methods:
falling or rolling ball, capillary, or rotational. The disadvantage of the falling or rolling ball viscometer is that it needs to be calibrated with a standard fluid that is similar in viscosity to the fluid of interest. Capillary viscometers measure the kinematic viscosity directly. In order to convert to absolute viscosity, the kinematic
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viscosity must be multiplied by the fluid density which requires additional experiments to determine fluid density so that the absolute viscosity can be calculated.
The last type of widely used viscometer is the rotational viscometer. These can adopt a variety of geometries including concentric cylinders, cone and plate, and parallel disks. Of the three geometries, a concentric cylinder is well suited for low viscosity fluids. All the three methods appear to provide equally high quality IL viscosity data. Most of IL viscosity data found in the literature were generated using the capillary method that is probably due to its low cost and relative ease of use.
However, the rotational viscometer has the potential to provide additional information by the Newtonian behavior of the ILs [37].
The density and viscosity measurements of the present synthesized ILs were carried out over a temperature range 293.15 to 353.15 K using Anton Paar viscometer (Model SVM3000). A built-in density measurement based on the oscillating U-tube principle allows the determination of kinematic viscosity from the measured dynamic viscosity. The measuring ranges for this instrument are as follows: dynamic viscosity 0.2-20000 mPas, kinematic viscosity 0.2-20000 mm2/s, density 0.65-3.0 gcm3, temperature range 20-105 °C (with cooling from -40 °C). The temperature was controlled to within ±0.01 C. The reproducibility of the measurements were 0.35 % and ± 510-4 gcm-3 for viscosity and density respectively. The uncertainties of the viscosity and density were ±0.5 % and ±0.0004 gcm-3 respectively. Required sample volume for both measuring cells is 2.5 ml. The instrument was calibrated before each series of measurements and checked using pure organic solvents with known viscosity and density and also by measuring the densities of atmospheric air. During the measurements the IL was transferred to a syringe and injected into the instrument. To prevent any air bubble, the tube was first filled with some of the contents and the first measurement was taken (after the temperature set point was reached). Another measurement followed after the liquid of the vibrating tube was replaced with the one that remained in the syringe. The agreement between both values is a measure of the effectiveness of the method.
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