Temperature molten salt viscosity

An ionic liquid (IL) , or classically a room-temperature molten salt , is an interesting series of materials being investigated in a drive to find a novel electrolyte system for electrochemical devices. ELs contain anions and cations, and they show a liquid nature at room temperature without the use of any solvents. The combination of anionic and cationic species in ILs gives them a lot of variations in properties, such as viscosity, conductivity, and electrochemical stability. These properties, along with the nonvolatile and flame-resistant nature of ILs, makes this material especially desirable for lithium-ion batteries, whose thermal instability has not yet been resolved despite investigations for a long time. In this chapter we discuss the efforts made for battery application of ILs. 

Matsumoto H, Matsuda T, Tsuda T, et al. The application of room temperature molten salt with low viscosity to the electrolyte for dye-sensitized solar cell. Chem. Lett. 2001. 26-27. 

Ionic Liquids are often very viscous. This is not unexpected. When the viscosities of classical high temperature molten salts, such as NaCl, is extrapolated down to room temperature, the viscosity of RTlLs can be estimated. These viscosities are in the range of the measured values. The viscosities can be many decades higher than the value of water. Many ILs are like honey [9,10]. Ionic Liquids are usually electrochemically stable, but they are often hygroscopic and chemically not always very stable. For example, at temperatures above 80 °C, choline salts rapidly decompose [11]. 

Room temperature molten salts based on imidazolium iodides have revealed very attractive stability features (Kubo, 2002 Wang, 2003). Despite their high viscosity impressive overall conversion efficiencies exceeding 6% have been obtained so far. This has been attributed to a Grothus mechanism which increases the diffusion coefficient of the triiodide ions in the melt and to a very effective mode of charge screening which is operative in this ionic liquids. 

Thus a large number of ambient temperature ionic liquids can be formed from a mixture of solid aluminium(ni) chloride and solid 1-ethyl-3-methylimidazolium chloride. Mixing these two salts together results in an exothermic reaction and generation of a clear, colorless liquid, and an ambient temperature molten salt with low viscosity. We can see... 

The dynamic viscosity of high temperature molten salts diminishes with increasing temperatures and generally follows the Arrhenius expression, where is the activation energy ... 

As in die case of die diffusion properties, die viscous properties of die molten salts and slags, which play an important role in die movement of bulk phases, are also very stiiicture-seiisitive, and will be refeiTed to in specific examples. For example, die viscosity of liquid silicates are in die range 1-100 poise. The viscosities of molten metals are very similar from one metal to anodier, but die numerical value is usually in die range 1-10 centipoise. This range should be compared widi die familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for die temperature dependence of die viscosity of liquids as an AiTlienius expression is... 

Ionic liquids are a class of solvents and they are the subject of keen research interest in chemistry (Freemantle, 1998). Hydrophobic ionic liquids with low melting points (from -30°C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also of interest. NMR and elemental analysis have characterized the molten salts. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1,2, 3, and 4(5)-positions on the imidazolium cation on these properties has been scrutinized. Viscosities as low as 35 cP (for l-ethyl-3-methylimi-dazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of l-ethyl-3-methyl-imidazolium bis((trifluoromethyl)sulfonyl)amide. The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications (Bon-hote et al., 1996). 

Cross-flow filters behave in a way similar to that normally observed in crossflow filtration under ambient conditions increased shear-rates and reduced fluid-viscosity result in an increased filtrate number. Cross-microfiltration has been applied to the separation of precipitated salts as solids, giving particle-separation efficiencies typically exceeding 99.9%. Goemans et al. [30] studied sodium nitrate separation from supercritical water. Under the conditions of the study, sodium nitrate was present as the molten salt and was capable of crossing the filter. Separation efficiencies were obtained that varied with temperature, since the solubility decreases as the temperature increases, ranging between 40% and 85%, for 400 °C and 470°C, respectively. These workers explained the separation mechanism as a consequence of a distinct permeability of the filtering medium towards the supercritical solution, as opposed to the molten salt, based on their clearly distinct viscosities. 

There are some density data for solid salts above ambient temperature which are given in the form of thermal expansion coefficients. These have been listed when they seemed reliable. Above the melting point, density data are scarce. Most are available for alkali halides but those available for salts are taken from the critically evaluated compilation Janz, G.J., Thermodynamics and transport properties for molten salts, correlation equations for critically evaluated density, surface tension, electrical conductance, and viscosity data,./. Phys. Chem. Reference Data, 17, Suppl. 2, 1988. 

Molten salts at room temperature, so-called ionic liquids [1, 2], attracting the attention of many researchers because of their excellent properties, such as high ion content, liquid-state over a wide temperature range, low viscosity, nonvolatility, nonflammability, and high ionic conductivity. The current literature on these unique salts can be divided into two areas of research neoteric solvents as environmentally benign reaction media [3-7], and electrolyte solutions for electrochemical applications, for example, in the lithium-ion battery [8-12], fuel cell [13-15], solar cell [16-18], and capacitor [19-21],... 

In fact, one can go further and make the following statement Molten salts look like water and not far above their melting points have viscosities, thermal conductivities, and surface tensions on the same orders ofmagnitude as those of water. In general, however, and with the important exception of some AlClj-complex organic systems, most fused salts are stable as liquids only at relatively high temperatures (500 to 1300 K) (Table 5.3). 

Ionic liquids (IL) are salts melting at low temperatures, and represent a novel class of solvents with non-molecular ionic character. In contrast to a classical molten salt, which is a high-melting, highly viscous, and very corrosive medium, an ionic liquid is already liquid at temperatures below 100 °C and is of relatively low viscosity [4]. In most cases, ionic liquids consist of combinations of cations such as ammonium, phosphonium, imidazolium, or pyridinium with anions such as halides, phosphates, borates, sulfonates, or sulfates. The combination of cation and anion has a great influence on the physical properties of the resulting ionic liquid. By careful choice of cation and anion it is possible to fine tune the properties of the ionic liquid and provide a tailor-made solution for each task (Fig. 1), and this is why ionic liquids are often referred to as designer solvents or materials. 

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