Gas solubilities and related thermodynamic properties

In this chapter, we will first discuss various experimental techniques that can be used to measure gas solubilities and related thermodynamic properties in ILs. Then, we will describe and compare literature data on the solubility of various gases in ILs. Most of these studies have been published since 2000. Finally, we will discuss the impact that gas solubilities in ILs have on the applications described above (reactions, gas storage, gas separations, separation of solutes from ILs) and draw some conclusions. 

In this section, we describe some of the various experimental techniques that can be used to measure gas solubilities and related thermodynamic properties. 

In general, gas solubilities are measured at constant temperature as a function of pressure. Permanent gases (gases whose critical temperatures are below room temperature) will not condense to form an additional liquid phase no matter how high the applied pressure. However, condensable gases (ones with critical temperatures above room temperature) will condense to form a liquid phase when the vapor pressure is reached. In normal liquids, the solubility of many gases is quite low and can be adequately described at ambient pressure or below by Henry s law. The Hemy s law constant is defined as 

Also of importance is the effect of temperature on the gas solubihty. From this information, one can determine the enthalpy and entropy change experienced by the gas when it changes from the ideal gas state (hf and to the mixed liquid state hi and Si). 

Ahi and Asi can be obtained from determining the pressure required to achieve a specified solubility at several different temperatures and constant composition, xi. In the Henry s law region, Ahi and Asi can be found directly from the temperature dependence of the Henry s law constant, as given by the familiar van t 

Joan F. Brennecke, Jennifer L. Anthony, EdwardJ. Maginn 

The enthalpy and entropy of gas dissolution in the IL provide information about the strength of the interaction between the IL and the gas, and about the ordering that takes place in the gas/IL mixture, respectively. 

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One of the important properties of surfactants that is directly related to micelle formation is solubilization. Solubilization may be defined as the spontaneous dissolving of a substance (solid, liquid, or gas) by reversible interaction with the micelles of a surfactant in a solvent to form a thermodynamically stable isotropic solution with reduced thermodynamic activity of the solubilized material. Although both solvent-soluble and solvent-insoluble materials may be dissolved by the solubilization mechanism, the importance of the phenomenon from the practical point of view is that it makes possible the dissolving of substances in solvents in which they are normally insoluble. For example, although ethylbenzene is normally insoluble in water, almost 5 g of it may be dissolved in 100 mL of a 0.3 M aqueous solution of potassium hexadecanoate to yield a clear solution. 

For the determination of a (and hence ii), the most direet method is arguably that based on inert gas solubility data. However, in view of the arduousness involved and the uncertainties in both the extrapolation procedure and the experimental solubilities, it is natural to look out for alternatives. From the various suggestions,a convenient way is to adjust a sueh that the computed value of some selected thermodynamic quantity, related to o, is consistent with experiment. The hitherto likely best method is the following To diminish effects of attraction, the property chosen should probe primarily repulsive forces rather than attractions. Since the low compressibility of the condensed phase is due to short-range repulsive forces, the isothermal compressibility Pr = -(lA0( V/ P)x might be a suitable candidate, in the framework of the generalized van der Waals (vdW) equation of state... 

The optimisation of manufacturing processes of cellular polymers involves the control of fluidodynamic behaviour of macromolecular viscoelastic materials containing a dissolved gas at high concentration and at thermodynamic conditions able to promote the formation of gas bubbles in the melt. Nucleation and growth rates, which determine the final morphology of the foam are related to the physical and the rheological properties of the polymeric melt surface tension, elongational viscosity of the polymer/gas system, solubility and diffusivity of the gas into the melt. 

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