Ionic liquids enthalpy change

In addition to combining self-diffusion coefficient measurements with impedance analysis to determine ionicity as describe above, a novel method has been developed to determine the fraction ofions in ILs exclusively from self-diffusion coefficients obtained using PFG-NMR [18]. EquiHbrium constants of the ionization process from measured ion self-diffusion coefficients were calculated using this method. Enthalpy and entropy changes of ionization and ion self-diffusion processes have been obtained for a series of ionic liquids using this method. 

In Chapter 5 we learned that the enthalpy change in a process can provide information about the extent to which a process will occur. (Section 5.4) Processes tirat are exothermic tend to proceed spontaneously. A solution will not form if AHsoin is too endothermic. The solvent-solute interaction must be strong enough to make AH3 comparable in magnitude to A Hi + A H2. This is why ionic solutes like NaCl do not dissolve in nonpolar liquids such as gasoline. The nonpolar hydrocarbon molecules of the gasoline would experience only weak attractive interactions with the ions, and these interactions would not compensate for the energies required to separate the ions from one another. 

Covalently bonded substances with a simple molecular structure, for example water and ammonia, are usually liquids or gases. This is because the forces between the molecules are weak. It does not take much energy to overcome these intermolecular forces, so these substances have low melting points, low boiling points and low enthalpy changes of vaporisation compared with ionic compounds. Some substances that have covalently bonded molecules maybe solids at room temperature, for example iodine and poly(ethene). These are usually molecules where the van der Waals forces are considerable. However, the melting points of these substances are still fairly low compared with ionic compounds or most metals. 

---