Noura Dawass

5 106 P ROPERTIES OF U REA -C HOLINE C HLORIDE M IXTURES Figure 5.3: Densities of urea–ChCl as a function of the mole fraction of urea computed using MD simulations at T = 343.15 K and P = 1 atm. Details of the MD simulations are provided in sec- tion 5.2.4 and Table 5.1. The experimental density [169] is reported at molar ratio of urea to ChCl of 1:2 ( x urea = 0.5). factors exhibit no distinct trend as a function of the mole fraction of urea. The variations are small and error bars are large. Although there is no certain trend shown in Figure 5.4, we can see that thermodynamic factors become minimum at a mole fraction of urea of 0.5. This can be attributed to the relatively increased affinity between urea-urea molecules at this mole fraction. P ARTIAL MOLAR VOLUMES The partial molar volume of a component represents the change of volume as a result of the addition of the same component in the mixture at a fixed tempera- ture, pressure and number of molecules. The relation between KBIs and partial molar volumes is provided in Eq. (1.6) . In Figure 5.5, we show the partial mo- lar volumes of urea and ChCl as a function of the mole fraction of urea. As the mole fraction of urea increases, the partial molar volume of urea approaches the molar volume of pure urea. For both components, the change in partial molar volumes with the increasing mole fraction of urea is not significant. This is inter- esting because the results of RDFs and KBIs show that the interactions between urea and ChCl are significantly affected by the change of the urea content. In Fig- ure 5.5, we also show the molar volumes of pure urea and pure ChCl, calculated from separate MD simulations of pure systems. Molar volumes for mixtures are consistent with those of pure components.