Noura Dawass

5 102 P ROPERTIES OF U REA -C HOLINE C HLORIDE M IXTURES are increasing with the mole fraction of urea. This indicates that urea-urea in- teractions become stronger as more urea is added to the system. Figure 5.2 also shows that the values of the KBIs G urea − ChCl and G ChCl − ChCl slightly decrease with increasing the content of urea. Consequently, urea-ChCl and ChCl-ChCl interac- tions become weaker with larger mole fractions of urea. Such trends obtained in KBIs are consistent with the results of RDFs of ChCl mixtures. It is interesting to examine how interactions between dissimilar components (i.e. ChCl-urea) differ from the interactions between similar components (i.e. urea-urea and ChCl-ChCl) at various mole fractions of urea. To study this, the term G f = G αα + G ββ − 2 G αβ can be used. The term G f is zero for an ideal mix- ture in which interactions of α and β are equal to the average interactions of molecules of the same type. The term G f can be computed by integrating the combined RDF g f ( r ) = g αα ( r ) + g ββ ( r ) − 2 g αβ ( r ) as shown in Eq. 1.25. The values of G f shown in Figure 5.2 are computed using this approach. Similar to the KBIs of urea-urea pairs, the values of G f increase dramatically between x urea = 0.25 and x urea = 0.4. This means that the molecular interactions of the system are shifting for these mole fractions. While the affinity between urea and ChCl is al- ways stronger than the average affinity between similar molecules, attractive in- teractions between urea and ChCl are stronger when x urea < 0.25. Interestingly, the values of G f do not significantly change beyond x urea = 0.41 as more urea is added to the system. When studying other microscopic properties of mixtures of ChCl and urea, Sun et al. [168] has correlated the change of interactions with the content of urea to the eutectic behaviour. The authors reported that the eutectic composition is at the molar ratio of ChCl to urea of 1:2 ( x urea = 0.5).