Noura Dawass

5.3. R ESULTS AND DISCUSSION 5 105 5.3.2. T HERMODYNAMIC PROPERTIES OF UREA – CHOLINE CHLORIDE BSBSB MIXTURES D ENSITY Densities of ChCl and urea mixtures at 343.15 K and 1 atm are shown in Figure 5.3 as a function of the mole fraction of urea (Eq. 5.1) . The density of the mixture increases as the mole fraction of urea increases. This trend was expected due to the high density of pure urea. We performed MD simulations of pure ChCl (400 molecules) and pure urea (400 molecules) systems at 343.15 K and 1 atm. The computed density of pure ChCl is 1021.2 kg/m 3 and the density of pure urea is 1453.2 kg/m 3 . Values of the densities of mixtures of ChCl and urea are bounded between the densities of the pure components. For a mixture with a molar ratio of 1:2 ( x Urea = 0.5), the computed density exhibits an excellent agreement with the density measured experimentally by Yadav et al. [169] , showing deviation of less than 1.5%. To the best of our knowledge, experimental data for other mole fractions of urea are not available in literature. T HERMODYNAMIC FACTORS To compute the thermodynamic factors of ChCl and urea mixtures, we use three different descriptions of KBIs: (1) the individual KBIs of G αα , G αβ and G ββ com- puted by integrating the corresponding RDFs, (2) the term G f computed by inte- grating the RDF g f ( r ) = g αα ( r ) + g ββ ( r ) − 2 g αβ ( r ), and (3) KBIs using the Cortes- Huerto finite-size correction [83] (see section 3.3.3) . In the first two approaches, RDFs are corrected for finite-size effects using the method reported by Ganguly and van der Vegt [167] . In the Cortes-Huerto approach, RDFs are corrected us- ing a correction that is independent of the interparticle distance. Based on the computed RDFs, KBIs of finite subvolumes are used to estimate KBIs in the ther- modynamic limit ( G ∞ αβ ) [83] . This is important to study the effect of the correc- tion method on the computed KBIs and thermodynamic factors. While RDFs are sampled using long and multiple simulations, still, uncertainties of the com- puted KBIs and the term G f are not very small. Naturally, these uncertainties re- flect on the accuracy of the estimation of thermodynamic factors. In Figure 5.4, we show the thermodynamic factors of ChCl and urea at T = 343.15 K, P = 1 atm and various mole fractions of urea. The different methods, used here to com- pute thermodynamic factors, present relatively similar results. This could be ex- plained by the fact that systems of large sizes were used to compute KBIs, which makes the choice of the RDF correction not very important. Figure 5.4 shows that all Γ values are found larger than 1 for all urea contents. This indicates that all mixtures of ChCl and urea studied here are not ideal, and that the interactions between urea and ChCl are more favorable than interactions between molecules of the same type (i.e. urea-urea and ChCl-ChCl). Computed thermodynamic