Noura Dawass

5 108 P ROPERTIES OF U REA -C HOLINE C HLORIDE M IXTURES 5.3.3. T RANSPORT PROPERTIES OF UREA – CHOLINE CHLORIDE MIXTURES In Figure 5.6, viscosities computed fromMD simulations are compared to the ex- perimental viscosity [169] , which is available for a molar ratio of urea to ChCl of 1:2 ( x urea = 0.5). For this molar ratio, the viscosity from MD simulations deviates from the experimental viscosity by ca. 9%. Also, Figure 5.6 shows the viscosities of mixtures of urea and ChCl at various mole fractions of urea computed using MD simulations. The viscosity of the mixture decreases almost linearly with the mole fraction of urea. Low viscosities result in a higher mobility and as a result, larger diffusivities are observed with the addition of more urea to the mixture. From MD simulations, self diffusion coefficients D self of urea, choline and chlo- rine were computed and corrected for finite–size effects using the Yeh–Hummer correction [72, 73, 97] . In Figure 5.7 (a), the self diffusion coefficients D self of urea, choline and chlorine are shown as a function of the mole fraction of urea. As expected, self diffusion coefficients of all species increase with the addition of more urea to the mixture. This is due to the fact that urea diffuses faster than choline and chloride as shown in Figure 5.7 (a). From the computed viscosities and thermodynamic factors, it is possible to correct the binary diffusion of urea– ChCl mixtures for finite–size effects using the correction proposed by Jamali and co–workers [72, 94] . The corrected MS and Fick diffusivities are presented in Fig- ure 5.7 (b) for urea and ChCl at different mole fractions. The Fick diffusivities are computed from the MS diffusion coefficients and the thermodynamic factors. As in the case of self diffusivities, the binary diffusion of urea and ChCl mixtures increases as the mole fraction of urea increases. Another transport property that is of interest for salt solutions is the ionic conductivity. Ionic conductivities can be estimated from the Nernst–Einstein (NE) equation [170] , κ = e 2 k B TV X i N i q 2 i D self, i (5.31) where q i is the charge of the molecule and e is the elementary charge. D self, i is the self diffusion coefficient of component i computed fromMD simulations. In Figure 5.8, ionic conductivities of urea and ChCl mixtures are presented. Larger diffusion was observed at high mole fractions of urea, but ionic conductivities remain relatively constant with composition. This is due to the fact that fewer ions are present in the system when more urea is added.