Influence of CO2 loading on diffusion coefficients of reaction product species in equilibrium


Fig. 8 shows the diffusion coefficients of all molecular species in DEAB at various loadings (for numerical data see Table S5 in the ESI). With increasing CO2-loading, the mobility of all compounds in the mixed solutions decreases. This is in agreement with the observed increase of viscosity (see above). The diffusion coefficients for DEABH+ and CO32− are identical expect for very high loading when the strong electrostatic interaction is persistent (with a lifetime of 1 ns) and the complex diffuses as a supermolecule. At low and intermediate CO2 loadings, the carbonate ions are preferably solvated by water molecules. The electrostatic interaction between the bicarbonate HCO3 and DEABH+ is weaker and thus it shows a faster diffusion. The slower diffusion of CO32− ions is determined its larger charge due to which the interactions with surrounding water molecules and DEABH+ are stronger and responsible a slower mobility.


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Fig. 8 Diffusion coefficients of reaction product species in equilibrium as a function of CO2 loading in the six-component mixture of aqueous DEAB with CO2.

The hydrophobic DEAB molecules within the self-aggregate clusters diffuse slower than charged DEABH+ molecules even when the latter are bound to anionic species such as carbonate and bi-carbonate. At very low CO2-loading, the DEAB molecules move even slower than in case of the pre-reaction solvent. As the CO2-loading increases, the DEAB cluster size becomes smaller and the diffusion coefficients of DEAB molecules thus increase with CO2 concentration and finally approach that of DEABH+. The free CO2 molecules are mostly localized within the DEAB clusters and are moving much faster than the DEAB molecules. The load-dependence of CO2 diffusion reflects on the one hand the acceleration of DEAB molecule diffusion and on the other hand the slower diffusion of other species in solution. Therefore, the CO2 diffusion coefficient is less sensitive to the CO2-loading than other mixture components. At a maximum loading of 0.815, the CO2 diffusion coefficient is still at 79% of the non-reacted ternary system. Water diffusion slows down upon increase of CO2 loading since the number of charged species DEABH+/HCO3/CO32− increased which possess a tight solvation shell.”

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