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

10.1039/C9CP03976G

“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.

	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.”