CO2 is more soluble in watera than CO [IAPWS], which is strange as it is both larger and without a dipole moment. This has been explained [166] by specific hydrogen bonding between the oxygen atoms in CO2 and water molecules. CO has a partial positive charge on the oxygen that inhibits hydrogen bonding.
The CO2 may form a hydration shell from a symmetrical dodecahedral arrangement of 18 water molecules where each CO2 oxygen atom is hydrogen bonded to three water molecules. This allows some cooperation between the hydrogen bonding at both ends of the CO2 molecule. Such a cluster can form the central part of an icosahedral water cluster (CO2(H2O)278) possessing just two defects (water molecules with only 3 rather than 4 hydrogen bonds). In this model, there are six water molecules closest to the CO2 in agreement with many studies [499].
The calculated [166] carbon-oxygen and carbon-hydrogen pair correlation functions (PCF) are remarkably similar to those predicted by the icosahedral model.
The red lines are the calculated pair correlation functions, between the carbon atom in CO2 and water, and the bars are the predictions from the model. Note that a similarly good fit is apparent if the central cluster is tetrakaidecahedral rather than dodecahedral. Such occupied 51262 inner-shell clusters are found to be more stable using theoretical modeling [876].
It is notable that calculated pair correlation functions between CO and water [166] are consistent with the CO molecules sitting centrally (clathrate-like) in expanded icosahedral water clusters; CO only forming extremely weak complexes to water (major, HOH---CO; minor H2O---OC). For interactive Figures of the central dodecahedral cluster, see Jmol.
At high pressures (for example, >2 MPa) and low temperatures (for example, <4°C) CO2 forms crystalline clathrates (type-I clathrate, 46 H2O:8 CO2 maximum), within a cubic arrangement of two dodecahedral (512) and six tetrakaidecahedral (51262) cages. In these structures there is no hydrogen bonding between the CO2 guest molecules and the water clathrate lattice and the CO2 molecules occupy both cages but prefer the tetrakaidecahedral cages.
a CO2 undergoes a slight hydration (~ 0.2%) to H2CO3 in solution with the resulting weakly acidic H2CO3 ionizing slightly. [Back]
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This page was last updated by Martin Chaplin on 23 April, 2012
