CO2 Dipole Moment: A Simple Model and Its Implications for CO2-Rock Interactions

Abstract

CO2 is a widespread fluid naturally occurring within the Earth crust or injected in deep strata for technological issues such as Carbon Capture and Storage (CCS). At STP conditions, CO2 is a gas, with a net zero dipole moment. Growing pressures produce an increase in its density. The reduced intermolecular distance causes a variation in the molecular structure, due to the intensification of mutual interactions. Some published spot data reveal the departure from the planarity of the bond angle while others provide few values of the CO2 dipole moment. Based on a small amount of literature-measured angle values, it was possible first to extrapolate a correlation between bond angle and density (R2 = 0.879). By fixing the partial charges distribution, we present a simple model that allows the calculation of the CO2 dipole moment directly from the geometry of the molecule, in the range of 179–162 degrees, 1-degree step. Results give values up to about 1 D. Being aware that this model is qualitative, it gives, however, an explanation of the experimental reactivity, and it also provides a valid tool in identifying zones in the crust where these reactions are likely to occur efficiently. Finally, we hypothesise the role of dry CO2 in the carbonate formation through the interactions with the basalts.