The simulation box with CO2 solvated in water. CO2 is displayed in ball-and-stick form while water molecules are shown in licorice representation. The three overlapping spherical volumes represent the inner-shell region with Ro = 3.3 Å (radius of the sphere centered on oxygen atoms) and Rc = 3.925 Å (radius of the sphere centered on the carbon atom).
Radial distribution function (RDF), g(r), of CO2 and water oxygen atoms. Blue line depicts the RDF of CO2 carbon and water oxygens. Green and red lines represent the RDFs of two CO2 oxygens and water oxygens. The shoulder of the CO2 carbon-water RDF is presumably due to the uneven distribution of the water molecules in the first hydration shell caused by the existence of the two CO2 oxygens.
Inner-shell free energy contribution based on corrdination number distribution (RT lnxn) with the inner-shell boundary at Rc = 3.925 Å (Ro = 3.3 Å). The red line with squares gives the observed data points while the black line is the extra/interpolation of a second-order polynomial function.
Packing contribution to the solvation free energy in terms of radius. Red squares are the observed points. The solid black line represents the extrapolation of a polynomial function.
Decomposition of solvation free energy of CO2 with respect to the radius of the inner shell. Blue diamonds represent the inner-shell chemical contribution; red squares represent the packing term; green triangles are the long-range contributions including electrostatics and van der Waals terms. The sum gives the total solvation free energy, denoted in purple crosses.
Coordination number distributions, p(n), for various inner-shell radii ranging from 2.7 to 3.3 Å. The most probable number of water molecules surrounding CO2 in liquid water ranges from n = 1 to n = 7, depending on the inner-shell observation volume.
QM-optimized structures of CO2-water clusters with various numbers of water ligands from n = 1 to n = 6.
Van der Waals parameters for CO2 and water interactions. The parameters were determined using the combination rule (Ref. 41) from the AMBER force field. ε ij is the well depth of the potential while σ ij is the radius. Ow represents the oxygen atom of water.
Hydration free energies of CO2 in terms of coordination number using the cluster approach. Individual terms are listed. μ IS , the total inner-shell contribution, includes the gas-phase binding free energy ΔG (0), anharmonicity correction G anharm, inner-shell dispersion effects, , the density term μ den = nRTln1354, the entropic penalty and the probability term RTlnp(n). The outer-shell components consist of the long-range electrostatic interaction, ; long-range van der Waals interaction, ; and molecular packing term, Δμ pac . μ ex is the total solvation free energy.
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