The Microscopic Mechanism Of Salt-effect On The Solubility Of CO₂ And Molecular Simulation Studies

Environment pollution and greenhouse effect are closely related to the content of CO2 in the nature. However, the ocean as the biggest carbon sink, the inorganic salts in it have a great influence on the solubility of CO2, salts make CO2 salt-in or salt-out, while the microscopic mechanism of salt-effect is unclear, there’s a great scientific significance to deeply discuss.In order to uncover the mechanism of salt-effect on the solubility of CO2, the structures, the dynamic properties and the solvation free energy of CO2 in aqueous KCl, KF, KI, KCl O4, KNO3, KSCN, Na Cl, Na F, Na I, Na Cl O4, Na NO3, Na SCN solutions, are explored by the molecular dynamics simulations. The results from our simulations are well consistent with the experimental measurements. The microscopic mechanism beyond the salt effect on the solubility of CO2 can be explained as:I. From the microscopic angle, the salt-effect on the solubility of CO2 in water solutions can be classified into two types. One is the direct association and specific effect of ion. The aggregation between ion and CO2 can enhance the solubility of CO2. For example, the direct aggregation of Cl O4- and CO2 results in the salt-in effect of KCl O4;The other is the indirect effect, which is mediated by the water. The mobility of water can be disturbed by the coexisting ion. The solubility of CO2 is closely related to the mobility of water. For example, because Floride ion has stronger retarding effect on the rotation and translation of water than Cloride ion, the stronger salt-out effect of Floride ion on the CO2 solubility can be observed than Cloride ion in the experiments and simulations.II. According to the solvation free energies of CO2 in electrolyte solutions and its subcompositions, the contribution from Van der Waals potential plays a more important role than from the electrostatic interactions. When inserting CO2 into the water, the creation of the cavity in bulk water causes a more evident increment of free energy due to the expelling effect of Van der Waals interaction than the electrostatic attraction between CO2 and surrounding molecules and ions. This is explained by the hydrophobic nature of CO2.III. The mobility of water has close connection with the solubility of CO2. Stronger the salt-out effect of ion is, more intensely retarding effect of ion can be observed. The tendencies of salt-out effect and the effect of ion on the mobility of water are well consistent with each other:Na F > Na Cl > Na I, KF> KCl> KI. However, the SCN- can accelerate the motion of water and results in a salt-in effect. This is in well agreement with the order of Hofmeister ions.