Solvation Free Energy And Acidity Model Of Nanoaerosols Based On Molecular Dynamics Simulation

Acidity is a key property of aerosol particles,affecting the thermodynamics of gas-particle distribution and the chemical kinetics of air-particle formation as well as transformation.Aerosol acidity is an important factor affecting the amount of particulate matter formation,and plays a key role in determining the reaction pathway of secondary aerosol formation,which affects the global climate and human health.The acidity of aerosol is closely related to particle size.It is important to study the size dependence of aerosol particle acidity from the microscopic perspective for understanding the new particle formation and related phenomena accurately.In this paper,the solvation free energies of NH₃,H₂O,HNO₃,H₃O⁺,NH₄⁺,OH^-,NO₃^-,HSO₄^-as well as SO₄^2-were obtained in nanodroplets with different diameters using the molecular dynamics simulation based on the polarization field and high-level quantum mechanics calculation.And the size dependence of aerosol acidity was also investigated at the nanoscale.The main study results are as follows:（1）The molecular dynamics simulations of NH₃,H₂O,HNO₃,H₃O⁺,NH₄⁺,OH^-,NO₃^-,HSO₄^-and SO₄^2-were carried out in nanodroplets with different diameters based on the polarization force field.It is found that the solvation free energies are linearly related to the solvent size(N^-1/3,N represents the water molecule numbers in the nanodroplets).And the solvation free energies become more negative with the increase of solvent size,which is favorable for the dissolution.（2）The deprotonation free energies of H₂O,HNO₃,HSO₄^-and NH₃ in the gas phase were obtained using the quantum mechanics calculation.Then the classical dissociation constants of H₂O,HNO₃,HSO₄^-and NH₃ could be calculated through the thermodynamic cycle.The pK_a values of water in nanodroplets are obviously higher than those in the bulk water.All the pK_avalues of HNO₃,HSO₄^-and H₂O as well as pK_b values of NH₃ have a linear relationship with the solvent size,which are decreased with increasing solvent sizes.It is proved that larger particles are more favorable for dissociation of HNO₃,HSO₄^-,H₂O and NH₃.According to the calculation of pH values,the HNO₃ and HSO₄^-aerosol acidity as well as NH₃ aerosol basicity increases with increasing solvent size in a certain range,compared with water nanodroplets.（3）From solvation free energies,it’s found that the Henry’s law constants increase with increasing solvent sizes.Considering the multiphase buffer theory,the effective dissociation constant（pK_a^*）is larger than the classical dissociation constant（pK_a）and decreases with the increase of nanodroplet diameters.By calculating the acidity,the HNO₃ and HSO₄^-aerosol acidity increases while the acidity of NH₄⁺aerosol decreases slightly with increasing solvent size in a certain range,compared with water nanodroplets.In general,the variation law of the solvation free energies of aerosol particles with the solvent size was studied by molecular dynamics simulations in this paper.Combined with quantum mechanical calculations,the variation of classical dissociation constants and aerosol acidity with solvent sizes was obtained through the thermodynamic cycle.The changes of effective dissociation constants and aerosol acidity with solvent size were gotten by the introduction of multiphase buffer theory.The size dependence of aerosol acidity was revealed from the microscopic point of view,which was of great significance for the study of pH-dependent multiphase chemical processes of aerosols.It has a guiding role for the accurate prediction of ecosystem and climate properties.