First-principles microscopic pH theory
2014.03.21 21:07
Despite being one of the most important thermodynamic variables, pH has yet to be incorporated into first-principles thermodynamics to calculate stability of acidic and basic solutes in aqueous solutions. By treating the solutes as defects in homogeneous liquids, we formulate a first-principles approach to calculate their formation energies under protonchemical potential, or pH, based on explicit molecular dynamics. The method draws analogy to first-principle calculations of defect formation energies under electron chemical potential, or Fermi energy, in semiconductors. From this, we propose a simple pictorial representation of the general theory of acid-base chemistry. By performing first-principles molecular dynamics of liquid water models with solutes, we apply the formulation to calculate formation energies of various neutral and charged solutes such as H+, OH−, NH3, NH4 +, HCOOH, and HCOO− in water. The deduced auto-dissociation constant of water and the difference in the pKa values of NH3 and HCOOH show good agreement with known experimental values. Our first-principles approach can be further extended and applied to other bio- and electro-chemical molecules such as amino acids and redox reaction couples that could exist in aqueous environments to understand their thermodynamic stability.
http://scitation.aip.org/content/aip/journal/jcp/136/13/10.1063/1.3700442
Y. -H. Kim, K. Kim, S. B. Zhang, J. Chem. Phys. - Apr. 6, 2012
