title: First-principles calculation of thermodynamic stability of acids and bases under pH environment: a microscopic pH theory.
abstract: 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 proton chemical 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(-), NH(3), NH(4)(+), HCOOH, and HCOO(-) in water. The deduced auto-dissociation constant of water and the difference in the pKa values of NH(3) 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.
authors.affiliation: Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Korea. yong.hyun.kim@kaist.ac.kr
authors.forename: Yong-Hyun
authors.initials: YH
authors.lastname: Kim
authors.forename: Kwiseon
authors.initials: K
authors.lastname: Kim
authors.forename: S B
authors.initials: SB
authors.lastname: Zhang