Titration in Canonical and Grand-Canonical Ensembles

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Thermodynamics

Background:

• Standard pH definitions are well-established for homogeneous systems, serving as a measure of hydrogen potential.
• However, applying these definitions to heterogeneous systems presents thermodynamic inconsistencies, violating fundamental principles like the Gibbs-Guggenheim equation.
• This issue is critical for modern computational methods modeling complex systems such as proteins and nanoparticles.

Purpose of the Study:

• To highlight the thermodynamic inconsistencies of pH definitions in heterogeneous systems.
• To investigate the impact of different simulation approaches on calculating titration isotherms for complex systems.

Main Methods:

• Analysis of thermodynamic principles governing electrochemical potentials in heterogeneous systems.
• Comparison of titration isotherms derived from semigrand canonical simulations versus canonical reactive Monte Carlo simulations.

Main Results:

• pH definitions based on chemical activity are thermodynamically inconsistent in heterogeneous systems.
• Semigrand canonical simulations yield significantly different titration isotherms compared to canonical reactive Monte Carlo simulations for systems with charge regulation.

Conclusions:

• The conventional pH concept requires re-evaluation for heterogeneous systems, especially in computational modeling.
• Different simulation methodologies can produce non-comparable results for titration isotherms, necessitating careful selection of methods for accurate thermodynamic analysis.