Classical simulation of acid and base dissociation constants in supercritical water at constant density

• Simple molecular models used to reproduce ionic dissociation in supercritical water.
• pKW decreases at 0.9 g cm−3 due to increasing S and falling U/T.
• pKa of HCl decreases at 0.9 g cm−3 with decreasing U/T and S.
• pKb of NaOH decreases slightly at 0.9 g cm−3 as both S and U/T increase.
• Qualitative agreement with experiment.

Molecular dynamics simulations were performed to investigate the dissociations of water, NaOH and HCl in water at constant density of 0.9 g cm−3 at near-critical and supercritical temperatures. Results were in good qualitative agreement with available data, showing increased temperature favouring all dissociations. The dissociation of water was favoured by more negative values of U/T and an increasing entropy tem, whereas the dissociation of HCl showed both decreasing U/T and entropy. NaOH showed an increasing value of U/T which was dominated by an increasing entropy term. Differences in the energy contributions were attributed to the change in solute charges upon dissociation.

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