Hydration of acetic acid and acetate ion in water studied by 1D-RISM theory

Structural and thermodynamic aspects of the hydration of acetic acid (CH3COOH) and acetate ion (CH3COO-) were studied with the 1D-RISM integral equation method. It was found that the average number of water molecules in the hydrophobic hydration shell of the -CH3 group is 8.9 for acetic acid and 10 for acetate. The average number of H-bonds formed by the -COOH group is 2.5, whereas that of -COO- is 6, indicating that deprotonadon of acetic acid leads to increased H-bonding of water molecules with the carboxylate moiety. This step involves significant reorientation of water molecules surrounding the carboxylic group. The hydration free energies and the aqueous ionization constant calculated with the set of semi-empirical corrections are in reasonable agreement with available experimental results. (C) 2011 Elsevier B.V. All rights reserved.