FREE-ENERGY OF HYDRATION OF A MOLECULAR IONIC SOLUTE - TETRAMETHYLAMMONIUM ION

We have performed Monte Carlo simulations of the tetramethylammonium i on, hydrated by 256 simple point charge (SPC) water molecules, as a fu nction of total charge on the ion. The total charge was varied between -3e to +3e at intervals of 1e, and was distributed equally on the fou r methyl sites on the ion. Derivatives of the foe energy with respect to charge were related to the fluctuations in the ion-water interactio n energies using a cumulant expansion. This derivative information for the different charge states was found to give an accurate description of the free energy of hydration. The calculated hydration foe energie s were also found to be only weakly dependent on system size and the m ethod used for calculating the electrostatic interactions (Ewald summa tion or generalized reaction field), when finite system size correctio ns are applied. The quadratic charge dependence was obtained for the f ree energy of hydration for both positive and negative ions as expecte d from the Born model. The hydration is, however, asymmetric. Negative ions are more favorably hydrated compared to positive ions. We relate this asymmetry of hydration to water structure; that is, to differenc es in the water oxygen and water hydrogen density profiles surrounding positive and negative ions. Another manifestation of this asymmetry i s seen in the positive electrostatic potential at the center of methyl sites in the uncharged state of the tetramethylammonium solute. (C) 1 998 American Institute of Physics.