250 6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



HYDRATION OF IONS 



Ions exist in aqueous solution surrounded by water molecules in various 

 degrees of orientation and restricted movement. Primary hydratioyi refers 

 to water molecules near an ion which have lost their translational degrees 

 of freedom and move as an entity with the ion during Brownian movement; 

 secondary hydration refers to those water molecules not included in the 

 above which undergo electrostatic interaction with the primarily hydrated 

 ion sufficient to affect certain thermodynamic quantities (Bockris, 1949). 

 Considerations of interactions between ions or ionic groups in solution 

 must take into account not only the ions but the water molecules asso- 

 ciated with them. Let us consider two ions of opposite charge approaching 

 one another. Below a certain separation distance, water molecules inter- 

 acting with the ions must be displaced from the region between the ions 

 and when the ions have reached their equilibrium position an appreciable 

 amount of energy may have been expended on this displacement. If the 

 ions come to rest with their primary water layers intact, then only dis- 

 placement of secondary hydration has occurred; if the ions approach more 

 closely, water molecules in the primary layers must also be displaced. It 

 is difficult to determine if ions in solution lose their primary hydration 

 layer upon interaction and it may well be that the minimum distance of 

 approach is often when the ions are separated by one or two water layers. 

 The water molecules that are displaced associate by hydrogen bonding 

 (or dipole-dipole interaction) with other water molecules and the energy 

 involved in this process also enters into the total interaction. The total 

 potential energy of two interacting ions in solution is thus: 



ft = <Pi-i + <Pi-w + <Pw-w (6-76) 



where 9?,_j is the potential energy resulting from the interaction of the two 

 ions (as calculated previously and including induction and dispersion 

 terms), 9?i_j^ is the energy associated with the displacement of water from 

 the ion and (p^^,_^^, is the energy of the water-water interaction. For ions of 

 opposite charge, (p,_^ and 99j„_^ will be negative and 9?j_j^ will be positive. 

 The crystallization of a salt from solution involves the same terms but 

 in that case much more water is displaced from the ions. 



The molar hydration energy is the change in energy when one mole of 

 ions in the gaseous state passes into an infinitely dilute solution (or iV times 

 the energy involved in a single ion being transferred from the gas phase to 

 an aqueous phase). Attending this process there are large decreases in heat 

 content, free energy, and entropy; these values and the primary hydration 

 numbers are given in Table 6-17. The relative contributions from primary 

 and secondary hydration are unknown but it is likely that well over 95% 

 of the hydration energy results from the primary water layer. The energy 



