Figure 4. Calculated pH dependence of the aqueous solubility of benzylamine, generated using the program PhysChem 7.0 (Advanced Chemistry Development, Toronto, CA).
for the example of benzoic acid. Similarly, the solubility of a free base is much less than the solubility of its protonated form, as shown in Figure 4 for benzylamine. Finally, for molecules containing more than one ionizable functional group, the pH dependence of the aqueous solubility can be fairly complicated. Figure 5 gives the example for 4-(aminomethyl)benzoic acid.
The solubility of a solid can be understood using a simple model. For a solid to dissolve, the forces of attraction between solute and solvent molecules must overcome the attractive forces holding the solid intact and the solvent aggregates together. In other words, the solvation free energy released upon dissolution must exceed the lattice free energy of the solid plus the free energy of cavity formation in the solvent for the process to proceed spontaneously. The balance of the attractive and disruptive forces will determine the equilibrium solubility of the solid in question (which is an exponential function of the free energy change of the system). The enthalpy change and the increase in disorder of the system (i.e., the entropy change) determine the Gibbs free energy change. Finally, the act of dissolution may be endothermic or exothermic in nature, so
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