Dissolution is the process by which a substance forms a solution in a solvent. For the dissolution of solids, the process of dissolution can be explained as the breakdown of the crystal lattice into individual ions, atoms or molecules and their transport into the solvent.
The outcome of the process of dissolution (the amount dissolved at equilibrium, i.e., the solubility) is governed by the thermodynamic energies involved, such as the heat of solution and entropy of solution, but the dissolution itself (a kinetic process) is not. Overall the free energy must be negative for net dissolution to occur. In turn, those energies are controlled by the way in which different chemical bond types interact with those in the solvent.
The graph below shows the dissolution of 5.6 mg of dipyridamole powder in 45 mL of USP SIF buffer solution at pH 6.6. The experiment was done on the SiriusT3 instrument.
Each blue point represents the weight of dipyridamole (pKa 6.25) in solution at a specific time. The black curve fitted to the points represents a first order exponential equation that relates the weight dissolved with the extrapolated solubility and the dissolution rate constant. Very little sample is still dissolving after 30 minutes.
The weight in solution at that time, divided by the volume is equivalent to the extrapolated solubility at that pH. The value determined, 5.3 µg/mL is similar to the value interpolated from a CheqSol measurement of dipyridamole solubility vs. pH.