Solubility of Substances in Various Solid State Forms

Many pharmaceutical solids are capable ofexisting in several different solid-state forms, such as polymorphs, solvatomorphs, and amorphous form (Brittain, 1999; Bernstein, 2002). Polymorphism is defined as the ability of a substance to exist in two or more crystalline phases that differ in the arrangement and/or conformation of the molecules in the crystal structure with the empirical formula of a polymorphic pair being identical. Polymorphism can arise from a different packing arrangement of molecules having the same conformation, or from the alternate assembly of different conformational states of the same molecule. Sol-vatomorphism (pseudopolymorphism) is defined as the ability of a substance to exist in two or more crystalline phases that differ in their empirical formulae with solvatomorphs being characterized by the presence of water (i.e., hydrates) or other solvent molecules (i.e., solvates) in the crystal structure. An amorphous solid is characterized by a disordered arrangement of molecules, where intermolecular forces impose short range order and where there is no long range order in the solid.

The different internal energies of these structural types are manifested in different magnitudes of lattice energy, and hence lead to the existence of different solubilities for the various forms. The solubility difference may be understood using the solution models that have been developed in the previous sections. For dissolution to take place, the solute-solvent attractive forces must be stronger than the solute-solute and solvent-solvent attractive interactions so that the latter may be overcome by the former. As always, the free energy change associated with the process determines the ultimate equilibrium solubility of the solute in the solution. Details of the internal structure of the various solid-state forms will determine the respective enthalpies of solution, and the differing enthalpies of solution associated with the various different solid-state forms will lead to the existence of differing solubilities. These phenomena will be considered using the basic thermodynamic theory.

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