Dissolution is the process by which a solid, liquid or gas 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 rate of dissolution quantifies the speed of the dissolution process.
The rate of dissolution depends on:
nature of the solvent and solute
temperature (and to a small degree pressure)
degree of undersaturation
presence of mixing
interfacial surface area
presence of inhibitors (e.g., a substance adsorbed on the surface).
The rate of dissolution can be often expressed by the Noyes-Whitney Equation or the Nernst and Brunner equation[1] of the form:
where:
m – amount of dissolved material, kg
t – time, seconds
A – surface area of the interface between the dissolving substance and the solvent, m2
D – diffusion coefficient, m2/s
d – thickness of the boundary layer of the solvent at the surface of the dissolving substance, m
Cs – concentration of the substance on the surface, kg/m3
Cb – concentration of the substance in the bulk of the solvent, kg/m3
For dissolution limited by diffusion, Cs is equal to the solubility of the substance.
When the dissolution rate of a pure substance is normalized to the surface area of the solid (which usually changes with time during the dissolution process), then it is expressed in kg/m2s and referred to as “intrinsic dissolution rate”. The intrinsic dissolution rate is defined by the United States Pharmacopeia.
Dissolution rates vary by orders of magnitude between different systems. Typically, very low dissolution rates parallel low solubilities, and substances with high solubilities exhibit high dissolution rates, as suggested by the Noyes-Whitney equation. However, this is not a rule.
