Dissolution is the process by which a solid or liquid forms a homogeneous mixture with a solvent (solution). This can be explained as a breakdown of the crystal lattice into individual ions, atoms or molecules and their transport into the solvent.
Dissolution testing is widely used in the pharmaceutical industry for optimization of formulation and quality control.
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Ionic Compounds
For ionic compounds, dissolution takes place when the component ions dissociate into its component parts. This normally occurs in polar solvents, such as Water or Ammonia.
NaCl(s) → Na+(aq) + Cl-(aq)
In a colloidal dispersed system, ion dissolution arises, where the dispersed particles exist in equilibrium with their saturated counter part, i.e.
NaCl(s) → Na+(aq) + Cl-(aq)
The behavior of this system is characterised by the components activity coefficients and solubility product, i.e.
The ability of an ion to preferentially dissolve (as a result of unequal activities) is classified as the Potential Determining Ion. This in turn results in the remaining particle possessing either a net positive/negative surface charge.
Polar Compounds
Other compounds experience dissolution simply as a breakdown of their crystal lattice, and due to their polarity, or non-polarity, mix perfectly with the solvent.
Rate of dissolution
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 equation 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.
The dissolution rate vary by orders of magnitude between different systems. Usually, substances exhibiting low solubility exhibit also low dissolution rates.
See also
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