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Comparatively Speaking: Solubility vs. Partition Coefficient
By: Anthony J. O'Lenick Jr., Siltech LLC
Posted: April 10, 2012
Solubility is a term used to describe a situation in which one material becomes clear and homogeneous in another. The two materials may be liquids, or they may be one liquid and one solid. Solubility relates to the balance between two opposing forces. The first is the force of attraction between the molecules of the material being dissolved to each other, and the second the force of attraction between the material being absorbed and the solvent. Simply put, if the force of attraction between the molecules of the solute is less than the force that is necessary to dissolve the material, the material dissolves.
To determine solubility, one must consider several factors, including temperature and concentration. Generally, a material is soluble at a concentration of 0.1M, as the material is clear at RT. Conversely, if a material is not clear at a concentration of 0.001M, it is insoluble.
The solubility of a material increases as the temperature increases, but not always, as shown in Figure 1.1 While most materials follow this rule such as sodium hydrogen arsenate, some materials actually decrease in solubility as temperature increases. Gases are more soluble at low temperatures than at high temperatures.
The solubility of materials in different solvents is dependent upon many things and can vary based upon temperature, concentration and formulation additives. This can provide the formulator a way to deliver materials. If a material is delivered in an aqueous solution spread on the skin, water evaporating can change the deposition on the skin. The concept of solubility equilibrium will be discussed in another column, but it relates to a dynamic equilibrium between the concentration of soluble and insoluble compounds. This equilibrium can be altered by temperature or other formulation ingredients.