Getting back to the basics, Tony O'Lenick asks: What is the difference between solubility and partition coefficient?
Solubility is the amount of a given substance or solute that can be dissolved into a solvent.1 It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution. The solubility of a solute is the maximum quantity of solute that can dissolve in a certain quantity of solvent, or quantity of solution, at a specified temperature.
Partition or distribution coefficient (KD) is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.2 Normally one of the solvents chosen is water, while the other solvent chosen is hydrophobic such as octanol,3 since cosmetic products are used on substrates like hair and skin, where oil and water are present.
The partition and distribution coefficient are measures of the hydrophilic or hydrophobic properties of a chemical substance. Hence, these coefficients are a measure of differential solubility of the compound between these two solvents. The difference between the two is that solubility measures the amount of solute that can be dissolved in one solvent, while partition coefficient measures the ability of the solute to go into two immiscible phases.
Partition coefficient determines how much of a given solute ends up in which phase and is of major importance to the cosmetic chemist.
1. J. De Paula and P.W. Atkins, Atkins' Physical Chemistry, 7th ed.
2. A Leo, C Hansch C and D Elkins D, Prtition coefficients and their uses, Chem Rev 71(6) 525-616 (1971)
3. J Sangster, Octanol-water partition coefficients: Fundamentals and physical chemistry, Vol. 2, Wiley Series in Solution Chemistry, Chichester: John Wiley & Sons Ltd (1997)