Solubility determines how an ingredient can be incorporated into a cosmetic formulation and whether it will remain uniformly distributed during storage.

An ingredient may be soluble in one solvent and insoluble in another. Water solubility, oil solubility and alcohol solubility can therefore be very different.

Factors Affecting Solubility

Temperature, pH, solvent polarity, ionic strength and the chemical structure of the ingredient can all influence solubility.

For ionizable ingredients, pH can be particularly important because the ionized and non-ionized forms may have very different solubilities.

Formulation Problems

Insufficient solubility can lead to precipitation or crystallization. This can cause visible particles, changes in texture, loss of active ingredient from the intended phase or inconsistent dosing.

Solubility should therefore be tested at realistic formulation concentrations and temperatures.

References

Barel, A. O., Paye, M. and Maibach, H. I. Handbook of Cosmetic Science and Technology.

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