Many cosmetic ingredients do not dissolve sufficiently in water or in a single solvent. A co-solvent system can improve solubility by combining solvents with different polarity characteristics.

Examples include combinations of water with ethanol, propylene glycol, propanediol or other compatible solvents.

Co-solvents are frequently used for botanical extracts, fragrances, active ingredients and preservatives.

Formulation Considerations

The selected co-solvent can affect skin feel, evaporation, viscosity, preservation and the stability of the formulation.

Increasing the concentration of a solvent may improve solubility but can also alter the sensory properties of the finished product.

The total solvent concentration should therefore be evaluated rather than considering each solvent independently.

Stability

An ingredient that is initially soluble may precipitate after dilution, cooling or evaporation. Solubility should therefore be checked under realistic processing and storage conditions.

References

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

European Commission. CosIng – Cosmetic Ingredients Database.

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