Co-Solvent – Cosmetic Uses in Solubilisation and Formulation

What Is a Co-Solvent?

A co-solvent is a solvent used together with another solvent to increase the solubility of an ingredient that may otherwise be difficult to incorporate into a formulation.

In cosmetic formulations, co-solvents are particularly useful when an active ingredient, fragrance component or other functional material has limited solubility in water or the main formulation solvent.

Cosmetic Uses of Co-Solvents

Co-solvents can be used in aqueous, hydroalcoholic and emulsion-based formulations to help incorporate ingredients that have limited water solubility.

Common cosmetic co-solvents include:

• Propylene glycol
• Propanediol
• Glycerin
• Ethanol
• Pentylene glycol
• Dipropylene glycol

The choice depends on the ingredient being solubilised, the target concentration, formulation type and desired sensory properties.

Formulation Considerations

Co-solvent selection should consider solubility, polarity, concentration, skin feel, compatibility and regulatory requirements. Increasing the amount of a co-solvent can improve solubility, but may also change viscosity, evaporation, sensory characteristics or skin tolerability.

A co-solvent system should therefore be evaluated in the complete formulation rather than by considering the solubility of the individual ingredients separately.

Co-solvents may also influence preservative performance and the partitioning of ingredients between the aqueous and oil phases. Stability testing should be performed to confirm that the chosen system remains effective throughout the product's intended shelf life.

References

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

European Commission. CosIng – Cosmetic Ingredients Database.