Phase Inversion – Cosmetic Uses in Emulsion Formulation

What Is Phase Inversion?

Phase inversion is a change in the structure of an emulsion in which the original dispersed phase becomes the continuous phase. For example, an oil-in-water (O/W) emulsion can invert to a water-in-oil (W/O) emulsion.

Phase inversion can occur when the composition, temperature, emulsifier system, or processing conditions of an emulsion change sufficiently to alter which phase forms the continuous phase.

Phase Inversion in Cosmetic Formulation

Phase inversion is an important consideration when developing creams, lotions and other emulsified cosmetic products. It can occur intentionally as part of a manufacturing process or unintentionally during formulation or storage.

In some formulation methods, controlled phase inversion is used to produce fine droplets and influence the texture and stability of the final emulsion. In other cases, uncontrolled inversion can result in changes in viscosity, appearance, sensory properties and product stability.

Factors that can contribute to phase inversion include:

• Internal phase concentration
• Emulsifier type and concentration
• Hydrophilic-lipophilic balance (HLB)
• Temperature
• Electrolyte concentration
• Mixing intensity and shear
• Order and rate of phase addition
• Changes in the composition of the formulation

Phase Inversion Temperature

Temperature can significantly affect the behaviour of some non-ionic emulsifiers. In temperature-sensitive systems, a change in temperature can alter the affinity of the emulsifier for the oil and water phases and promote phase inversion.

This principle is used in some emulsion manufacturing processes to control droplet formation and improve the physical characteristics of the finished product.

Formulation Considerations

During formulation development, the risk of unwanted phase inversion should be evaluated together with other emulsion stability factors such as droplet size, viscosity, flocculation, coalescence and creaming.

The formulation should be evaluated under different temperatures and storage conditions because an emulsion that appears stable immediately after production may undergo structural changes during long-term or accelerated stability testing.