Emulsions are thermodynamically unstable systems that require formulation and processing strategies to maintain acceptable physical stability.

Instability can occur through several mechanisms, including creaming, sedimentation, flocculation, coalescence, Ostwald ripening and phase inversion.

These mechanisms should not be treated as identical because each requires a different troubleshooting approach.

Factors Affecting Stability

Important factors include droplet size, droplet-size distribution, emulsifier concentration, interfacial-film strength, phase ratio, continuous-phase viscosity, temperature and electrolyte concentration.

Processing conditions are equally important. Homogenization intensity and mixing conditions can significantly influence droplet size and therefore long-term stability.

Stability Testing

Emulsion stability can be evaluated using real-time storage, accelerated temperature studies, freeze-thaw cycles, centrifugation, microscopy, droplet-size analysis, viscosity measurements and pH monitoring.

A formulation that looks stable immediately after production may still develop instability during storage.

References

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

Lachman, L., Lieberman, H. A. and Kanig, J. L. The Theory and Practice of Industrial Pharmacy.

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