How to Stop Emulsions Separating: The Science Behind Creating Stable Emulsions

Introduction

Most cosmetic products are emulsions, from light lotions, roll on deodorants, serums to rich creams. Despite careful formulation, emulsions can still separate over time if factors such as emulsifier choice, droplet size, temperature, or pH are not properly managed. Understanding why emulsions separate and how to prevent it, is essential for creating products that remain smooth, stable, and effective. This blog will explore the science behind emulsion stability and provide practical tips for formulators.

What is an Emulsion?

An emulsion is a mixture of two immiscible substances - oil and water, stabilised by emulsifiers. In cosmetic formulation there are two types of emulsions:

• Oil in water (O/W) emulsions, where oil droplets are dispersed in water (common in lotions and creams).

• Water in oil (W/O) emulsions, where water droplets are dispersed in oil (used for richer, occlusive products).

Without proper stabilisation, gravity and molecular forces cause these two phases to separate over time.

What is an emulsifier?

An emulsifier, chemically a surfactant, contains a water loving (hydrophilic) head and a oil loving (hydrophobic) tail. This chemical makeup allows it to bind to both water and oil and create an emulsion. It is important to understand that although emulsifiers are chemically surfactants in cosmetic formulation they come under their own category - separate to surfactants. The key distinction is that emulsifiers are used to make creamy emulsions such as creams and lotions. Whilst surfactants are what we use in cleansing products such as shower gels and shampoos; they provide foam, bubbles and cleansing action.

Understanding Why Emulsions Separate

The separation of emulsions especially those made of oil and water is a complex process affected by many factors. These can be grouped into physical, chemical, and mechanical influences.

1. Physical Factors

Gravity and Droplet Size: Gravity pulls heavier ingredients down and lighter ones up, this is why oil often rises to the top of an unstable emulsion. The bigger the droplets are, the faster they move. By creating smaller droplets through proper mixing or homogenisation, you slow down this movement and help keep the emulsion stable.

Temperature: Heat changes how an emulsion behaves. Warmer temperatures make the liquid thinner (less viscous), so droplets can move around and bump into each other more easily. If it gets too hot, droplets may merge and cause separation. Temperature also affects how well the emulsifier works, so stability tests under different conditions are essential.

2. Chemical Factors

Emulsifiers and Protective Layers: Emulsifiers are the ingredients that help oil and water mix and stay mixed. They sit at the boundary between oil and water, lowering surface tension and forming a thin layer around each droplet. This coating stops the droplets from sticking together. The type and amount of emulsifier you choose have a big impact on how long the emulsion stays stable.

pH and Other Ingredients: The pH (how acidic or alkaline the product is) can change how well an emulsifier works. If the pH moves too far from the emulsifier’s ideal range, droplets can start to stick together. Other ingredients like salts, acids, or thickeners also influence stability. For example, too much salt can cause droplets to clump, while a thickener can help keep them suspended.

3. Mechanical Factors

Mixing and Shear: The way you mix an emulsion affects droplet size and stability. Gentle stirring might not break the oil into small enough droplets, while very high shear can create too much heat or introduce air. Finding the right balance gives smooth, uniform emulsions that last longer.

4. Other Things to Consider

Tiny Forces Between Droplets: Even at a microscopic level, droplets attract or repel each other due to invisible forces, such as electrical charges and molecular interactions. Emulsifiers and stabilisers help control these forces so droplets stay evenly spaced instead of clumping together.

Testing Stability: To check how stable an emulsion is, formulators test it under different conditions, like centrifuge testing or freeze and thaw testing. Measuring how much the product changes over time helps identify weaknesses and improve the formulation.

Putting It All Together

Every factor, temperature, pH, emulsifier choice, and mixing works together to determine whether an emulsion stays stable or separates. Small changes in one area can make a big difference in the end result. By understanding these principles, you can create products that not only look and feel great but also stay beautifully blended for their entire shelf life.

The Chemistry Behind Stability

To keep your emulsion stable, you need to manage interfacial tension - the energy at the boundary between oil and water.

Emulsifiers and the HLB System

Emulsifiers reduce interfacial tension and form a protective film around droplets. Their balance is often guided by the Hydrophilic–Lipophilic Balance (HLB) system. HLB (Hydrophilic–Lipophilic Balance) is a numerical system ranging from 0 to 20, used in cosmetic formulation to describe how soluble an emulsifier is in water or oil. Low HLB values (around 3–6) mean the emulsifier is more oil loving and suitable for water in oil (W/O) emulsions, while high HLB values (around 8–18) are more water loving and ideal for oil in water (O/W) emulsions.

• Low HLB (3–6): favours W/O emulsions

• High HLB (8–18): favours O/W emulsions

  
  

Blending emulsifiers with complementary HLB values often yields better results and texture.

Co-Emulsifiers and Stabilisers

Use waxy emulsifiers and fatty alcohols, such as cetearyl alcohol, to give your emulsion a rich, creamy texture. These ingredients help build viscosity, which not only improves the sensory feel of the product but also contributes to better emulsion stability by strengthening the internal structure.

• Incorporate co-emulsifiers or a blended emulsifiers containing two or more materials like Olivem 1000 (Cetearyl Olivate and Sorbitan Olivate). Using a blend of emulsifiers in emulsions is crucial for long term stability.

• Add polymers and gums, for example xanthan gum or carbomer for oil in water emulsions, or trihydroxystearin or oil compatible grades of silica for water in oil emulsions. These materials increase the viscosity of the continuous phase, which helps to suspend droplets evenly and prevent phase separation over time, ensuring the emulsion remains smooth and uniform throughout its shelf life.

Electrolytes and pH

pH shifts can destabilise emulsifiers, especially ionic ones. Always verify the pH range of your emulsifier and maintain it throughout formulation. Electrolytes such as salts can also screen the charge of ionic emulsifiers, leading to droplet merging.

How to Prevent Emulsion Separation

Here’s a formulation checklist to help you achieve long lasting stability:

1. Select the correct emulsifier system: Ensure the charge and HLB of the emulsifier is compatible with your emulsion type eg O/W or W/O.

2. Verify your phase ratios: very high oil content may require a W/O system or additional stabilisers. Do not exceed 15% oils / butters in W/O emulsions.

3. Add a co-emulsifier: such as cetearyl alcohol or glyceryl stearate for structural support.

4. Control process temperature: heat both phases to similar temperatures (usually 70–75 °C) before combining.

5. Use appropriate shear: insufficient mixing creates large droplets; excessive shear can break the emulsion. Usually high shear is used when creating emulsions - if you are a home formulator make use of pulse blenders.

6. Check and adjust pH: at the end of formulation and ensure the pH is compatible with the emulsifiers used.

7. Increase viscosity: with polymers or thickeners to limit droplet movement.

8. Conduct stability testing: perform accelerated storage and freeze–thaw cycles to observe any separation.

Conclusion

Preventing emulsion separation is about understanding and controlling a combination of physical, chemical, and mechanical factors. Varying factors from the type and amount of emulsifier to droplet size, pH, viscosity, and processing method can influence stability. By carefully managing these variables, conducting stability testing, and following best practices in emulsifier selection and formulation, cosmetic formulators can create products that maintain their texture, appearance, and performance throughout their shelf life. Mastering emulsion science ensures your creams and lotions stay beautifully blended and effective.

Additional / Further Reading

Mohamed Reda Akdim & Thomas Krebs. (2022). Emulsion Separation. Accessed: [10/11/25]. Available at: https://www.sciencedirect.com/science/chapter/edited-volume/abs/pii/B978012823891200003X?via%3Dihub

Frising, T, et al. (2008). Contribution of the Sedimentation and Coalescence Mechanisms to the Separation of Concentrated Water-in-Oil Emulsions. Accessed: [10/11/25]. Available at: https://www.tandfonline.com/doi/abs/10.1080/01932690701781501

Amir, M. Mahamad, et al. (2024). Development of a Novel Modelling Tool for Condensate Emulsions: Key Factors Influencing Oil-In-Water Emulsions. Accessed: [10/11/25]. Available at: https://onepetro.org/SPEAPOG/proceedings-abstract/24APOG/24APOG/D011S004R006/570550

Angardi, V, et al. (2021). Critical Review of Emulsion Stability and Characterization Techniques in Oil Processing. Accessed: [10/11/25]. Available at: https://asmedigitalcollection.asme.org/energyresources/article-abstract/144/4/040801/1112321/Critical-Review-of-Emulsion-Stability-and?redirectedFrom=fulltext

Tian, Y, et al. (2022). The Formation, Stabilization and Separation of Oil–Water Emulsions: A Review. Accessed: [10/11/25]. Available at: https://www.mdpi.com/2227-9717/10/4/738

Martin Wingrove & Sunil Kokal. (2000). Emulsion Separation Index: From Laboratory to Field Case Studies. Accessed: [10/11/25]. Available at: https://onepetro.org/SPEATCE/proceedings-abstract/00ATCE/00ATCE/SPE-63165-MS/132163

Raya, et al. (2022). Investigation of the synergistic effect of nonionic surfactants on emulsion resolution using response surface methodology. Accessed: [10/11/25]. Available at: https://pubs.rsc.org/en/content/articlelanding/2022/ra/d2ra04816g

Baron & Manor. (2024). Contributions of Colloidal Forces to the Heterogeneous Separation of Stable Oil-In-Water Emulsions. Accessed: [10/11/25]. Available at: https://pubs.acs.org/doi/10.1021/acs.langmuir.4c03056