Emulsions as Engineering Problems
Surfactants and Interfacial Chemistry
Rheology Modifiers and Structural Scaffolding
Pigment Stability and Dispersion Science
Protecting Against Oxidation
Environmental Resilience Through Formulation Design
Conclusion
References and Further Reading
A foundation that separates in its bottle, a lipstick that bleeds at the edges, or a cream that turns watery after two weeks. These are all indications of material engineering failures. Cosmetics are surprisingly complex multiphase systems, and the ingredients that keep them stable rely on the same scientific principles found in industrial colloids, pharmaceutical dispersions, and food emulsions. Understanding these principles is key to creating effective and long-lasting cosmetic products.
Image Credit: Panint Jhonlerkieat/Shutterstock.com
Saving this article for later? Grab a PDF here.
Emulsions as Engineering Problems
Most makeup and skincare products are either oil-in-water (O/W) or water-in-oil (W/O) emulsions. The mixtures are thermodynamically unstable and require deliberate structural support to remain homogeneous. Without intervention, the immiscible oil and water phases naturally seek to minimize interfacial energy by coalescing and separating.
To maintain a stable blend, formulation scientists design barriers that prevent droplet fusion (coalescence) and density-driven separation, known as creaming or sedimentation. The difficulty is sustaining these barriers across varying temperatures, pH levels, and mechanical stress throughout a product's shelf life.1,2,3
The stability of an emulsion begins at the nanoscale. Emulsifier molecules, which carry both hydrophilic and hydrophobic segments, adsorb at the oil-water interface and form a thin, mechanically resistant film around each droplet.
This film reduces attraction between droplets, preventing their fusion. Polysaccharides such as gum Arabic and pectin enhance this barrier by creating a hydrated steric layer that keeps droplets apart. The effectiveness of this protective film is important in determining a product's shelf life, as it determines whether it stays smooth for up to 18 months - or begins to weep within weeks.3,4
Surfactants and Interfacial Chemistry
Surfactants play a key role in managing interfacial chemistry.
Their molecular geometry, described by the hydrophilic-lipophilic balance (HLB) value, determines which type of emulsion they favor and how effectively they reduce interfacial tension. Non-ionic surfactants are widely used in cosmetics for their mildness and stability across pH levels, as they mainly stabilize emulsions through steric repulsion.
In contrast, anionic surfactants create negative charges, leading to electrostatic repulsion. This repulsive force, measured as zeta potential, serves as a quantifiable indicator of colloidal stability. Emulsions with zeta potential magnitudes exceeding ± 30 mV demonstrate significantly greater resistance to flocculation and coalescence.2,5
Zeta potential acts as a diagnostic tool during formulation development. When zeta potential values drift toward neutral, it signals weakening electrostatic repulsion and potential instability.
This is highly important in cationic emulsions, where a negative zeta potential at higher pH predicts failure. Moreover, measuring zeta potential with particle size distribution allows formulators to detect instability immediately after production, rather than waiting for visible changes to appear over weeks of accelerated stability testing.2
Rheology Modifiers and Structural Scaffolding
Rheology modifiers govern how a product flows, spreads, and resists deformation. In cosmetics, controlling viscosity can improve sensory experience while maintaining physical stability.
By increasing the viscosity of the continuous phase, these modifiers effectively slow the movement of dispersed droplets and suspended particles. This reduction in movement decreases creaming and sedimentation. Ingredients like carbomers, acrylate crosspolymers, and cellulose derivatives create a structured, viscoelastic matrix that holds droplets in place within a gel-like network.6,7
Advanced rheological scaffolds extend this principle further. Instead of using conventional surfactants, Pickering emulsions use solid particles, such as silica, to create strong barriers at the oil-water interface. These particles form strong barriers at the oil-water interface, which are much less likely to break apart than molecular films.
Additionally, lamellar gel networks made from polyglyceryl esters and fatty alcohols form organized liquid-crystalline structures around oil droplets that withstand shear and temperature changes. This approach blends materials science with formulation chemistry, where careful design at the mesoscale directly translates into macroscopic product stability.6,7
Pigment Stability and Dispersion Science
Color cosmetics introduce an additional challenge of maintaining uniform pigment distribution throughout the product's usable life. Pigment particles are denser than their carrier liquids and can settle if not properly suspended. There are two main strategies to solve this problem.
One is electrostatic stabilization, which uses the same surface charges on pigment particles to repel them from each other. The other is steric stabilization, which involves using polymer chains or surfactants to create physical barriers that prevent particles from clumping together.8
For stability, dispersants are essential as they maintain separation through both electrostatic and steric effects, ensuring pigments stay separated in both water-based and solvent-based products. Particle size also becomes critical as smaller particles settle more slowly, while larger ones can create noticeable color variations.
Techniques like high-shear mixing and bead milling break down clumps of pigment into smaller sizes, providing consistent color from production to application.8
Protecting Against Oxidation
Image Credit: Nomad_Soul/Shutterstock.com
Physical stability alone does not guarantee a cosmetic product's longevity. Many formulas contain unsaturated fats and natural oils that can degrade when exposed to oxygen and light. This leads to bad odors, color changes, and reduced effectiveness.
Antioxidants, especially tocopherols from vegetable oils, help stop this degradation by reducing free radicals. Chelating agents like EDTA and citric acid can bind to trace metals like iron and copper that speed up oxidation.9,10
Formulating for oxidative stability requires a systems-level perspective. Blending antioxidants like tocopherols and ascorbic acid works better than single antioxidants, disrupting oxidation at various stages.
In cosmetics like foundations and lipsticks, encapsulated antioxidants provide targeted protection, preventing harmful interactions with pigments. The combination of chelation, radical scavenging, and encapsulation further improves oxidative stability throughout the product’s lifespan.9,10
Cosmetic products need to remain stable despite changes in storage and usage conditions such as temperature, UV light, and humidity. To achieve this, special polymeric rheology modifiers, like acrylate crosspolymers, are used. These modifiers resist salt and maintain consistent viscosity, even in the presence of active ingredients and preservatives.
Another promising solution comes from biopolymers like cellulose nanocrystals. These materials offer significant stability. They have a high level of crystallinity and small particle size, resulting in stable emulsions even under pH changes and temperature fluctuations.
Such advances in formulation science aim to create more reliable cosmetic systems that perform well under various conditions.11
Conclusion
Cosmetic stability results from deliberate decisions made at the molecular and colloidal level. Emulsifiers, surfactants, rheology modifiers, antioxidants, and dispersants each address a specific failure mode, and their combined actions sustain product performance across various real-world conditions.
These materials are functional engineering components rather than passive ingredients. This perspective positions formulation design as a precise, science-based discipline in which consistency, safety, and shelf life are integrated into the product from the outset.
Why do nanomaterials matter in makeup? Click here to read more.
References and Further Reading
- Badruddoza, A. Z. M. et al. (2023). Assessing and Predicting Physical Stability of Emulsion-Based Topical Semisolid Products: A Review. Journal of Pharmaceutical Sciences. DOI:10.1016/j.xphs.2023.03.014. https://jpharmsci.org/article/S0022-3549(23)00114-4/abstract
- Gasparelo, A. P. et al. (2014). Zeta Potential and Particle Size to Predict Emulsion Stability. Cosmetics & Toiletries. https://www.cosmeticsandtoiletries.com/testing/method-process/article/21835272/zeta-potential-and-particle-size-to-predict-emulsion-stability
- Thy, L. T. M. et al. (2025). Applications of lecithin in emulsion stabilization and advanced delivery systems in cosmetics: A mini-review. Results in Surfaces and Interfaces, 19, 100543. DOI:10.1016/j.rsurfi.2025.100543. https://www.sciencedirect.com/science/article/pii/S2666845925001308
- Xiao, T. et al. (2025). Advances in emulsion stability: A review on mechanisms, role of emulsifiers, and applications in food. Food Chemistry: X, 29, 102792. DOI:10.1016/j.fochx.2025.102792. https://www.sciencedirect.com/science/article/pii/S259015752500639X
- Sarella, P. N. K. et al. (2023). The Expanding Scope of Emulgels: Formulation, Evaluation and Medical Uses. International Journal of Current Science Research and Review. Volume 06 Issue 05. DOI: 10.47191/ijcsrr/V6-i5-42. https://ijcsrr.org/wp-content/uploads/2023/05/42-29-2023.pdf
- Guzmán, E. et al. (2022). Pickering Emulsions: A Novel Tool for Cosmetic Formulators. Cosmetics, 9(4). DOI:10.3390/cosmetics9040068. https://www.mdpi.com/2079-9284/9/4/68
- Rheology Modifiers, Thixotropy, and Advanced Emulsion Control in Cosmetic Formulation. (2025). SKD Pharmaceuticals. https://skdpharmaceuticals.com/rheology-modifiers-thixotropy-and-advanced-emulsion-control-in-cosmetic-formulation/
- Key Factors for Stable Effect Pigment Formulations. (2025). Allan Chem. https://allanchem.com/stable-effect-pigment-formulations-factors/
- Stockstrom, M. et al. (2022). Oxidation Stability – a New Holistic Approach for Cosmetic Product Protection. SOFW Journal. https://sofw.com/en/sofw-journal/articles-en/48-personal-care/3516-oxidation-stability-a-new-holistic-approach-for-cosmetic-product-protection
- How to avoid oxidation in cosmetics and improve color stability with natural antioxidants. Btsa. https://www.btsa.com/en/oxidation-cosmetics-natural-antioxidants/
- Franceschini, M. et al. (2025). On the Key Role of Polymeric Rheology Modifiers in Emulsion-Based Cosmetics. Cosmetics, 12(2). DOI:10.3390/cosmetics12020076. https://www.mdpi.com/2079-9284/12/2/76
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.