Two moisturisers can contain almost identical ingredients yet feel – and sometimes perform – completely differently on the skin. One may absorb quickly and leave a silky finish, while the other feels rich, protective, and stays noticeable for hours.
A major reason is the emulsion system behind the formula.
Most creams and lotions contain ingredients that naturally do not mix well together, especially water and oil. Formulators use emulsifiers, stabilisers, thickeners, and carefully controlled manufacturing processes to keep these phases arranged in a usable system.
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But an emulsion does much more than stop a cream from separating. Its internal structure can affect texture, spreadability, product stability, ingredient solubility, skin penetration, hydration, and even how an active ingredient is released after application.
Research on topical formulations shows that emulsion type, droplet size, emulsifiers, emollients, and internal organisation can all influence dermal delivery.
Understanding these systems makes skincare formulation much easier to appreciate – and explains why the ingredient list never tells the whole story.
What Is an Emulsion in Skincare?
An emulsion is a mixture of two liquids that normally resist mixing, most commonly oil and water.
One phase is broken into small droplets and dispersed throughout the other. Because these droplets naturally want to join together again, formulators use emulsifiers to stabilise the interface between oil and water.
The result can be a lotion, cream, serum-like emulsion, sunscreen, or medicated topical product.
The exact arrangement matters because water-soluble ingredients prefer the aqueous phase while oil-soluble ingredients tend to remain in the lipid phase. That means the formulation acts like a delivery environment rather than simply a container for ingredients.
Reviews of topical emulsions show that interactions between the active ingredient, emulsion components, and skin can significantly influence how much material reaches or passes through the stratum corneum.
Oil-in-Water vs Water-in-Oil Emulsions
1. Oil-in-water systems
In an oil-in-water, or O/W, emulsion, tiny oil droplets are suspended within a continuous water phase.
These formulas commonly feel lighter, spread easily, and wash off relatively easily. Many everyday facial lotions and lightweight moisturisers use this type of architecture.
Because the outer phase is water-based, O/W systems can also provide the fresh or cooling sensory experience consumers often associate with lighter skincare.
2. Water-in-oil systems
A water-in-oil, or W/O, emulsion reverses the arrangement. Water droplets are dispersed inside a continuous oil phase.
These formulations often feel richer and can provide stronger occlusive properties because oil forms the external phase touching the skin. This can make W/O systems useful where greater water resistance or reduced moisture evaporation is desired.
Neither structure is automatically “better.” The appropriate system depends on the intended use, active ingredients, desired skin feel, environment, and target consumer.
Emulsion Structure Can Change Active Ingredient Delivery
Putting an ingredient into a cream does not guarantee that the skin receives all of it.
An active must first leave the formulation, partition into the skin surface, and then move through or into the relevant layers. The emulsion system influences each stage.
For example, an oil-soluble ingredient may strongly prefer the oil phase of a formulation. If it is held too comfortably there, its tendency to leave the vehicle and enter the skin may actually decrease.
On the other hand, carefully selected oils, surfactants, and cosurfactants can sometimes improve solubility or modify interactions with the stratum corneum, increasing delivery.
Research therefore shows that the type of emulsion, emollient choice, surfactant organisation, and physicochemical properties of the active all influence cutaneous absorption.
This is why two products containing the same percentage of an active may not behave identically.
Percentage matters, but vehicle design matters too.
Droplet Size Changes More Than Appearance
Droplet size is another important part of emulsion design.
Traditional emulsions usually contain larger dispersed droplets, while nanoemulsions use much smaller droplets.
Nanoemulsion systems have attracted particular interest because they can improve the solubilisation of poorly water-soluble compounds and offer useful delivery properties.
Recent reviews describe nanoemulsions with droplet sizes commonly in the tens to hundreds of nanometres, although definitions can vary between publications.
These systems are being explored in cosmetics for antioxidants, moisturising ingredients, sunscreens, and other bioactive compounds.
Smaller does not automatically mean better, however.
Droplet size influences interfacial area, physical stablity, manufacturing requirements, ingredient release, and the amount of surfactant that may be needed.
The best droplet size is therefore the one that fits the complete formulation – not simply the smallest number a laboratory can produce.
Emulsifiers Can Help the Formula but Affect the Skin
Emulsifiers are essential because oil and water would otherwise tend to separate over time.
These molecules contain both water-attracting and oil-attracting regions, allowing them to sit around dispersed droplets and reduce interfacial tension.
But emulsifier selection requires balance.
Some surfactants can interact with proteins and lipids in the stratum corneum. Depending on their chemistry, concentration, formulation environment, and exposure, these interactions may contribute to irritation or barrier changes.
That does not mean emulsifiers are inherently harmful.
Modern formulas often use combinations of emulsifiers, fatty alcohols, polymers, lipids, and other stabilisers to produce systems that remain stable while maintaining acceptable skin tolerance.
The complete formula matters more than judging one emulsifier seperately from everything around it.
Rheology Shapes How a Product Feels and Performs
Have you ever used a cream that feels thick inside the jar but suddenly becomes easy to spread?
That behaviour is part of rheology, the study of how materials flow and deform.
Many cosmetic emulsions are shear-thinning. Their viscosity decreases when force is applied during rubbing, allowing them to spread across the skin more easily.
Thickeners and rheology modifiers can influence firmness, pickup from the container, spreadability, rub-in behaviour, and the sensory impression left behind.
A study comparing thickening systems in oil-in-water cosmetic emulsions found significant differences in several sensory characteristics, with measurable relationships between rheological properties and how participants perceived the formulas.
Rheology also contributes to practical stablity by helping keep dispersed droplets from moving, colliding, and eventually separating.
So texture is not merely decorative. It can be part of functional product engineering.
The Formula Changes After You Apply It
One easily overlooked fact about skincare is that the formula inside the bottle is not necessarily the same formula that remains on your skin five minutes later.
After application, water and other volatile ingredients can evaporate. The concentration of oils, polymers, surfactants, and active ingredients increases.
Scientists call this transformation formulation metamorphosis.
Research on topical semisolid products shows that evaporation after application can change viscosity, particle or globule characteristics, ingredient concentration, thermodynamic activity, and ultimately drug release or skin penetration.
Imagine a lotion containing an active at a relatively comfortable concentration while sitting inside the bottle. Once much of its water evaporates, the remaining film becomes more concentrated.
That evolving structure can influence what the skin actually experiences.
Advanced formulation therefore considers not only shelf behaviour but also what happens after the product meets warm, living skin.
Nanoemulsions and Microemulsions Offer Advanced Delivery Options
Conventional emulsions are only part of the picture.
Microemulsions and nanoemulsions use much smaller dispersed structures and have become widely studied for topical delivery.
Microemulsions are generally thermodynamically stable systems formed from suitable combinations of water, oil, surfactants, and often cosurfactants. They can solubilise substantial amounts of both hydrophilic and lipophilic compounds.
Nanoemulsions are different. They are generally kinetically stable rather than truly thermodynamically stable and often require specific high-energy or low-energy manufacturing methods.
Both can be engineered to influence ingredient release and skin delivery. However, formulation components matter enormously, and increasing penetration is not automatically desirable in every skincare product.
A moisturiser designed to support the surface barrier has different goals from a pharmaceutical system designed to deliver an active deeper into the skin.
More penetration does not always equal better skincare.
Stability Determines Whether the Product Still Works Months Later
A skincare formula must perform not only on production day but also after months of storage, shipping, temperature changes, and repeated consumer use.
Poorly designed emulsions can experience creaming, droplet aggregation, coalescence, phase separation, changes in viscosity, or ingredient crystallisation.
Physical instability can change appearance and texture, but it may also affect how ingredients are distributed and delivered.
This is why formulators test properties such as droplet size, viscosity, pH, temperature stability, centrifugation behaviour, and storage performance.
Newer approaches such as Pickering emulsions use solid particles rather than conventional molecular surfactants to stabilise interfaces. Research is exploring these systems for their distinctive physical stability and potential topical applications.
Innovation continues, but the fundamental objective remains the same: keep the disperison controlled enough that the product behaves consistently throughout its intended shelf life.
Why the Ingredient List Cannot Tell You Everything
Consumers often compare skincare products ingredient by ingredient.
That can be useful, but it has limitations.
An ingredient list does not reveal droplet size, manufacturing temperature, homogenisation pressure, internal phase structure, rheological behaviour, ingredient partitioning, or how the formula changes after application.
Even two formulas containing water, glycerin, ceramides, oils, and the same active could perform differently if their emulsion architectures are different.
This is one reason skincare performance should be evaluated as a complete system.
Ingredient quality matters. Concentration matters. But formulation science determines whether those ingredients remain stable, reach the right location, feel pleasant enough for consistant use, and continue performing throughout the product’s lifespan.
Emulsion systems influence skincare product performance far beyond simply keeping oil and water mixed. They shape texture, stability, ingredient solubility, hydration, sensory feel, active release, and interactions with the skin.
Oil-in-water and water-in-oil systems offer different advantages, while nanoemulsions, microemulsions, and emerging technologies provide even more ways to control delivery.
Droplet size, emulsifiers, rheology, and post-application evaporation can all change how the final product behaves.
So when comparing skincare, do not judge performance from the ingredient list alone. Pay attention to how the complete formula spreads, feels, remains stable, and performs with regular use.
Good skincare is not just about choosing good ingredients. It is also about engineering the right system to deliver them.
