Emulsions are one of the most widely used cosmetic forms in the industry, found in facial creams, body lotions, sunscreens and hair care products. From a physicochemical perspective, an emulsion is a dispersed system composed of two immiscible phases (typically water and oil), where one phase is distributed as small droplets within the other.
The stability of this system depends on the presence of agents capable of reducing interfacial tension and preventing phenomena such as coalescence or phase separation. In this context, cosmetic emulsifiers play a key role: they are responsible for binding and stabilizing both phases, while also determining critical formulation properties such as texture, viscosity and sensory profile during application.
An inappropriate selection of the emulsifier can compromise not only the stability of the product, but also its acceptance by the consumer.

For this reason, the selection of the appropriate emulsifier should be approached by considering parameters such as the desired type of emulsion (O/W or W/O), the hydrophilic-lipophilic balance (HLB), compatibility with other ingredients, and the manufacturing process.

1. Definition and Mechanism of Action

When incorporated into a system, emulsifiers spontaneously migrate to the oil–water interface, where they orient themselves so that each end interacts with its compatible phase. This phenomenon reduces the free energy associated with the interface and stabilizes the dispersion.

Furthermore, above a critical concentration, many emulsifier molecules tend to self-organize into micelles: structures in which the hydrophobic groups are protected inside, surrounded by a hydrophilic “shield” in contact with water.

This dynamic equilibrium between free molecules and micelles contributes to maintaining system stability and controlling droplet size. The smaller the droplet diameter and the larger the interfacial area, the greater the amount of emulsifier required to cover it. Otherwise, the system tends to minimize the exposed surface, promoting coalescence phenomena and eventual phase separation.

2. Types of Emulsions in Cosmetics

Depending on which phase acts as the continuous phase and which as the dispersed phase, different types of emulsions can be distinguished:

• Oil-in-water emulsions (O/W): the oil phase is dispersed as droplets within a continuous aqueous phase. They are lightweight, fast-absorbing and commonly used in facial creams, body lotions and hair care products.
• Water-in-oil emulsions (W/O): water is dispersed as droplets within a continuous oil phase. They offer greater resistance to water and help prevent skin dehydration, making them suitable for sunscreens, hand creams and barrier formulations.
• Multiple emulsions (W/O/W or O/W/O): more complex systems where droplets of one emulsion are encapsulated within another. Unlike simple emulsions, they present greater stability challenges due to the presence of multiple interfaces and, therefore, more potential destabilization processes. In cosmetics, they are used for their versatility: enabling controlled release of fragrances, prolonged moisturization, protection of sensitive actives (such as vitamins, lipids or natural extracts) or the development of sustained-action formulations. They also provide novel textures that enhance sensory experience and product differentiation in the market.

The selection of the emulsion type directly determines the required emulsifier. Formulating a lightweight O/W system is not the same as developing a water-resistant W/O emulsion.

Classification and Selection of Emulsifiers

According to ionic charge

The chemistry of an emulsifier defines its solubility, compatibility and function within cosmetic formulations.

Based on their charge, emulsifiers can be classified as follows:

• Anionic emulsifiers: negatively charged, commonly used and particularly suitable for cleansing formulations (soaps, foams). However, they are sensitive to salts and pH changes, which requires specific stability testing. A widely used example is Potassium Cetyl Phosphate, an anionic O/W emulsifier that provides good stability in creams and lotions.
• Cationic emulsifiers: positively charged, they strongly adhere to the stratum corneum or hair, making them essential in sunscreens, long-lasting makeup and hair care products. They can also function as conditioning agents.

• Non-ionic emulsifiers: carry no electrical charge and are the most versatile and widely used. They are tolerant to pH changes and electrolytes and generally present a lower risk of irritation. They are recommended when formulations involve high ionic content in water or include sensitive actives (such as salicylic acid or vitamin C). Among non-ionic emulsifiers, widely used for their versatility and tolerance to pH and electrolyte variations, examples include Polyglyceryl-10 Laurate, PEG-120 Methyl Glucose Dioleate and PEG-7 Hydrogenated Castor Oil, which are suitable for formulating O/W emulsions.
• Amphoteric emulsifiers: their charge varies depending on pH, offering broad compatibility with different formulations, mildness and low irritation potential. They are ideal for delicate and versatile products such as shampoos, facial cleansers and baby care formulations.

According to Hydrophilic-Lipophilic Balance (HLB)

The HLB (Hydrophilic-Lipophilic Balance) concept remains a classical and widely used tool to guide emulsifier selection.

• A low HLB (<8) indicates higher lipophilicity, favoring water-in-oil (W/O) emulsions.
• A high HLB (>12) reflects a more hydrophilic character, suitable for oil-in-water (O/W) emulsions.

In practice, formulators do not rely on a single emulsifier but instead combine surfactants with different HLB values. For example, it is common to combine a highly hydrophilic surfactant (HLB 16–20) with one of intermediate HLB (9–15) and a more lipophilic one (<8), adjusting the proportions to obtain an emulsifying system capable of stabilizing oils with different polarities. This approach allows modulation of viscosity, improved tolerance to temperature changes, and the achievement of more robust emulsions.

The combination of fatty alcohols with non-ionic emulsifiers is also a common strategy. For instance, blends such as Cetearyl Alcohol and Cetearyl Glucoside act as self-emulsifying O/W systems, providing high stability and a pleasant creamy texture in facial and body creams.

According to Origin

The origin of the emulsifier defines both its technical profile and its market perception:

• Natural emulsifiers: derived from plant or animal sources (lecithins, lanolin, waxes). They appeal to eco-conscious consumers, although they may require more robust preservation systems and can be less effective at low dosages.
• Synthetic emulsifiers: chemically engineered with precise structural control (polymers, PEGs, silicone-based). They offer consistency, industrial availability and predictable performance.
• Biotechnological / bio-based emulsifiers: represent a modern bridge between efficacy and sustainability, derived from fermentation or modified processes to improve their sensory and environmental profile.

3. Required HLB and Stability

What is HLB?

The Hydrophilic–Lipophilic Balance (HLB) system was developed by Griffin in the mid-20th century to facilitate emulsifier selection. It is a numerical index (ranging from 0 to 20 in the original method) that reflects the relative affinity of a surfactant for water (hydrophilic) or oils (lipophilic).

• Low HLB (<9): more lipophilic molecules, with a tendency to form W/O emulsions.
• Intermediate HLB (9–11): balanced behavior, capable of stabilizing both systems depending on the formulation composition.
• High HLB (>11): more hydrophilic molecules, suitable for O/W emulsions.

Subsequently, Davies proposed an extension of the system by considering the contribution of different functional groups within the molecule, allowing the scale to be extended up to values of 40 in some cases (especially for ionic surfactants).

Real HLB vs Required HLB

In cosmetics, it is important to distinguish between:

• Real HLB: the value assigned to the emulsifier based on its chemical structure.
• Required HLB: the value corresponding to the oil phase (oils, esters, triglycerides) that is intended to be emulsified.

The basic principle is straightforward: oils with a low required HLB need emulsifiers with low HLB, and vice versa. If there is a mismatch, the system loses stability, leading to phase separation or undesirable textures.

Emulsifier Blending Strategy

Although it is possible to use a single emulsifier that matches the required HLB, in practice it is often more effective to combine a high-HLB emulsifier with a low-HLB one.

• This allows for a more complete coverage of the oil/water interface.
• “Gaps” in the interfacial film are reduced.
• The overall stability of the emulsion increases against changes in temperature, pH or composition.

4. Formulation with Emulsifiers

The formulation of emulsions in cosmetics requires consideration of both process parameters and ingredient compatibility. Emulsifiers do not act in isolation: their effectiveness depends on the balance between phases, the order of addition and the emulsification conditions.

Oil/Water Phase Ratio

The first step in designing an emulsion is to define the balance between the aqueous phase and the oil phase.

• O/W emulsions are typically formulated with an oil phase around 10–30%, resulting in lightweight, fresh textures with fast absorption. This profile is highly valued in daily-use facial creams.
• W/O emulsions, on the other hand, contain a higher proportion of the oil phase. This gives them a denser character, with greater occlusive properties and water resistance, making them particularly useful in sunscreens, long-lasting makeup and formulations for very dry skin.

In both cases, the ratio between water and oil not only determines the sensory profile but also the final viscosity and overall robustness of the emulsion. Therefore, this balance must always be considered together with emulsifier selection and the manufacturing process.

Emulsification Temperature

Stability depends on working at appropriate temperatures

• In O/W emulsions, the oil and aqueous phases are heated separately to a similar temperature (typically 60–75 °C) before mixing.
• Temperature-sensitive emulsifiers (e.g. polymeric or naturally derived) may require cold processing or lower temperatures to avoid degradation.
• Adequate agitation or high-shear homogenization at this stage ensures smaller droplet size and improved stability.

Primary and Secondary Emulsifiers

In practice, it is common to combine:

• Primary emulsifiers → responsible for forming the emulsion (e.g. non-ionic emulsifiers such as polysorbates, glyceryl esters).
• Secondary emulsifiers or co-emulsifiers → enhance viscosity, improve stability against temperature changes and optimize sensory properties (e.g. fatty alcohols, thickening polymers).
  

The synergy between both defines the emulsion’s resistance to variations in pH, temperature and storage conditions.

In W/O emulsions, in addition to primary emulsifiers, co-emulsifiers such as Glyceryl Caprylate can play a dual role: increasing system viscosity and improving resistance to coalescence, while also providing an emollient touch that enhances the final sensory profile of the product.

Compatibility with Actives and Desired Texture

The selection of the emulsifying system must also be adapted to:

• Compatibility with actives: certain actives (e.g. acids, electrolytes) can destabilize emulsions, requiring the selection of salt-resistant emulsifiers or specific combinations.
• Final product pH: not all surfactants remain stable across a wide pH range; cationic emulsifiers, for example, perform better under slightly acidic conditions, which are common in hair care formulations.
• Final texture: fluid lotions require low-viscosity systems, whereas richer creams or body butters need co-emulsifiers that increase consistency.

6. Emulsion Destabilization Factors

It is important to understand that an emulsion is not a naturally stable system, but a transient state that can be maintained for months or years thanks to emulsifiers. These ingredients act as a barrier that delays phase separation, although they cannot completely prevent it.

Figure 3 helps to visualize this concept. In the initial state, with the phases separated, the system is at a low free energy level (G₀). Upon applying energy during emulsification (stirring, homogenization), the system transitions to a metastable state (G₁), where water and oil are dispersed as droplets.
Between these two states, there is an energy barrier (ΔG*): the higher this barrier, the longer the emulsion will resist breakdown. If this barrier is sufficiently large compared to the available thermal energy, the emulsion can remain stable over extended periods.

Meanwhile, dispersed droplets are in constant motion due to thermal agitation or gravity, colliding with one another. Over time, these interactions promote destabilization processes, which manifest through different mechanisms:

• Creaming and sedimentation
  Density differences between phases cause oil droplets to rise (creaming) or settle (sedimentation). Although this is not an irreversible breakdown, it affects appearance and may promote further destabilization processes.
• Flocculation
  Droplets aggregate into clusters without merging. This phenomenon increases the likelihood of coalescence and reduces product homogeneity.
• Coalescence
  Droplets of the dispersed phase merge to form larger droplets. As this process progresses, the emulsion loses viscosity and moves closer to phase separation.
• Ostwald ripening
  Smaller droplets dissolve and their material redistributes toward larger droplets due to differences in osmotic pressure. This leads to uneven growth and progressive loss of stability.
• Phase separation (breaking)
  This is the final stage of instability: the system breaks down and the aqueous and oil phases become clearly separated.

Factors that accelerate these processes

• Temperature: high temperatures fluidize the oil phase, accelerating coalescence; at low temperatures, fatty alcohols may crystallize, altering viscosity.
• Salts and pH: affect surfactant solubility and viscosity, promoting instability.
• Oil phase composition: solid or linear oils tend to crystallize, expelling emulsifier and weakening the interfacial film.
• Incompatible interactions: poorly combined ionic surfactants with polymers or other surfactants may induce precipitation or phase separation.

La estabilidad y eficacia de una emulsión cosmética dependen en gran medida de la correcta elección del emulsionante. Este ingrediente no solo mantiene unidas fases inmiscibles como agua y aceite, sino que también determina aspectos clave de la fórmula: desde su textura y sensorialidad, hasta la liberación de activos y la experiencia final del consumidor.

The stability and performance of a cosmetic emulsion largely depend on the proper selection of the emulsifier. This ingredient not only keeps immiscible phases such as water and oil together, but also defines key aspects of the formulation, from texture and sensory profile to active delivery and the overall consumer experience.

At Ismael Quesada Personal Care, we provide our clients with high-quality emulsifiers and raw materials to support the development of cosmetic formulations tailored to current market needs.

References

• Kawakatsu, T. (2017). Emulsion and emulsification. En H. Iwata & H. Shimada (Eds.), Cosmetic Science and Technology: Theoretical Principles and Applications (pp. 617–638). Elsevier
• Benson, H. A. E. (2019). Cosmetic formulation: principles and practice,(pp. 87-102).