Stability of Emulsifier: from interface state to formulation control
The HLB value of the emulsifier is not the same concept as the emulsifying stability.What really determines the success or failure of the formula is the arrangement state and dynamic balance of emulsified molecules at the interface.For example, the same Water in oil formula is stable at room temperature, but once it encounters temperature fluctuations, stratification and demulsification may occur.
A batch of emulsions that appear to be homogeneous may experience water-oil separation or paste effluent after long-term standing.The formula with the active substance added may also change from a delicate emulsion to a rough tofu residue due to changes in ion strength.
These phenomena are all related to the stability of the emulsifying system, but this is more than just “choosing the right HLB value”.
01 / Five states of instability of the emulsifying system
When the emulsifying system is out of balance, problems will appear in five different forms, and they often occur incrementally.
1. Flocculation: Oil droplets or water droplets in the emulsion gather into loose clusters because of the attraction of Van der Waals forces, but the individual boundaries of each droplet still exist.
-This is reversible. By gently stirring, the clusters can be re-dispersed and return to a uniform state.
-It is a warning signal of system instability. At this time, the viscosity of the formula may change slightly.
-Long-term flocculation will lay hidden dangers for subsequent coalescence.
2. Sedimentation and Creaming: This is a phenomenon caused by gravity.Oil droplets float up because their density is smaller than that of water, forming a thicker oil layer (stratification) in the upper layer, and water droplets sink because their density is larger than that of oil, forming a lower water layer (sedimentation).
-This is reversible and can be temporarily restored by shaking, but if it occurs repeatedly, it will accelerate the convergence of droplets.
-The root cause is the difference in the density of the two phases and the excessive particle size of the droplets.
-The insufficient viscosity of the formula and the lack of an effective thickening system are one of the main reasons.
3. Coalescence: The interfacial membrane between the flocculated droplets breaks down, and the small droplets merge into a large droplet.
-This is irreversible.Once the droplets merge, they cannot be restored to their original fine droplets.
-It will cause the particle size distribution of the emulsion to widen, the gloss to decrease, and the skin feeling to become rough.
-Convergence is a prelude to demulsification. Once it occurs, it is difficult for the system to regain stability.
4. Ostwald Ripening: The process of dissolving small droplets and growing large droplets.Due to the higher solubility of small droplets, they will gradually disappear, and the dissolved components will precipitate on the surface of the large droplets, making the large droplets larger.
-This is irreversible and mainly occurs in systems where the two phases of oil and water have a certain degree of miscibility.
-It is different from poly. The interfacial membrane between the droplets has not broken, but is completed by dissolving-re-precipitating.
-This is the culprit of many low-viscosity, high-fluidity emulsions that become rough after long-term storage.
5. Phase Separation: This is the final form of instability of the emulsion system.The two phases of water and oil are completely separated, forming a clear oil-water interface, and the emulsion is completely destroyed.
-Irreversible and cannot be restored by any physical means.
-This is a sign of a complete failure in the stability of the formula.
02 | Engineering Significance of Emulsifier selection
The “HLB value” is usually listed in the formula manual. For example, the HLB requirement for mineral oil is about 10-12, while the HLB requirement for silicone oil is much lower.However, just comparing these numbers can easily mask the key information behind the stability of the formula.
Oil in water (O/W) system: With water as the continuous phase and oil as the dispersed phase, the formula is refreshing and easy to push away. It is the choice for most lotions and creams.Engineers need to find a balance between refreshing skin feeling and system stability, because excessive water phase ratio will reduce the viscosity of the system and increase the risk of stratification.
Water in oil (W/O) system: With oil as the continuous phase and water as the dispersed phase, the formula is moisturizing and has strong water-locking ability. It is commonly used in heavy cream-like products or sunscreen formulas.However, it lacks the thickening effect of the aqueous phase, and the water droplets in the internal phase are easier to merge, so it is more prone to instability in low temperature or high ionic strength environments.
The HLB value of the emulsifier reflects its affinity to the oil-water phase, but this is not the only criterion for determining stability.When choosing an emulsifier, the following factors must be considered: the compatibility of the emulsifier with the oil phase, the strength of the film formed by the emulsifier at the interface, and whether its interaction with other components in the system (such as active substances, thickeners) is acceptable.
03 / Comparison Table of commonly used emulsifying systems
| System Type | Representative Emulsifier | System Characteristics |
| Non-ionic O/W System | PEG-100 STEARATE and GLYCERYL STEARATE | A classic oil-in-water emulsification system, highly versatile and offering a balanced skin feel; it is widely utilized in standard skincare formulations—such as creams and lotions—as a key cosmetic raw material. |
| Non-ionic W/O System | PEG 30 Dipolyhydroxystearate | Highly moisturizing with excellent water resistance; suitable for sunscreen and makeup products. |
| Cationic Emulsification System | Behentrimonium chloride | Provides conditioning effects; commonly used in hair care products; demonstrates synergistic effects when combined with cationic polymers. |
| Polymeric Emulsification System | Ammonium Acrylates Copolymer | Dual thickening and emulsifying action; results in high system viscosity; exhibits strong tolerance to temperature fluctuations. |
| Silicone Emulsification System | Dimethicone Crosspolymer | Imparts a silky-smooth, non-sticky skin feel; demonstrates excellent compatibility with silicone-based ingredients. |
04 / Key influencing factors of emulsification stability
Temperature effect
Temperature is the most direct factor affecting the stability of the emulsifying system.For nonionic emulsifier systems, increasing temperature will destroy the hydrated layer of the emulsifier, reduce the strength of the interfacial membrane, cause the viscosity of the system to decrease, and the droplets will more easily merge.The sudden temperature change will exacerbate the phase separation trend of the system, which is also the main reason why many products fail in the high and low temperature cycle test.
Effect of ion strength
When electrolytes, plant extracts, or active ingredients are added to the formula, the ionic strength of the system will increase.This will compress the double electrical layer at the interface of the emulsifier, reduce the electrostatic repulsive force between the droplets, and lead to an increased risk of flocculation and convergence.This is why many formulas with high content of active substances need to choose a polymer emulsion system that is more tolerant to electrolytes.
Effects of mechanical stress
During filling, transportation and use, the product will be subjected to mechanical stresses such as vibration and shear.These external forces will destroy the interfacial membrane of the emulsion, causing the droplets to merge.Therefore, the rheological characteristics of the system are essential-the appropriate viscosity and thixotropic properties allow the system to remain stable at rest and flow easily when subjected to shear.
Compatibility impact
The compatibility of emulsifiers with the oil phase and other additives is also essential.If the emulsifier is incompatible with the oil phase composition, a stable film cannot be formed at the interface, resulting in excessive droplet particle size and decreased stability.For example, the silicone oil phase needs to choose an emulsifier compatible with the siloxane structure instead of the traditional fatty alcohol ether emulsifier.
05 / Practical methods to improve the stability of emulsification
1. Scientific collocation of emulsifiers
It is often difficult for a single emulsifier to balance skin feeling and stability, so it is a better choice to use it together.
-Main emulsifier: Choose ingredients that provide the main emulsifying ability and interfacial membrane strength, such as
Emulsifying wax.
-Co-emulsifiers: Select ingredients that can assist in the formation of a mixed film and enhance the viscosity of the system, such as fatty alcohols and fatty acids.They can form a denser composite film on the interface, which significantly improves stability.
-Polymer stabilizers: When used in combination with polymer components such as
Ammonium acrylate copolymer, they can form a three-dimensional network in the system, prevent the movement and convergence of droplets, and improve the overall stability of the system.
2. Refined control of processing technology
The details of the process often determine the success or failure of the formula.
-Phase transition temperature control: For nonionic emulsifying systems, emulsification near the phase transition temperature can obtain an emulsion with finer particle size and more uniform distribution, and better stability.
-Optimization of homogenization parameters: The speed and time of homogenization directly affect the particle size of the droplets.The speed is too low, the particle size is too large, it is easy to stratify, the speed is too high, and too many air bubbles may be introduced, destroying the system.
-Cooling process control: The cooling rate after emulsification should not be too fast, and it should be cooled slowly so that the emulsifier has enough time to rearrange to form a stable interfacial film.
3. Optimized design of formula system
-Thickening system collocation: Adding a suitable thickener to the aqueous phase can improve the viscosity of the continuous phase and slow down the sedimentation and stratification of droplets.
-Electrolyte concentration control: Try to avoid adding excessive concentrations of electrolytes to the formula. If it must be added, the tolerance of the emulsion system should be evaluated in advance, or a polymer emulsion system that is stable to the electrolyte should be selected.
-Oil phase optimization: The type and proportion of the oil phase will also affect the stability.Choose an oil phase that is compatible with the emulsifier, or reduce the risk of austenitic aging by adjusting the polarity and viscosity of the oil phase.
Emulsification stability is not a fixed value, but the dynamic equilibrium characteristics of the formula at the molecular level.To judge whether a formula is stable, you can't just look at the HLB value, but also pay attention to: the state of the emulsifier on the interface, the key factors affecting the stability, and the specific requirements of the application scenario for stability.
Hubei Mingya New Material Technology Co., Ltd.
