Fundamentals of Emulsion Technology in Cosmetics OEM | Formulation Design, Emulsification Equipment & Stability Evaluation

Emulsification is the technology of dispersing two inherently immiscible phases—water and oil—as fine droplets through the action of surfactants to form a system with a uniform appearance. Over 70% of cosmetic products are emulsion-based systems, making emulsion technology a cornerstone of cosmetics OEM manufacturing.

Emulsion systems are classified into three main types based on the relationship between the continuous phase (external phase) and the dispersed phase (internal phase).

• O/W (Oil-in-Water): Water is the continuous phase and oil is the dispersed phase. Used in most skincare products including toners, milky lotions, serums, and sunscreens. Provides a light, refreshing feel and makes it easy to incorporate water-soluble active ingredients. Characterized by high electrical conductivity and easy rinse-off with water.
• W/O (Water-in-Oil): Oil is the continuous phase and water is the dispersed phase. Used in foundations, concealers, and waterproof sunscreens. Offers excellent water resistance and is suitable for delivering oil-soluble ingredients. Provides a rich, moisturizing feel upon application.
• W/O/W (Multiple Emulsion): A three-phase structure in which a W/O emulsion is further dispersed in a water phase. Water-soluble active ingredients (ascorbic acid, niacinamide, etc.) can be encapsulated in the inner water phase, protected by an oil film, while maintaining the light feel of an O/W system. Enables controlled release (slow release) of ingredients, making it suitable for high-performance serums and sensitive skin products.

The choice of emulsion type is determined by the solubility of the active ingredients, the target sensory experience (texture), the final product category, and whether water resistance is needed. Clarifying the direction of the emulsion type is an important first step in formulation design discussions with an OEM manufacturer.

Emulsifier (surfactant) selection is the most critical design factor affecting emulsion stability and sensory feel. The fundamental metric for emulsifier selection is the HLB value (Hydrophilic-Lipophilic Balance).

The HLB value quantifies the hydrophilic-lipophilic balance of an emulsifier on a scale of 0–20. HLB 3–6 is suitable for W/O emulsification, and HLB 8–18 is suitable for O/W emulsification. Matching the Required HLB of the oil phase with the HLB value of the emulsifier yields the most stable emulsion system. For example, to emulsify squalane (Required HLB approximately 12), an emulsifier system with an HLB value around 12 is optimal.

Major emulsifiers commonly used in cosmetics

• Polyglyceryl fatty acid esters (INCI: Polyglyceryl-10 Laurate, etc.): HLB value can be adjusted over a wide range with low irritation potential. Frequently used in sensitive skin formulations. Major suppliers include Taiyo Kagaku and Daicel.
• PEG-60 Hydrogenated Castor Oil (INCI: PEG-60 Hydrogenated Castor Oil): HLB around 14. High solubilizing power; used for solubilizing oily ingredients in clear toners and serums.
• Hydrogenated Lecithin (INCI: Hydrogenated Lecithin): A natural phospholipid-based emulsifier. Easily forms lamellar structures and is essential for ceramide-containing formulations and liposome preparations. Supplied by Q.P. Corporation and Lipoid.
• Glyceryl Stearate (SE) (INCI: Glyceryl Stearate SE): A self-emulsifying emulsifier. HLB around 11; widely used as a base emulsifier for O/W creams.
• Alkyl Glucosides (INCI: Decyl Glucoside, etc.): Sugar-derived nonionic surfactants. Highly biodegradable, with increasing adoption in natural and organic cosmetics formulations.

In practice, rather than using a single emulsifier, the standard approach is the "mixed HLB method" combining two or more emulsifiers with different HLB values. This increases interfacial film strength and significantly improves long-term stability. The total emulsifier loading is typically 2–5% for O/W systems and 3–8% for W/O systems.

The choice of emulsification equipment directly affects the achievable emulsion particle size, production scale, and formulation characteristics. The equipment an OEM manufacturer owns is an indicator of their manufacturing capabilities and a critical checkpoint during partner selection.

Main types of emulsification equipment and their features

• Homomixer (high-speed rotor type): Uses a rotor-stator structure to apply high-speed shear to the liquid. At 3,000–10,000 rpm, emulsions with particle sizes of 1–10 μm are produced. Widely used for general skincare products such as milky lotions, creams, and body lotions. Major manufacturers include Tokushu Kika Kogyo and Primix. Benefits include relatively low equipment costs and easy scale-up.
• High-pressure homogenizer: Liquid is forced through a valve gap at 50–200 MPa, creating submicron emulsions (0.1–1 μm / 100 nm–1 μm) through cavitation and shear forces. Major manufacturers include GEA Niro Soavi and Izumi Food Machinery. High-pressure homogenization yields a narrow particle size distribution and excellent long-term stability.
• Microfluidizer: Liquid streams collide at high pressure (up to 200+ MPa) inside an interaction chamber, stably producing nanoemulsions with particle sizes of 50–200 nm. Microfluidics Corporation is the representative manufacturer. While expensive, the equipment offers extremely high reproducibility and very narrow particle size distributions (PDI < 0.2).
• Membrane emulsification: The dispersed phase is pressed through a porous membrane (such as SPG membrane—Shirasu Porous Glass) into the continuous phase. Particle size can be precisely controlled by membrane pore size (CV < 10%), yielding monodisperse emulsions with uniform particle size distribution. Enables minimal emulsifier usage, making it suitable for low-irritation formulations. Developed and supplied by Miyazaki Prefectural Industrial Technology Center and SPG Techno.

When evaluating an OEM manufacturer's equipment, check not only the types of emulsifiers they own but also their scale range (lab scale 1–5 kg, pilot scale 10–50 kg, production scale 100+ kg) and whether they have CIP (Clean-in-Place) capability. For multi-product small-lot manufacturing, CIP-capable equipment is valuable for reducing changeover time and preventing cross-contamination.

A nanoemulsion refers to an ultra-fine emulsion with dispersed particle sizes of 50–200 nm (0.05–0.2 μm). It is a kinetically stable system distinct from conventional macroemulsions (particle size 1–100 μm) and microemulsions (thermodynamically stable systems), produced through high-energy emulsification methods.

Characteristics and advantages of nanoemulsions

• Transparent to semi-transparent appearance: When particle sizes fall below the wavelength of visible light (380–780 nm), light scattering is suppressed, resulting in a clear, gel-like or water-like appearance. Ideal for product designs requiring a refreshing, clean aesthetic such as serums and essence waters.
• Enhanced percutaneous absorption: Fine particle sizes improve penetration through the stratum corneum. Research has reported that retinol and Coenzyme Q10 (ubiquinone) encapsulated in nanoemulsions achieve 1.5–3 times greater skin permeation compared to conventional emulsions.
• Superior stability: Brownian motion overcomes gravity-driven creaming and sedimentation, preventing phase separation over extended periods. With proper formulation design, nanoemulsions can pass 6-month accelerated stability testing at 40°C.
• Protection of unstable ingredients: Smaller oil droplet size increases the specific surface area, enhancing the antioxidant effect of the interfacial film. Effective for stabilizing oxidation-sensitive ingredients such as tetrahexyldecyl ascorbate (INCI: Tetrahexyldecyl Ascorbate) and retinol.

Manufacturing considerations

Nanoemulsion production requires a high-pressure homogenizer or microfluidizer, typically with 2–5 passes (multiple processing rounds) to reach the target particle size. In terms of formulation, emulsifier concentration is often set higher than in conventional emulsions (3–8%). To ensure adequate interfacial film thickness and elasticity, co-emulsifiers such as cetyl alcohol (Cetyl Alcohol) or behenyl alcohol (Behenyl Alcohol) are commonly added at 0.5–2%. When commissioning OEM manufacturing, verify that the manufacturer supports nanoemulsions and owns particle size distribution measurement equipment (dynamic light scattering compatible), such as the Malvern Zetasizer Pro.

Stability evaluation of emulsion systems is a critically important process that forms the basis for product quality assurance and shelf-life determination. In cosmetics OEM, the following evaluations are systematically conducted during the prototyping stage.

1. Centrifugation test (accelerated separation test)

The product is centrifuged at 3,000–10,000 rpm for 15–30 minutes to observe whether creaming (oil droplet rising) or sedimentation occurs. As a general guideline, no separation at 3,000 rpm for 30 minutes is considered equivalent to approximately 6 months of stability at room temperature. However, this is a preliminary screening method, and actual storage testing is required for final stability determination.

2. Temperature cycling test (including freeze-thaw testing)

Repeated cycles between −5°C and 40°C (or −10°C and 45°C) every 24 hours, with evaluation of appearance changes, viscosity changes, and separation after 3–6 cycles. Special attention is paid to crystal precipitation below 5°C and viscosity reduction or discoloration above 40°C. Cosmetics GMP recommends stability data at a minimum of three temperature ranges (5°C, 25°C, 40°C).

3. Laser diffraction particle size distribution measurement

Representative instruments include the Horiba LA-960V2 and the Malvern Panalytical Mastersizer 3000. The median particle size (D50) and span value ((D90−D10)/D50) are recorded as baseline values and compared with values after accelerated stability testing. A progressive increase in particle size over time (Ostwald ripening) is a sign of emulsion destabilization. For nanoemulsions, DLS (dynamic light scattering) measurement is appropriate, with the Malvern Zetasizer Pro being widely used.

4. Real-time storage test (long-term stability test)

Products are stored at 25°C/60% RH (ambient conditions) and 40°C/75% RH (accelerated conditions) for 6+ months, with periodic evaluation (at 1-month, 3-month, and 6-month intervals) of appearance, pH, viscosity, color difference (ΔE), and microbial limits. The standard approach for setting shelf life is to use 6-month accelerated data to estimate approximately 3 years of quality retention at ambient temperature.

When selecting an OEM manufacturer, confirm the availability of stability evaluation equipment (constant temperature/humidity chambers, centrifuges, particle size distribution analyzers) and the format of their test reports.

Here are two representative formulation design examples utilizing emulsion technology in cosmetics OEM. Both serve as useful references for formulation design discussions with an OEM manufacturer.

Case Study 1: W/O/W Multiple Emulsion Serum

Niacinamide (INCI: Niacinamide) at 3% and ascorbyl glucoside (INCI: Ascorbyl Glucoside) at 2% are dissolved in the inner water phase. The oil phase contains squalane (INCI: Squalane) at 10% and meadowfoam seed oil (INCI: Limnanthes Alba Seed Oil) at 5%. Primary emulsification (W/O) uses polyglyceryl-2 dipolyhydroxystearate (INCI: Polyglyceryl-2 Dipolyhydroxystearate, HLB 3–4) at 3% with a homomixer at 6,000 rpm for pre-emulsification. Secondary emulsification (W/O/W) uses polyglyceryl-10 laurate (HLB 12–14) at 2% with a paddle mixer at 500 rpm for gentle phase inversion. If stirring speed during secondary emulsification is too high, the inner water phase will be destroyed—gradual water-phase addition at low speed is the key.

Case Study 2: O/W High-SPF Sunscreen (SPF50+ PA++++)

UV filters include ethylhexyl methoxycinnamate (INCI: Ethylhexyl Methoxycinnamate) at 7.5%, diethylamino hydroxybenzoyl hexyl benzoate (INCI: Diethylamino Hydroxybenzoyl Hexyl Benzoate, commonly known as DHHB) at 3%, titanium dioxide (INCI: Titanium Dioxide, microfine, surface-treated) at 5%, and zinc oxide (INCI: Zinc Oxide, microfine) at 10%. Emulsifiers include PEG-30 dipolyhydroxystearate (INCI: PEG-30 Dipolyhydroxystearate) at 3% and aluminum stearate (INCI: Aluminum Stearate) at 1%.

Microfine titanium dioxide and zinc oxide are pre-dispersed into a paste using a bead mill or triple-roll mill to prevent the white cast caused by aggregation. Finish emulsification using a high-pressure homogenizer (100 MPa, 2 passes) achieves uniform dispersion and high SPF values simultaneously.

These formulation examples serve as baselines. In practice, texture adjustment, preservative system design (e.g., phenoxyethanol 0.8% + ethylhexylglycerin 0.3%), and fragrance selection are refined through collaboration with the OEM manufacturer's technical team.

Here is a checklist of equipment and capabilities to confirm when evaluating cosmetics OEM manufacturers from an emulsification technology perspective.

Emulsification Equipment

• Availability and capacity of vacuum emulsification vessels: Vacuum emulsification simultaneously achieves deaeration and oxidation prevention. Confirm capability at each scale: lab (1–10 L), pilot (10–100 L), and production (100–1,000 L). For multi-product, small-lot manufacturing, the availability of small-scale vessels is particularly important.
• Maximum processing pressure of high-pressure homogenizer: If 100 MPa or above is supported, nanoemulsion production is possible. In Japan, equipment from Nihon Seiki Seisakusho and APV Gaulin is commonly installed.
• Dispersion and mixing equipment: Check for bead mills (powder dispersion), planetary mixers (high-viscosity paste), and dispersers (pre-dispersion). These are essential for formulations requiring powder dispersion, such as foundations and sunscreens.
• Temperature control precision: Emulsification is typically performed at 70–80°C, but heat-sensitive ingredients (vitamin C derivatives, enzymes, etc.) require low-temperature emulsification below 50°C. Cooling rate control (rapid vs. gradual cooling) also affects texture.

Quality Evaluation Equipment

• Particle size distribution analyzer: Check whether both laser diffraction (for macroemulsions) and DLS (for nanoemulsions) instruments are available.
• Viscometer: In addition to rotational viscometers (Brookfield or cone-plate type), a manufacturer with a rheometer (dynamic viscoelasticity measurement) indicates high precision in texture design.
• Constant temperature/humidity chamber: Equipment capable of running storage tests simultaneously at three temperature ranges (5°C, 25°C, 40°C). Also check capacity and the number of samples that can be accommodated.
• pH meter and conductivity meter: Used for emulsion type identification (O/W type has high conductivity) and monitoring changes over time.

Beyond equipment, also confirm the number of formulation developers (formulators) with emulsification expertise on staff. Even with identical equipment, the ability to optimize operating conditions and troubleshoot formulation issues depends on personnel. Manufacturers that demonstrate strong formulation proposal capabilities during the prototyping stage tend to experience fewer problems during full-scale production.