Emulsions are at the heart of countless formulated products — from skin creams and sunscreens to paint, mayonnaise, and pharmaceutical topicals. Yet an emulsion is never truly stable: it is a thermodynamically unfavourable system that will always tend to revert to its separated oil and water phases. The role of the formulator is to slow this process down — ideally for the full commercial shelf life of the product.
Rheology modifiers are among the most powerful tools available for doing so. They work by altering the flow behaviour of the continuous phase, building network structures that physically inhibit droplet movement and coalescence. But choosing and using them correctly requires a clear understanding of what makes emulsions fail in the first place, and what each modifier chemistry actually does at a molecular level.
This article walks through the science — from instability mechanisms to modifier selection — and is intended as a practical guide for formulators across cosmetics, pharmaceuticals, coatings, and food.
1. Why Emulsions Fail: The Four Instability Mechanisms
Before selecting a rheology modifier, it is essential to identify the dominant destabilisation route in your system. The scientific literature identifies four primary mechanisms:
- Creaming and sedimentation arise from density differences between the dispersed and continuous phases. In oil-in-water (O/W) emulsions, the lighter oil droplets rise to the surface; in water-in-oil (W/O) emulsions, denser water droplets settle to the bottom. The process is governed by Stokes' law: droplet size and the viscosity of the continuous phase are the two levers a formulator can pull.
- Flocculation involves droplets aggregating into clusters without merging. While reversible, flocculation accelerates subsequent coalescence and creaming if left unchecked.
- Coalescence is the irreversible merging of droplets into progressively larger ones, ultimately leading to complete phase separation. It is often the consequence of inadequate interfacial stabilisation by surfactants or polymers
- Ostwald ripening occurs when differences in Laplace pressure between droplets of unequal size drive the diffusion of material from smaller to larger droplets, causing a gradual coarsening of the droplet size distribution.
In practice, these mechanisms act in concert and can reinforce one another. A rheology modifier primarily addresses gravitational separation and flocculation and can indirectly slow coalescence by restricting droplet mobility.
2. How Rheology Modifiers Stabilise Emulsions
Rheology modifiers stabilise emulsions through two complementary mechanisms:
2.1 Viscosity Enhancement of the Continuous Phase
A higher viscosity in the continuous phase directly slows the creaming or sedimentation rate of dispersed droplets, as Stokes' law predicts. The increased resistance to droplet movement reduces the frequency of inter-droplet collisions, limiting flocculation and coalescence. Hydroxyethyl cellulose (HEC), for instance, is well documented for providing both colloidal protection and thickening effects that stabilise emulsions through the formation of three-dimensional network structures with strong elastic properties.
2.2 Three-Dimensional Network Formation
Beyond simple viscosity, many rheology modifiers form physical or chemical gel networks that provide a yield stress — a threshold force below which the system behaves as a solid, physically "locking" droplets in place. This is particularly effective against gravitational separation. Acrylic polymers such as carbomers, for example, act through swelling or chemical cross-linking to form a polymeric three-dimensional network that heavily modifies the rheology of the system while offering increased resistance to destabilisation mechanisms.
2.3 Steric and Electrostatic Barrier Effects
Unadsorbed macromolecules within the emulsion create steric hindrance, acting as physical barriers that reduce direct contact between droplets. In the case of charged polymers, electrostatic repulsion between droplets of the same charge further prevents aggregation. Hydrophilic macromolecules adsorbed at the droplet interface form a stabilising layer that creates a stable spatial structure.
3. Classes of Rheology Modifiers and Their Mechanisms
3.1 Acrylic Polymers (Carbomers / Carbopol)
Carbomers — cross-linked polyacrylic acid polymers — are among the most widely used rheology modifiers in cosmetics and pharmaceuticals. They build viscosity and form gels upon neutralisation with a base (typically sodium hydroxide or triethanolamine), becoming anionic and swelling considerably in water to create a dense, entangled network).
Their mechanism of emulsion stabilisation is primarily through gel network formation: the resulting structure physically immobilises oil droplets and provides a yield stress that resists gravitational separation. Carbomers are highly efficient — low concentrations (typically 0.1–1%) achieve high viscosity — and are particularly favoured for their ability to create clear gels.
Formulation considerations: Carbomers require neutralisation to develop viscosity. They are sensitive to electrolytes (salts reduce viscosity) and perform best at pH 6–10. High-electrolyte or salt-tolerant formulations may require specialised grades or alternative modifiers
Why the industry is actively moving away from carbomer
What was once a no-brainer choice has become, in 2026, a strategic liability. Three converging pressures are driving brands and contract manufacturers to seek alternatives — and the substitution window is narrower than many realise.
The regulatory clock is ticking — and it's non-negotiable. Under Commission Regulation (EU) 2023/2055, in force since 17 October 2023, carbomers are officially classified as microplastics (Synthetic Polymer Microparticles, SPM). The technical reasoning is subtle but decisive: the solubility test set out by the regulation must be conducted on the material in its commercialised form at pH 7. Carbomers are supplied in their solid acidic form and only become soluble after in-situ neutralisation by the formulator — so they fail the solubility test as defined, unlike pre-neutralised solid synthetic polymers that remain unchanged at pH 7.
3.2 Polysaccharides: Xanthan Gum
Xanthan gum is a high-molecular-weight microbial polysaccharide produced by fermentation of Xanthomonas campestris. It is notable for its pseudoplastic (shear-thinning) behaviour: at rest, it forms a rigid structured network with high viscosity; under shear (pumping, spreading), viscosity drops dramatically, then recovers when shear is removed.
This shear-thinning behaviour is ideal for emulsion stabilisation: the product remains structured during storage (resisting gravitational separation) but flows easily on application. Xanthan gum is stable across a broad pH range (4–10), tolerant of heat, freeze-thaw cycles, and compatible with electrolytes — making it suitable for formulations where carbomers would underperform.
Xanthan gum also acts synergistically with other polysaccharides such as guar gum, locust bean gum, and konjac mannan, with the result typically being an increased viscosity or gel formation, depending on concentration and the ionic environment.
Formulation considerations: At concentrations above 1%, xanthan gum may impart a "stringy" texture. It is anionic and incompatible with cationic surfactants. For natural and Ecocert-certified formulations, it is frequently the first-choice modifier
Beyond xanthan gum, several other polysaccharides contribute significantly to emulsion stability through viscosity enhancement, gel formation, or interfacial interactions. Gellan gum is particularly effective at low concentrations, forming fluid gels that can suspend droplets and improve physical stability without excessively increasing viscosity. Carrageenans (especially κ- and ι-types) interact with proteins and form structured networks that help prevent phase separation in emulsified systems. Alginates, derived from brown algae, create ionically crosslinked gels in the presence of divalent cations (e.g., calcium), enhancing droplet immobilization. Konjac glucomannan is known for its high water-binding capacity and synergistic gel formation with xanthan gum, contributing to improved texture and stability. Additionally, pullulan, although less commonly used as a primary stabilizer, can contribute to film-forming properties and viscosity modulation in certain formulations. These polysaccharides, alone or in combination, provide formulators with versatile tools to tailor rheology and stability, particularly in systems where natural or biodegradable ingredients are preferred.
3.3 Cellulose Derivatives: HEC and HPMC
Hydroxyethyl cellulose (HEC) and hydroxypropyl methylcellulose (HPMC) are water-soluble polymers that increase the viscosity of aqueous solutions through chain entanglement. HEC in particular provides both colloidal protection and thickening via three-dimensional network structures with strong elastic properties.
Hydrophobically modified HEC (HMHEC) is also used as an associative thickener in waterborne coatings (ScienceDirect, Associative thickeners for waterborne paints).
Cellulose derivatives are generally easier to incorporate than carbomers (no neutralisation required) but may not deliver the same level of formulation clarity or gel strength.
3.4 Inorganic Rheology Modifiers: Clays and Fumed Silica
Clays such as bentonite, hectorite (Laponite), and attapulgite form structured suspensions in water through electrostatic interactions between clay platelets, creating a "house-of-cards" network that exhibits a yield stress. Fumed silica operates similarly via hydrogen bonding between silanol groups. Both categories are widely used in waterborne coatings and cosmetics for precise rheological control, and are derived from mineral sources.
4. Selecting the Right Modifier: Key Formulation Variables
An important practical consideration highlighted in research: a modifier that works in a waterborne paint may be incompatible with a high-electrolyte cosmetic formula, or may lose performance in a system with a free surfactant concentration that disrupts its network structure.
For cosmetic emulsions specifically, the ideal rheological profile during the product's lifecycle is shear-thinning: high viscosity at rest during storage (10,000–100,000 mPa·s) to prevent phase separation, and low viscosity under shear during application (50–100 mPa·s) for easy spreadability.
5. The Role of Modifier Combinations
Single-modifier systems are often not enough. The scientific literature consistently shows that combining rheology modifiers with other stabilising ingredients — or combining two modifiers — can yield synergistic effects.
The cellulose formation by modified HEC, when combined with polysaccharides or cross-linked polymers, produces formulations with enhanced stability profiles. Carbomer–xanthan gum combinations are used to balance the gel strength of carbomer with the electrolyte tolerance of xanthan. Similarly, work on AS–Carbomer940 systems showed a strongly synergistic action, with the combined network providing gel-like behaviour comparable to commercial cosmetic creams.
6. Characterising Emulsion Stability: What to Measure
Selecting a modifier is only the first step; verifying its performance requires appropriate characterisation methods. The key tools are:
- Oscillatory rheology (G', G''): measures the viscoelastic properties of the formulation, including whether it behaves more like a solid (elastic, G' dominant) or a liquid (viscous, G'' dominant) at rest.
- Flow curves (shear stress vs. shear rate): quantifies shear-thinning behaviour and yield stress.
- Thixotropy loop tests: measure structural recovery after shear — critical for products that must recover viscosity after application or pumping.
- Turbiscan / backscattering methods: track droplet migration (creaming, sedimentation) in real time without dilution.
- Dynamic light scattering (DLS): monitors droplet size distribution over time to detect Ostwald ripening or flocculation.
7. Practical Guidance: Key Points for Formulators
- Identify your dominant instability mechanism first. Gravitational separation calls for yield stress and viscosity modification; Ostwald ripening requires a different approach (e.g., ripening inhibitors, narrower droplet size distribution).
- Match modifier chemistry to your system's pH and electrolyte profile. Carbomers are excellent in low-salt, neutral-to-alkaline systems; xanthan gum is the workhorse for electrolyte-tolerant, natural formulations.
- Consider the full lifecycle of the product. Viscosity needs differ between manufacturing (pumping), storage (anti-settling), and application (spreadability). Shear-thinning modifiers address all three stages simultaneously.
- Explore synergistic combinations. Single-modifier systems may fall short; co-formulation with a complementary modifier often improves both stability and sensory profile.
- Always test in your specific matrix. Modifier performance is highly system-dependent: surfactant interactions, temperature, pH, and ionic strength all affect the final rheological outcome.