I want to be honest with you here: when I started studying this ingredient, I discovered a lot of new things, and there were some properties of azelaic acid I hadn't even suspected. Azelaic acid is a powerful active, loved by dermatologists for its multifaceted action. The task of a formulator at Walker Formulation Academy is to deliver all these effects effectively and aesthetically. But as soon as we move to high concentrations, a familiar problem arises: product pilling. In this guide, we will break down everything from mechanisms of action to five strategies for combating pilling.
How azelaic acid acts on the skin
Azelaic acid is a saturated dicarboxylic acid with 9 carbons (nonanedioic acid). Before we discuss texture, let's remind ourselves why we put up with all the inconveniences of this active in the first place.
Antibacterial action
It disrupts energy metabolism and protein synthesis in bacteria, helping to control Cutibacterium acnes Cutibacterium acnes and other microorganisms associated with acne.
Comedonolytic action
It normalizes the proliferation and differentiation of keratinocytes in the follicle, reduces the formation of microcomedones, and helps keep pores clear.
Antimelanogenic action
It competitively inhibits tyrosinase in hyperactive melanocytes. It is useful for melasma and post-inflammatory hyperpigmentation (PIH).
Anti-inflammatory
It reduces the formation of reactive oxygen species (ROS) and softens inflammatory signaling pathways — a key factor in rosacea and inflammatory acne.
Most of the effects are realized within the epidermis, not just on the skin surface. Therefore, it is critical to ensure penetration through the stratum corneum by skillfully managing solubility and pH.
Interesting facts for formulators
Nylon from cosmetics?
The same molecule that treats acne and rosacea is used in industry to produce polyamides like nylon and plasticizers. A versatile dicarboxylic acid!
“Soluble in boiling water”
Azelaic acid dissolves well in hot water, but at room temperature, its solubility drops to ~0.2–0.3% 0.2–0.3%. An ideal example of why it is important to look at solubility at working temperature.
Why azelaic acid is so difficult to work with
Parameter | Value |
|---|---|
INCI | Azelaic Acid |
Type | Saturated dicarboxylic acid (C9) |
Appearance | White crystalline powder |
Melting point | ~105°C |
Water solubility (25°C) | ~0.2–0.3% |
pKa₁ / pKa₂ | ~4.5 / ~5.3–5.5 |
Working pH | 4.0–5.5 |
Cosmetic concentration | 10–20% (suspension) |
Very low solubility
In water at room temperature — only 0.2–0.3%. Anything above that is no longer a solution, but a suspension of crystals.
High melting point
105°C — it cannot be “melted and dissolved” in a standard emulsion. It crystallises back upon cooling.
Solid phase
At 10–20% — it is almost always a crystal suspension. You need polymers and thickeners for stabilisation → and this leads to pilling.
Solubility and pH: the hidden lever
Azelaic acid has two carboxyl groups. In water, it exists in three forms: H₂A (free acid, lipophilic), HA⁻ / A²⁻ (azelate salts, hydrophilic). The lower the pH, the more free acid; the higher the pH, the more salts. This directly affects both solubility and penetration.
pH | Predominant form | Solubility | Penetration | Formula |
|---|---|---|---|---|
< 4 | H₂A (free acid) | Very low | Maximum | Suspension, “gritty” |
4–5.5 | Mixture of H₂A + HA⁻ + A²⁻ | Medium (with co-solvents) | Good balance | Optimal range |
> 6 | Salts (azelates) | Good | Weak | Transparent solutions, gentle action |
Acid vs salt: which penetrates better
Free acid (pH 4–5)
More lipophilic and uncharged → enters the lipids of the stratum corneum more easily. Works more effectively for acne, rosacea, and pronounced hyperpigmentation. Cons: can cause burning, stinging, and dryness; crystals, risk of pilling, pasty texture.
Salts and derivatives (pH 6+)
Highly soluble → aesthetic transparent serums. Gentle, comfortable, suitable for sensitive skin. Gradual evening of skin tone, sebum normalisation. Cons: do not replace the potency of free acid for severe acne; the effect is slower.
The golden compromise: pH 4–5.5 — part of the azelaic acid is in salt form (solubility increases), part is in neutral form (penetration is maintained). Gentle on the skin, compatible with most polymers. More on pH balance in our pH guide.
Why azelaic acid formulas pill
Pilling is a mechanical problem: the dried product forms a brittle film or one overloaded with solid phase, which breaks and clumps under friction, creating “pills”. Four main factors:
Excess of rigid polymers
High-molecular-weight xanthan gum and “rigid” carbomers create an inelastic film when dry. At high concentrations of azelaic acid, more thickeners are required → an even more rigid film → pilling.
Drying too quickly
Too much water/alcohol, too few emollients → the polymer network 'sets' before it can adapt to the skin's micro-relief. A thin, brittle 'shell'.
High solids load
Solid azelaic acid + polymers + gums + (starch, pigments) → after the water evaporates, a layer of dry solids remains, which fragments easily.
Large crystals
Poorly wetted crystals feel like 'sand' and behave like micro-beads in the film. The film is mechanically weaker and breaks around them.
The consumer also makes it worse: even a good cream formula will pill if applied in too thick a layer, if the skin is rubbed vigorously, if layered under silicone primers/SPF, or applied to very dry, rough skin.
How to prevent pilling: 5 strategies
Preventing pilling is about managing film mechanics and the solids load, not just choosing a polymer.
Strategy 1: Partial neutralisation. Partial neutralisation (NaOH, AMP, TEA) + glycols → part of the azelaic acid dissolves (salts), the rest remains as a finely dispersed suspension. This reduces the 'powdery load' in the dry film, improves texture, and maintains the solubility–delivery balance.
Strategy 2: Flexible polymers instead of rigid ones.
Approach | Recommendation | Why |
|---|---|---|
Acrylate systems (Aristoflex) | Network base | Effective at low dosages, soft elastic films |
Modified starches | Co-thickener | 'Powdery' feel, non-ionic, tolerates electrolytes |
Combination of 2 polymers (low doses) | Polymer blend | Better than a single polymer at a high dose |
Xanthan gum, carbomers | Use with caution | Rigid film → pilling at high % |
Important: Aristoflex is sensitive to high levels of electrolytes and glycols — always test compatibility with your buffer/humectant system.
Strategy 3: Optimising the base.
More emollients
Caprylic/capric triglyceride, coco-caprylate/caprate, C12-15 alkyl benzoate, light volatile silicones. These plasticise the film and slow down evaporation.
Smart humectants
Glycerin, 1,3-propanediol, butylene glycol (2–5%). Slip, slowing down drying. Propanediol additionally dissolves part of the acid.But: excess glycerin → stickiness → pilling.
Emulsion type
O/W emulsions and gel-creams are better than purely aqueous gels. The oil phase + emulsifiers → a cushioning, flexible film, rather than a brittle “shell”.
Strategy 4: Particle size and dispersion. Use micronised azelaic acid — smaller particles are easier to suspend, with less “grit” and fewer points for crack formation.
Pre-wetting in a blend of glycols (propanediol + butylene glycol) with a non-ionic surfactant (polysorbate, poloxamer)
Use a homogeniser / rotor-stator mixer for high-quality dispersion
Add the azelaic acid slowly to the already formed base while stirring well
Goal: a homogeneous creamy suspension, not a chalky, clumping mass
Strategy 5: Co-solvents and balance. The goal is not a completely transparent 10–20% solution (this is rarely needed), but a balance: part of the azelaic acid is dissolved (available immediately), the rest is a finely dispersed “reservoir”. The total solid load in the dried film is low enough not to crack or pill.
Azelaic acid is a truly outstanding active: it works against acne, rosacea, hyperpigmentation, and inflammation. But it also demonstrates just how subtle and demanding the art of cosmetic formulation can be. If you understand the mechanisms of action and pH-dependent solubility, consciously choose flexible polymers, optimise the base, and control particle size — you can turn a “prone to pilling” active into stable, comfortable products that stay on the skin and work, rather than rolling off under your fingers.
Read also: pH in cosmetics • Triethyl citrate
Oksana Walker
Cosmetic Chemist, founder of the Walker Formulation Academy
IFSCC • SCS • IAA • IAC
