A serum is one of the most technically complex and, at the same time, most sought-after products in home cosmetic formulation. This is where the formulator first encounters the need to simultaneously manage viscosity, pH, active ingredient compatibility, and system stability. A competent face serum formula is built not on intuition, but on an understanding of the chemistry of each component — and that is exactly what this article is dedicated to. The online school "Walker Formulation Academy" has broken down the key principles of formulation: from choosing base thickeners to incorporating peptides and acids into a finished system.
How a serum differs from a cream: the fundamental difference in formulation
Before moving on to specific recipes, it is important to understand what makes a serum a serum. It is not just a "liquid cream" — it is a fundamentally different delivery system for active substances.
Concentration of actives and molecular weight
Serums work due to a high concentration of active ingredients (usually 5–30% of the formula) and the low molecular weight of the components, which ensures deeper penetration. A cream, by contrast, creates an occlusive barrier and retains moisture on the surface. This is why serums use sodium hyaluronate with a molecular weight of 5–50 kDa, niacinamide, ascorbic acid, and peptides — substances capable of interacting with the cells of the epidermis and dermis.
Texture and rheology
A serum should be light, absorb quickly, and not leave a film. This imposes strict limitations on the choice of thickeners: polymers with high content must produce a fluid gel, not a dense substance. The rheology of a serum is thixotropic: upon application, it thins under mechanical stress and quickly restores its structure. Read more about the behavior of gelling agents under load in our article Tribology, gums, and gelling agents.
Face serum recipe: the basic structure of the formula
Any aqueous serum is built according to one principle: water phase → thickener → actives → preservative → pH adjustment. Violating this sequence is one of the most common mistakes made by beginner formulators.
Water phase: not just distillate
The base of a serum is purified or distilled water (60–90% of the formula). However, experienced formulators replace part of the water with functional liquids:
- Hydrolats (rose, chamomile, witch hazel) — add mild active properties without changing the rheology
- Aloe vera gel (diluted) — provides additional hydration and a soothing effect
- Panthenol (5–10%) — a regenerating agent that is highly soluble in water
- Betaine (2–5%) — an osmoprotectant and softener with excellent compatibility
- Glycerin (3–8%) — a classic humectant, but can feel sticky in excess
Thickeners: the choice determines everything
The thickener is exactly what defines the texture and behavior of the serum on the skin. For aqueous serums, the most commonly used are:
- Carbomer (Carbopol 940, 980) — 0.3–0.8%, neutralized to pH 5.5–6.5, produces a transparent gel
- Hydroxyethyl cellulose (HEC) — 0.5–2%, pH-neutral, highly compatible with electrolytes
- Xanthan gum — 0.1–0.5%, excellent compatibility with salts, but can create "strings" during application; for a detailed analysis, see the article Xanthan and guar gums: comparison, synergy, and the unexpected winner
- Sclerotium Gum — 0.3–1%, a biopolymer with a silky glide, ideal for luxury serums
- Polyglutamic acid (γ-PGA) — 0.5–2%, a functional thickener with powerful moisturizing effects
Important: carbomer is sensitive to electrolytes — if your formula contains salts (for example, sodium ascorbate or niacinamide in high concentrations), it is better to switch to Carbopol Aqua SF-1 or a combination of HEC + xanthan.
pH and compatibility of active ingredients in a serum
pH is not just a technical parameter "for show," but a key factor in the efficacy and stability of a formula. An incorrect pH destroys actives, reduces the effectiveness of the preservative, and irritates the skin.
Optimal ranges for popular actives
Each active ingredient requires its own pH environment:
- Ascorbic acid (vitamin C) — pH 2.5–3.5; at higher values, it oxidizes within weeks
- Niacinamide — pH 5.0–7.0; at a pH below 3.5, it hydrolyzes into nicotinic acid and causes redness
- Retinol — pH 4.5–6.0; sensitive to light and oxygen
- Hyaluronic acid — pH 5.0–8.0; stable over a wide range
- AHA acids (glycolic, lactic) — pH 3.0–4.0 for a peeling effect
- Peptides — pH 4.5–7.0 depending on the type; for more on incorporating peptides into a formula, see our article pH in cosmetics: a basic guide for formulators
The practical conclusion is: ascorbic acid and niacinamide must not be combined in the same serum at a pH below 4.0 — you will end up with either ineffective niacinamide or unstable vitamin C. This is one of the most common misconceptions in the DIY community.
Buffer systems for pH stabilization
For acid-based serums, use a citrate buffer (citric acid + sodium citrate) or a lactate buffer. For neutral serums, adjustment with an NaOH solution or lactic acid is sufficient. A detailed analysis of buffer systems can be found in our material on managing pH in cream formulas.
Active ingredients: what to add and how much
This is where real cosmetic chemistry begins. The concentration of an active ingredient determines not only its effectiveness but also the product's tolerability.
Moisturizing actives: building the skin's water reservoir
A basic recipe for facial serums with a moisturizing effect includes several levels of humectants:
- Hyaluronic acid (HMW, 1500–1800 kDa) — 0.1–0.5% — creates a film on the surface
- Hyaluronic acid (LMW, 5–50 kDa) — 0.05–0.2% — penetrates the upper layers of the epidermis
- Glycerin — 3–5% — a classic humectant
- Erythritol or trehalose — 1–3% — new-generation osmoprotectants
- Sodium PCA — 1–3% — a component of the Natural Moisturizing Factor (NMF)
Brightening and antioxidant actives
If the goal is to even out skin tone and provide antioxidant protection, the following are added to the formula:
- Niacinamide — 2–10%; at concentrations above 5%, temporary redness is possible for some people
- Alpha-arbutin — 0.5–2%; stable at pH 4.0–6.0
- Ascorbyl glucoside — 1–3%; a stable form of vitamin C, works at pH 5.0–6.5
- Ferulic acid — 0.5%; a synergist for vitamins C and E
Read about how oils and oil-soluble actives complement water-based serums in the article How to choose oils and butters for your skin type.
Preservation and stability of water-based serums
An aqueous environment with a high nutrient content is an ideal medium for the growth of microorganisms. Preserving a serum is not an option; it is a mandatory requirement for product safety.
Choosing a preservative based on formula pH
The effectiveness of most preservatives depends directly on the pH:
- Phenoxyethanol + Ethylhexylglycerin — pH 3.0–8.0; a universal choice, 0.5–1.0%
- Sodium benzoate + Potassium sorbate — pH up to 5.5; a natural option, 0.5% each
- Levulinic acid + Anisic acid — pH 3.5–5.5; suitable for "clean" cosmetics
- Caprylyl Glycol + Ethylhexylglycerin — a booster to enhance the main preservative
Remember: the preservative is added to the cooled phase (below 40°C) and after the final pH adjustment. Changing the pH after adding the preservative is one of the causes of microbial contamination of a batch.
Stability testing
Even a perfectly formulated serum requires verification. The minimum set of tests for a home laboratory includes visual inspection (separation, color change), pH measurement at 1, 4, and 8 weeks, and an accelerated stability test at 40°C. A detailed testing protocol is described in our article on cream formula stability on the Formula cream website.
Practical example: basic moisturizing serum recipe
Below is an educational recipe demonstrating the principles described above. It is not a commercial formula and is intended for understanding the structure.
Moisturizing serum with niacinamide (pH 5.5)
- Distilled water — up to 100%
- Glycerin — 5.0%
- Betaine — 3.0%
- Panthenol — 3.0%
- Niacinamide — 5.0%
- Hyaluronic acid (HMW) — 0.2%
- Hyaluronic acid (LMW) — 0.1%
- Sodium PCA — 1.0%
- Hydroxyethyl cellulose (HEC) — 0.8%
- Phenoxyethanol + ethylhexylglycerin — 0.8%
- Lactic acid (10% solution) — to pH 5.5
Procedure: Disperse HEC in cold water while stirring and let it swell for 30 minutes. Dissolve the water-soluble components (glycerin, betaine, panthenol, niacinamide, hyaluronic acid, Sodium PCA). Heat to 70°C and hold for 5 minutes. Cool to 35°C, add the preservative, and adjust the pH.
Want to master more complex techniques — creating multi-phase serums, emulsion serums, and anionic polymer systems? Join the Formula cream Club, where new educational recipes with chemical breakdowns are released every month.
Frequently asked questions about serum formulation
Can I add oil to a water-based serum without an emulsifier?
Technically, no, if you want a stable product. Oil and water without an emulsifier will separate within a few hours or days. The exception is solubilizers (Polysorbate 20, Caprylyl/Capryl Glucoside), which allow you to incorporate up to 1–3% of light oils or essential oils into a clear gel. For a significant oil component, it is better to create a separate anhydrous oil or an emulsion serum. Read more about anhydrous systems in our guide Anhydrous Products: A Complete Guide for Beginners.
Why did my carbomer serum turn cloudy after adding niacinamide?
Niacinamide at concentrations above 5% acts as a weak electrolyte and can disrupt the structure of a carbomer gel, causing cloudiness or a decrease in viscosity. The solution: use Carbopol Aqua SF-1 (Acrylates/C10-30 Alkyl Acrylate Crosspolymer) — it is resistant to electrolytes. An alternative is to switch to HEC or Sclerotium Gum.
How long does a homemade serum with a preservative last?
With proper preservation, adherence to sanitary production standards, and storage in a cool, dark place — 6–12 months. However, without a professional Challenge Test (microbiological stability testing), it is impossible to guarantee a shelf life. For personal use, we recommend making small batches (50–100 ml) and using them within 3 months of opening.
Serum formulation is a discipline that combines knowledge of rheology, organic chemistry, dermatology, and production technology. Understanding how each ingredient interacts with the others allows you to create products that not only look professional but also work as intended. A facial serum recipe is not a random set of components, but a carefully balanced system where pH, concentrations, and the order of ingredient addition matter. If you want to move from kitchen experiments to real formulation with a scientific basis, learn more about our courses in the Walker Formulation Academy Club or check out the training programme on the instructor's page.
