02 — FORMULATION

How does the product actually work as a system?

A cosmetic formula is not simply a collection of ingredients. It is an engineered system in which water, oils, surfactants, polymers, solvents, emulsifiers, active ingredients, preservatives, and packaging interact continuously. The performance of a product depends not only on what ingredients are present, but on their concentration, chemical state, physical environment, interactions, stability, and ability to reach the intended site of action.

Two products can contain the same active ingredient and produce very different sensory, stability, and delivery characteristics because their formulation architecture is different.

For skin, the central formulation question is therefore not simply:

What is in the bottle?

It is:

What happens to each component from the moment the product is applied until it interacts with the skin?

SERUM VS CREAM: WHY THEY BEHAVE DIFFERENTLY

A serum and a cream are not merely different textures. They are different physical systems.

Many serums are relatively low-viscosity formulations designed to spread rapidly and provide a concentrated or elegant delivery vehicle. They may be aqueous, hydroalcoholic, anhydrous, or lightly emulsified. Their continuous phase and rheology determine how quickly they spread, evaporate, absorb, and leave material on the skin.

A cream is commonly an emulsion: a system containing two normally immiscible phases, usually oil and water, stabilized by emulsifiers and often structured by fatty alcohols, polymers, waxes, or other rheology modifiers.

In an oil-in-water (O/W) cream, oil droplets are dispersed throughout a continuous aqueous phase. In a water-in-oil (W/O) cream, water droplets are dispersed within a continuous oil phase.

This distinction affects:

• viscosity and spreadability;
• evaporation;
• occlusivity;
• ingredient solubility;
• release from the formulation;
• interaction with the stratum corneum;
• sensory properties;
• preservation requirements;
• and physical stability.

A cream can therefore function as both a delivery vehicle and a skin-conditioning system. Its lipids, humectants and occlusives can modify the environment of the stratum corneum independently of any specifically designated active ingredient.

COSMETIC CHEMISTRY

Cosmetic chemistry is applied chemistry at an interface between materials science, colloid science, physical chemistry, microbiology and skin biology.

A formulation may contain several chemically and physically distinct classes of materials.

Humectants attract and retain water through interactions such as hydrogen bonding. Examples include glycerol, propanediol and certain polyols.

Emollients improve the feel and flexibility of the skin surface by filling irregularities and modifying interfacial properties.

Occlusives reduce water loss by forming a relatively water-resistant film at the surface. Petrolatum is a classic example.

Surfactants contain both hydrophilic and lipophilic regions and can reduce interfacial tension. They are fundamental to cleansing, emulsification, solubilization and dispersion.

Emulsifiers stabilize interfaces between immiscible phases.

Polymers can control viscosity, suspension, film formation, texture and sensory characteristics.

Chelating agents bind metal ions that can otherwise accelerate undesirable chemical reactions.

Antioxidants can slow oxidative degradation of susceptible components.

Preservatives control microbial growth in formulations where microorganisms could otherwise proliferate.

The formula works because these materials occupy particular chemical and physical roles. Changing one component can alter the behaviour of the entire system.

CONCENTRATION: WHAT DOES IT ACTUALLY MEAN?

Concentration describes how much of a substance is present relative to the total formulation, but the numerical percentage alone does not tell the complete scientific story.

A formula containing 2% of an ingredient means that approximately 2 parts of that ingredient are present per 100 parts of finished product, assuming a conventional weight/weight percentage.

But concentration must be interpreted alongside:

• molecular identity;
• molecular weight;
• purity;
• chemical form;
• solubility;
• ionization;
• vehicle;
• partitioning;
• stability;
• skin compatibility;
• and the biological mechanism being targeted.

A higher concentration does not automatically mean greater efficacy.

An ingredient may reach a point at which increasing concentration produces little additional benefit while increasing irritation, instability, crystallization, sensory problems, or other formulation challenges.

Conversely, a relatively small concentration can be highly consequential when the ingredient is potent, biologically active, catalytic, or functions through a threshold-dependent mechanism.

Concentration is therefore only one variable in the relationship:

dose → availability → exposure → biological response.

DELIVERY SYSTEMS

A delivery system determines how an ingredient is presented to the skin.

The same molecule can behave differently depending upon whether it is dissolved in water, dissolved in oil, dispersed as a particle, incorporated into an emulsion, associated with a polymeric system, or encapsulated within a carrier.

Important delivery considerations include:

Solubility — Can the ingredient remain molecularly dispersed in the formulation?

Partitioning — Does it preferentially associate with water, oil, or the skin's lipid environment?

Diffusion — Can it move through the formulation and toward the skin interface?

Release — Can it leave its vehicle at an appropriate rate?

Partition into skin — Does the molecule preferentially enter the stratum corneum?

Stability — Does it remain chemically intact during storage and use?

A sophisticated formulation therefore attempts to control not simply the amount of an ingredient, but its chemical environment and availability at the interface.

ENCAPSULATION

Encapsulation places an ingredient within or associated with a carrier structure.

Examples include:

• liposomes;
• lipid nanoparticles;
• polymeric particles;
• microcapsules;
• cyclodextrin complexes;
• and other structured carrier systems.

Encapsulation can potentially improve:

• protection from oxidation or degradation;
• compatibility with other formula components;
• dispersion of poorly soluble materials;
• controlled or modified release;
• sensory properties;
• and delivery characteristics.

However, encapsulated does not automatically mean better absorbed.

The carrier must remain sufficiently stable during storage while also releasing its payload under the appropriate conditions. Particle size, surface chemistry, composition, loading efficiency, release kinetics and interaction with the formulation and skin can all influence performance.

Encapsulation is therefore an engineering strategy—not a guarantee of superior biological activity.

BIOAVAILABILITY AND SKIN PENETRATION

For topical products, the phrase bioavailability must be used carefully.

An ingredient may be present in a formulation without being biologically available at its intended site.

The skin presents a substantial barrier, particularly the stratum corneum, whose structure can be conceptualized as corneocytes embedded within a lipid-rich extracellular matrix.

For a molecule to move through this barrier, several processes become important:

Release from the vehicle → partitioning into the stratum corneum → diffusion through the barrier → possible movement into viable epidermis or dermis.

Molecular properties strongly influence this process, including:

• molecular size;
• lipophilicity;
• polarity;
• hydrogen-bonding capacity;
• ionization;
• melting behaviour;
• and chemical stability.

The formulation also matters. Solvents, surfactants, penetration-modifying excipients, occlusion, hydration and vehicle composition can alter the thermodynamic activity and partitioning of an ingredient.

But skin penetration is not synonymous with efficacy.

More penetration is not inherently better. The desired outcome depends on the intended biological target. For a surface-conditioning ingredient, remaining near the surface may be appropriate. For another ingredient, access to viable epidermal structures may be relevant.

The goal is therefore appropriate delivery to the appropriate biological compartment, not simply maximum penetration.

VEHICLE SYSTEMS

The vehicle is the formulation environment carrying the functional ingredients.

Common vehicle architectures include:

Aqueous systems — predominantly water-based formulations containing dissolved or dispersed materials.

Anhydrous systems — formulations containing little or no water, often based on oils, esters, silicones, waxes or other lipophilic materials.

Emulsions — combinations of aqueous and oil phases stabilized through interfacial structures.

Gels — systems in which polymers or other structuring agents create a three-dimensional network that immobilizes or thickens a liquid phase.

Suspensions — systems in which insoluble particles are dispersed throughout a continuous phase.

Hydroalcoholic systems — formulations containing water and alcohols or related solvents.

The vehicle influences spread, evaporation, residue, release, stability, sensory properties and ingredient partitioning.

In other words, the vehicle is not an inert background. It is part of the product's mechanism.

STABILITY

A formulation must remain sufficiently safe, effective and physically acceptable throughout its intended shelf life.

Stability has several dimensions.

Chemical stability concerns degradation reactions such as hydrolysis, oxidation, reduction, isomerization or photodegradation.

Physical stability concerns changes such as phase separation, creaming, sedimentation, crystallization, viscosity loss, particle aggregation or changes in appearance.

Microbiological stability concerns the ability of the formulation to resist microbial contamination and growth.

Important variables include:

• temperature;
• oxygen;
• light;
• water activity;
• pH;
• metal ions;
• packaging;
• processing conditions;
• ingredient interactions;
• and repeated consumer exposure during use.

Stability testing therefore asks a deeper question than Does the product still look good?

It asks:

Does the formulation still possess the intended chemical, physical, microbiological and functional characteristics?

Packaging is part of this system. A light-sensitive ingredient may require light protection. An oxidation-sensitive formula may require reduced oxygen exposure. A volatile component may require an appropriately sealed package.

The container is therefore an extension of the formulation.

pH

pH is a measure of hydrogen-ion activity, expressed on a logarithmic scale.

A one-unit change in pH represents approximately a tenfold change in hydrogen-ion activity under the conventional approximation used for dilute aqueous systems.

pH can influence:

• ingredient ionization;
• solubility;
• preservative performance;
• polymer behaviour;
• emulsifier performance;
• chemical degradation;
• skin compatibility;
• and formulation viscosity.

Many cosmetic ingredients are weak acids or bases whose ionization changes with pH.

The Henderson–Hasselbalch relationship provides a useful conceptual framework for weak electrolytes:

pH = pKa + log([A⁻]/[HA])

The ratio between ionized and unionized forms can affect solubility and membrane partitioning.

But there is no universal "ideal cosmetic pH." The appropriate pH depends on the complete formulation and intended use, including the stability and compatibility requirements of its ingredients.

A formula should therefore be designed around a scientifically justified pH range—not simply adjusted toward a fashionable numerical value.

INGREDIENT COMPATIBILITY

Ingredient compatibility is one of the most important—and least visible—aspects of formulation.

Two ingredients may be individually desirable yet incompatible when combined.

Interactions can be:

Chemical — one ingredient reacts with another or accelerates its degradation.

Physical — precipitation, crystallization, phase separation, aggregation or viscosity changes occur.

Interfacial — ingredients alter the structure or stability of an emulsion or other colloidal system.

Microbiological — formulation changes alter preservative effectiveness or microbial susceptibility.

Sensory — an otherwise stable system becomes tacky, greasy, pilling-prone or unpleasant to use.

Compatibility can also be concentration-dependent and pH-dependent.

For example, an ingredient may remain soluble at one pH but precipitate after pH adjustment. A polymer may thicken effectively within one ionic environment but lose viscosity when exposed to electrolytes. A botanical extract may introduce compounds that interact with preservatives, metals or other actives.

This is why simply combining a long list of "good ingredients" does not necessarily produce a good formula.

THE FORMULA AS A SYSTEM

The most useful way to understand formulation is as a chain of interconnected events:

INGREDIENT → CHEMICAL FORM → CONCENTRATION → VEHICLE → RELEASE → PARTITIONING → SKIN INTERACTION → BIOLOGICAL RESPONSE

Every stage can influence the next.

An ingredient can be exceptionally well researched yet contribute little if it is unstable in the formula.

A highly concentrated ingredient can perform poorly if it precipitates.

An encapsulated ingredient can be impressive on paper but ineffective if the carrier does not release its payload appropriately.

A beautifully designed serum can fail if its preservation system is inadequate.

A scientifically appropriate active can become irritating when the surrounding vehicle changes its exposure.

And a technically excellent formulation can still fail commercially if its texture prevents consistent use.

Formulation is therefore the science of making all of these variables work together.

THE URIBHO PRINCIPLE

At URIBHO, formulation should be understood beyond the ingredient list.

A product is not what its label says it contains. A product is the physical and chemical system that determines what those ingredients actually do.

Understanding formulation means asking:

What is the molecule?
How much is present?
In what chemical form?
Where does it reside in the vehicle?
Is it stable?
Can it be released?
Can it interact with the skin?
Where does it go?
And does the finished system deliver the intended biological effect safely and consistently?

That is where cosmetic chemistry becomes beauty science.

SCIENTIFIC REFERENCES

1. Barel, A. O., Paye, M., & Maibach, H. I. Handbook of Cosmetic Science and Technology. CRC Press.
2. Draelos, Z. D. Cosmetic Dermatology: Products and Procedures. Wiley-Blackwell.
3. Tadros, T. Emulsion Formation and Stability. Wiley-VCH.
4. Barry, B. W. Dermatological Formulations: Percutaneous Absorption. Marcel Dekker.
5. Benson, H. A. E., Watkinson, A. C. Transdermal and Topical Drug Delivery: Principles and Practice. Wiley.
6. Williams, A. C. Transdermal and Topical Drug Delivery: From Theory to Clinical Practice. Pharmaceutical Press.
7. Elias, P. M. The skin barrier as an adaptive interface between the body and environment. Clinical Dermatology and related foundational work on stratum-corneum barrier biology.
8. Madison, K. C. Barrier function of the skin: “La raison d'être” of the epidermis. Journal of Investigative Dermatology.
9. Lodén, M. Role of topical emollients and moisturizers in maintaining skin barrier function. American Journal of Clinical Dermatology.
10. Walters, K. A. Dermatological and Transdermal Formulations. Marcel Dekker.