The Chemistry of Cosmetics
Cosmetics are chemical formulations applied to the body for cleansing, beautifying, promoting attractiveness, or altering appearance. From moisturizers and sunscreens to lipsticks and anti-aging serums, every cosmetic product relies on specific chemical ingredients chosen for their interactions with skin, hair, and light.
Skin as a Chemical Barrier
To understand cosmetics, you must first understand their target. The outermost layer of skin — the stratum corneum — is a 10-20 micrometer thick barrier composed of dead keratinocytes (corneocytes) embedded in a lipid matrix, often described as a "bricks and mortar" structure. The lipid matrix contains ceramides, cholesterol, and free fatty acids arranged in lamellar bilayers.
The stratum corneum is slightly acidic, with a surface pH of 4.5-5.5 (the "acid mantle"), which inhibits pathogenic bacteria and maintains skin barrier integrity. This pH must be respected: cleansers with pH above 7 can disrupt the acid mantle, leading to dryness and irritation.
Moisturizers: Emollients, Humectants, and Occlusives
Moisturizers combat transepidermal water loss (TEWL) through three mechanisms:
Emollients fill the gaps between desquamating corneocytes, smoothing skin texture. Common emollients include dimethicone (a silicone), isopropyl myristate, and natural oils (jojoba, squalane). They "soften" skin by lubricating the surface and improving light reflection.
Humectants are hygroscopic molecules that attract water from the dermis and, in humid conditions, from the atmosphere. Key humectants include: - Glycerin (glycerol) — the most widely used humectant. Three hydroxyl groups provide strong hydrogen bonding with water. - Hyaluronic acid — a glycosaminoglycan that can hold up to 1,000 times its weight in water. Used at 0.1-2% in serums. - Urea (2-10%) — at low concentrations it hydrates; at higher concentrations (>10%) it acts as a keratolytic, loosening dead skin cells.
Occlusives form a physical barrier that traps moisture. Petrolatum (petroleum jelly) reduces TEWL by up to 99% and remains the gold standard occlusive. Beeswax, lanolin, and mineral oil are also effective occlusives.
Most effective moisturizers combine all three types. A typical formulation might include glycerin (humectant) + dimethicone (emollient) + shea butter (occlusive + emollient) in a water-based emulsion.
Sunscreen Chemistry: UV Filters
Ultraviolet radiation is classified by wavelength: UVA (320-400 nm) penetrates deep into the dermis, causing photoaging and contributing to skin cancer; UVB (280-320 nm) affects the epidermis, causing sunburn and direct DNA damage.
Sunscreens use two categories of UV filters:
Organic (chemical) filters absorb UV photons and dissipate the energy as heat through photochemical processes: - Avobenzone (Parsol 1789) — broad UVA absorber, but photounstable unless paired with stabilizers like octocrylene. - Octinoxate (octyl methoxycinnamate) — UVB absorber, widely used but facing regulatory scrutiny over endocrine disruption concerns and coral reef toxicity. - Ecamsule (Mexoryl SX) — photostable UVA filter.
Inorganic (physical) filters — zinc oxide (ZnO) and titanium dioxide (TiO2) — reflect, scatter, and absorb UV radiation. They are photostable, broad-spectrum, and generally well-tolerated. Modern formulations use nanoparticles (10-100 nm) to reduce the white cast that larger particles create.
SPF (Sun Protection Factor) measures protection against UVB only. An SPF 30 sunscreen allows 1/30th of the UVB radiation through, blocking approximately 96.7%. SPF 50 blocks ~98.0%. The marginal gain diminishes rapidly above SPF 30, making proper application thickness (2 mg/cm2) and reapplication far more important than chasing higher SPF numbers.
Active Ingredients in Anti-Aging Products
Retinoids are vitamin A derivatives — the most evidence-backed anti-aging actives. Retinoic acid (tretinoin) binds to nuclear retinoic acid receptors (RARs), upregulating collagen synthesis, accelerating cell turnover, and reducing hyperpigmentation. Over-the-counter retinol is a less potent precursor that must be enzymatically converted to retinoic acid in the skin. Retinoids can cause irritation, dryness, and photosensitivity, so gradual introduction and sunscreen use are essential.
Alpha-hydroxy acids (AHAs) — glycolic acid (from sugarcane), lactic acid (from milk) — exfoliate by weakening the ionic bonds between corneocytes in the stratum corneum. At concentrations of 5-10% and pH 3-4, they improve skin texture, reduce fine lines, and even out tone. Higher concentrations (20-70%) are used in professional chemical peels.
Niacinamide (vitamin B3) at 2-5% reduces sebum production, improves barrier function, reduces hyperpigmentation, and has anti-inflammatory effects. It is well-tolerated and compatible with most other actives.
Product Stability and Formulation
Cosmetic formulations are typically oil-in-water (O/W) emulsions stabilized by emulsifiers (polysorbate 60, cetearyl glucoside). Preservatives (phenoxyethanol, parabens, sodium benzoate) prevent microbial growth. Chelating agents (disodium EDTA) sequester metal ions that could catalyze oxidation or destabilize the emulsion. Antioxidants (tocopherol, ferulic acid) protect oxidation-sensitive actives like vitamin C.
Safety and Regulation
In the United States, the FDA regulates cosmetics under the Federal Food, Drug, and Cosmetic Act but does not require premarket approval for most cosmetic ingredients. In the European Union, the Cosmetics Regulation (EC No 1223/2009) requires a safety assessment by a qualified person before any product reaches the market and maintains a list of over 1,300 banned or restricted substances.