Chemistry, Ingredients, and Product Safety
pH and why it matters
The scale runs 0–14; skin's acid mantle sits near pH 4.5–5.5. Alpha hydroxy acids need a low pH (roughly 3–4) to exfoliate, which is why a "20% glycolic" at pH 6 does little while a 10% at pH 3.5 stings. Bar soaps and alkaline cleansers raise surface pH and temporarily compromise the barrier; that is the chemistry behind post-cleansing tightness.
Solubility
"Like dissolves like." Water-soluble ingredients need a water base; oil-soluble ingredients (salicylic acid, vitamin E, many essential oils) need an oil or alcohol carrier. This explains why an oil cleanser lifts sebum and waterproof makeup better than a foaming gel.
Mixtures
- Solution - fully dissolved, clear.
- Suspension - particles dispersed, will settle; shake before use.
- Emulsion - oil and water held together by an emulsifier; oil-in-water is lighter, water-in-oil is heavier and more occlusive.
- Gel - thickened by a gelling agent, water-based.
Surfactants and preservatives
Surfactants have a water-loving head and oil-loving tail; anionic cleans strongly, non-ionic and amphoteric cleanse gently. Any product containing water needs a preservative system (parabens, phenoxyethanol, benzyl alcohol) or it grows microbes. Antioxidants such as vitamin C and E oxidize and turn brown - discard discolored product.
Cosmetic versus drug
Classification follows the claim, not the ingredient. A moisturizer that only hydrates is a cosmetic; the same base claiming to treat acne or prevent sunburn is a drug. That is why sunscreens carry drug facts panels and why "repairs DNA" claims are regulatory problems.
Label reading
INCI names are listed in descending order of concentration down to about 1%, after which order is unrestricted. PAO symbols (the open-jar icon) state months of safe use after opening.
Worked example
A client wants to layer an AHA serum, a retinol, and a benzoyl peroxide spot treatment nightly. Reasoning: the acid and retinol both provoke turnover, and benzoyl peroxide oxidizes retinol and can inactivate it, so the combination multiplies irritation without adding benefit. Alternate nights, buffer with moisturizer, and add SPF.
Traps
Judging an acid by percentage alone; assuming "natural" means non-allergenic; storing vitamin C in a clear jar; mixing products in the treatment room; using a water-based product past its PAO.