Chemistry, Ingredients, and Product Selection
Chemistry questions here are practical: pH, emulsions, and how an ingredient list predicts performance. You do not need organic chemistry - you need to know why a product stings, oxidizes, or separates.
pH
Scale 0–14; below 7 is acidic, above 7 alkaline. The skin's acid mantle is about 4.5–5.5. Alpha hydroxy acids need low pH (roughly 3–4) to exfoliate; raising pH reduces efficacy. Lye-based soap is alkaline and can disrupt the barrier.
Mixtures
- Solution: fully dissolved (toner).
- Suspension: particles dispersed and prone to settling (calamine).
- Emulsion: oil and water held together by an emulsifier - most lotions and creams. Oil-in-water is lighter; water-in-oil is heavier.
Functional ingredients
- Humectants pull water in: glycerin, hyaluronic acid, sodium PCA.
- Emollients soften and fill cracks: fatty acids, esters, squalane.
- Occlusives block water loss: petrolatum, dimethicone, beeswax.
- Exfoliants: AHAs are water-soluble and surface-acting; BHA (salicylic acid) is oil-soluble and works in pores.
- Antioxidants: L-ascorbic acid is unstable - it oxidizes yellow-orange and performs best near pH 3–3.5 with vitamin E and ferulic acid.
- Retinoids speed cell turnover and increase photosensitivity.
- Preservatives (parabens, phenoxyethanol) prevent microbial growth and are required in water-containing products.
Reading a label
Ingredients are listed in descending order down to 1%; below 1% they may appear in any order, so botanicals at the end are often marketing. Check the period-after-opening symbol and discard expired products. Natural and organic are not safety guarantees; fragrance and essential oils are common sensitizers.
Worked example
A vitamin C serum has turned dark orange. It has oxidized, the active is degraded, and it may irritate. Discard it and advise storage away from light and heat.
Traps
Assuming a higher percentage is always better; layering AHAs with retinoids in one routine; believing any product is chemical-free; and thinking preservatives are optional in water-based formulas.