Chemistry and Product Knowledge
Chemistry questions connect formulation to what happens on skin: why a cleanser strips, why an acid exfoliates, why a serum turns brown.
pH and the acid mantle
pH is logarithmic - each unit is a tenfold change in hydrogen ion concentration. Skin sits near pH 4.5–5.5. High-pH cleansers swell the corneum and leave it feeling tight; low-pH formulas favor barrier lipids.
Ingredient families
- Surfactants lift oil and debris; harsh ones over-strip.
- Emulsifiers keep oil and water mixed in creams and lotions.
- Humectants bind water (glycerin, hyaluronic acid); emollients soften and fill gaps; occlusives slow water loss (petrolatum, dimethicone).
- Alpha hydroxy acids (glycolic, lactic) exfoliate; effectiveness depends on concentration, pH, and time, not the label percentage alone. Beta hydroxy acid (salicylic) is oil-soluble and suits congested pores.
- Antioxidants such as L-ascorbic acid neutralize free radicals but oxidize with light and air, turning yellow-brown.
Labeling and claims
Cosmetics cleanse, beautify, and alter appearance. Products that treat disease or change structure are drugs by definition. "Natural" and "chemical-free" are marketing terms, not safety data; preservatives prevent microbial growth and are chosen for a reason.
Worked example
A client's vitamin C serum has darkened and smells sharp. Reasoning: L-ascorbic acid oxidizes to dehydroascorbic acid, which is ineffective and irritating. Discard it and advise opaque packaging and cool storage rather than using it up.
Traps
Assuming a lower pH is always better; believing botanical means hypoallergenic; ignoring that acids and retinoids stacked together cause irritation; expecting a cream to reach the dermis.