Skin Care Products, Chemistry, and Ingredient Function
Formulation basics
Skin sits near pH 4.5–5.5; bar soap is alkaline and stripping, most gel cleansers are closer to skin. An emulsion is oil plus water held together by an emulsifier, either oil-in-water (light, spreadable) or water-in-oil (rich). Any product containing water needs a preservative system; anhydrous products generally do not.
Functional families
A humectant draws and holds water (glycerin, hyaluronic acid, sorbitol). An emollient softens and fills cracks between cells (plant oils, esters, fatty acids). An occlusive seals in moisture (petrolatum, dimethicone, lanolin). Surfactants lift oil and debris: anionic cleans hardest, nonionic is mild, amphoteric is gentle and foams, cationic conditions. Chelating agents like EDTA handle hard-water minerals.
Actives
AHAs (glycolic, lactic) are water-soluble surface exfoliants; BHA (salicylic) is oil-soluble and reaches pores. Enzymes (papain, bromelain) digest dead cells gently. Vitamin C is an unstable antioxidant and brightener; niacinamide supports barrier and sebum control; retinoids speed turnover and are photosensitizing; azelaic acid targets redness and tone; hydroquinone is regulated and not an over-the-counter free-for-all.
Sun protection
SPF chiefly measures UVB; 'broad spectrum' signals UVA coverage. Inorganic filters are zinc oxide and titanium dioxide. Reapply about every two hours outdoors.
Traps
'Chemical-free' (impossible), 'natural equals safe,' higher percentage equals better, confusing humectant with emollient, and ignoring that an acid only exfoliates at the right pH.
Rule of thumb
Water in it, preservative in it. And match the molecule to the layer it can reach.