Chemistry, Ingredients, and Product Knowledge
pH drives behavior
The scale runs 0–14; below 7 is acidic, above is alkaline. Skin's acid mantle sits around 4.5–5.5, which is why alkaline cleansers leave skin tight and why high pH swells hair and lifts the cuticle. Buffering, acids donating hydrogen ions, and bases accepting them are worth understanding conceptually rather than memorizing equations.
Vehicles and forms
A solution is dissolved; a suspension has undissolved particles; an emulsion combines oil and water with an emulsifier. Serums are light and water- or silicone-based; creams are emulsions; ointments are largely anhydrous and occlusive. Surfactants clean because one end loves water and the other loves oil. Humectants (glycerin, hyaluronic acid) draw water in; emollients (fatty acids, oils) soften and fill gaps; occlusives (petrolatum, dimethicone) block water loss.
Actives to know
AHAs (glycolic, lactic) are water-soluble surface exfoliants; BHA (salicylic acid) is oil-soluble and works into pores. Retinoids speed cell turnover and cause photosensitivity. L-ascorbic acid is an unstable antioxidant needing low pH. Niacinamide supports barrier and tone; peptides signal repair. Physical sunscreens (zinc oxide, titanium dioxide) sit on skin; chemical filters absorb UV.
Preservation
Any formula containing water needs a preservative system, because water supports microbial growth. Anhydrous products generally do not. Antioxidants slow rancidity of oils. Separated emulsions, off odors, and texture changes mean discard.
Traps
"Natural" does not mean safe or hypoallergenic. A higher percentage is not automatically stronger - pH and delivery matter.
Rule of thumb
Water in the formula means preservative needed; oil-only formulas do not support microbes.