5.1 Basic Chemistry: pH, Acids & Alkalinity

Basic Chemistry: pH, Acids & Alkalinity

Understanding basic chemistry gives you the "why" behind product selection, exfoliation, and skin reactions — knowledge the NY written exam tests directly under the curriculum's Chemistry and Chemistry as Applied to Esthetics units.

The pH Scale

pH stands for "potential of hydrogen" and measures how acidic or alkaline (basic) a substance is on a scale of 0 to 14:

• 0–6.9 = acidic (lower numbers = more acidic)
• 7 = neutral (pure water)
• 7.1–14 = alkaline/basic (higher numbers = more alkaline)

The scale is logarithmic, meaning each whole-number step represents a tenfold change in acidity or alkalinity. A substance at pH 4 is ten times more acidic than a substance at pH 5, and one hundred times more acidic than a substance at pH 6.

The Skin's Acid Mantle

The surface of healthy skin is covered by a thin, slightly acidic film called the acid mantle. It forms when sebum (oil), sweat, and dead skin cells combine on the skin's surface. Normal, healthy skin typically measures roughly pH 4.5 to 5.5 — mildly acidic, not neutral.

The acid mantle isn't incidental; it does real protective work:

• Defends against pathogens. Most bacteria, viruses, and fungi grow best in a more neutral-to-alkaline environment, so an intact, mildly acidic surface makes it harder for many pathogens to thrive.
• Regulates moisture. The acid mantle helps support the lipid barrier that slows transepidermal water loss (TEWL), keeping skin hydrated.
• Maintains barrier integrity. A stable surface pH supports the enzymes involved in normal skin-cell shedding and barrier repair.

Why Product pH Matters

Because the acid mantle sits in a fairly narrow, mildly acidic range, products that are significantly more alkaline than skin — some traditional bar soaps run pH 9-10 — can strip the acid mantle on contact. Repeated exposure to overly alkaline products can:

• Disrupt the lipid barrier, increasing water loss and dryness
• Leave skin more vulnerable to irritation, sensitivity, and breakouts
• Create a surface environment more favorable to certain problem bacteria

Skin has some ability to buffer and restore its own pH after a single alkaline exposure, but chronic use of harsh, high-pH products can overwhelm that self-correcting capacity over time. This is why estheticians favor cleansers and other leave-on products formulated close to the skin's natural pH range, and why a toner is often recommended immediately after cleansing — partly to help rebalance surface pH if an alkaline cleanser was used.

States of Matter & Mixtures in Cosmetic Chemistry

Cosmetic products are built from combinations of ingredients that behave differently once mixed. Three mixture types come up constantly in esthetics:

• Solution. One substance (the solute) fully dissolves into another (the solvent), forming a uniform mixture that will not separate on standing. A toner with dissolved humectants in a water base is a solution.
• Suspension. Solid particles are dispersed throughout a liquid but do not dissolve. Because the particles are not chemically dissolved, they will eventually settle out, which is why suspensions like calamine lotion carry "shake well before use" instructions.
• Emulsion. A mixture of two liquids that would not normally combine (are immiscible) — most often oil and water. Left alone, oil and water separate into layers, the same way oil floats on vinegar in a salad dressing.

Emulsifiers: Making Oil and Water Mix

Because oil and water naturally repel each other, a stable emulsion requires an emulsifier — an ingredient with a water-loving (hydrophilic) end and an oil-loving (lipophilic) end. The emulsifier positions itself between the two phases, allowing tiny droplets of one liquid to stay evenly dispersed throughout the other instead of separating out.

There are two common emulsion structures in skin care:

• Oil-in-water (O/W): small droplets of oil are dispersed within a continuous water phase. These formulas tend to feel lighter, absorb faster, and are less greasy — a common choice for oily, combination, and normal skin.
• Water-in-oil (W/O): small droplets of water are dispersed within a continuous oil phase. These formulas feel richer and more occlusive, and are often used in heavier night creams, barrier-repair creams, and products designed for very dry skin.

Common Acids Used in Esthetics

Two acid families dominate professional and retail exfoliating products:

Alpha-hydroxy acids (AHAs) are water-soluble acids historically derived from fruit and milk sugars. Glycolic acid (from sugar cane) has the smallest molecule of the common AHAs, which lets it penetrate more readily; lactic acid (from milk) is slightly larger and also has humectant properties, making it a gentler option for drier or more sensitive skin. AHAs work at the skin's surface, loosening the "glue" (desmosomes) that holds dead corneocytes together so they shed more readily, which exfoliates and smooths the epidermis. Because AHAs are water-soluble, they act primarily on the surface. They are generally most effective in a formulation pH of roughly 3 to 4, and because they thin the outer layer of dead cells, they increase sun sensitivity — making SPF counseling essential for any client using them.

Salicylic acid is the beta-hydroxy acid (BHA) most used in esthetics, derived from willow bark or wintergreen. Unlike AHAs, salicylic acid is oil-soluble (lipophilic), which allows it to travel into the lipid-filled lining of the follicle/pore itself rather than staying on the surface. That makes it especially effective for oily and acne-prone skin, blackheads, and congested pores. Like AHAs, salicylic acid is generally formulated in a similarly low, acidic pH range (roughly 3-4) to remain chemically active as a "free acid" available to exfoliate — if a product's overall pH is buffered too high, the acid becomes less effective even when it's listed prominently on the label.

Both AHAs and BHA are considered chemical exfoliants, distinct from mechanical/physical exfoliation methods (scrubs, microdermabrasion) covered elsewhere in this guide.