The Complete Guide to Acids

THE SCIENCE BEHIND IT…
OVERALL CLINICAL DATA ON SPECIFIC INGREDIENT(S) & HOW THEY FUNCTION:

Acids can be incredibly versatile in skincare. Their structure, size, and chemical properties all influence how they function on the skin and what benefits they offer. Below, we break down our favorite acids by their structural family and how these features shape their cosmetic behavior.

How Acids Interact With Skin

The skin’s outer layer—the stratum corneum—acts as a natural barrier. Its layered lipid structure slows the movement of charged, water-loving, or large molecules while allowing smaller, uncharged, or more lipid-friendly molecules to move more freely. This is especially important when discussing acids, as their pH, charge, and molecular structure influence how effectively they interact with the skin. Each acid has a unique pKa, the point at which half the molecules are uncharged (“free acid”) and half are ionized. The free acid form is generally better able to pass through the skin barrier, which is why pH plays a key role in how exfoliating an acid feels. Lower pH typically means more free acid, and therefore a stronger exfoliating effect. However, formulas must balance efficacy, comfort, and stability.¹

Carboxylic acids in Skincare

Many well-known skincare acids—alpha hydroxy acids (AHAs), beta hydroxy acids (BHAs), polyhydroxy acids (PHAs), tranexamic acid, azelaic acid, and retinoic acid—share a common carboxyl group (COOH). What differentiates them is how the rest of the molecule is structured.

Alpha Hydroxy Acids(AHAs)

AHAs have a hydroxyl (OH) group on the carbon next to the carboxyl group. They are known for softening and loosening the outer layers of the skin, revealing a smoother, more polished appearance. Common AHAs include:

Lactic acid is a gentle AHA known for its smoothing and hydrating qualities. Because it has a larger molecular size than glycolic acid, it tends to penetrate more slowly, making it a great option for those seeking mild exfoliation.

Mandelic acid is an aromatic AHA, meaning it has a ring-like structure that makes it more oil-soluble. This feature allows for gradual exfoliation and supports a balanced-looking complexion.

Glycolic acid is the smallest AHA, allowing it to move more readily through the skin’s surface layers. It’s often used to help refine the look of texture, support radiance, and visibly smooth the skin.

Beta Hydroxy Acids

BHAs have a hydroxyl group located two carbons away from the carboxyl group. They are especially helpful for refining the look of pores and texture.

Salicylic Acid, is the most common BHA and has an aromatic ring that makes it oil-soluble. This allows it to work within the look of pore congestion and help visibly smooth the skin’s surface. Its exfoliating activity is less dependent on pH than AHAs.

Polyhydroxy Acids(PHAs)

PHAs contain multiple hydroxyl groups, giving them both exfoliating and hydrating characteristics. They are often described as next-generation AHAs. Because PHAs have larger molecular structures, they tend to penetrate more slowly, making them a great option for sensitive skin. Their multiple hydroxyl groups also attract water, helping support a well-hydrated feel and reinforcing the skin’s moisture barrier. They also pair easily with other actives.

Azelaic Acid

Azelaic acid is a dicarboxylic acid, meaning it has two carboxyl groups. Unlike AHAs and BHAs, it does not contain hydroxyl groups along its chain. Because of its molecular size and structure, azelaic acid offers gentle exfoliation and helps support a balanced-looking complexion. It is often chosen for its ability to visibly smooth texture and help the skin look calm and even.

Tranexamic Acid

Tranexamic acid is a monocarboxylic acid structurally similar to the amino acid lysine. It is frequently used in cosmetic formulas to support a more uniform-looking complexion. It pairs well with ingredients like kojic acid, which helps enhance brightening routines through different molecular pathways.

Choosing the Right Acids for Your Routine

There is an acid suited for nearly every skin type. Whether you want gentle exfoliation, help refining the appearance of pores, added radiance, or support for more even-looking tone, acids can be tailored to your needs. Because of their diversity in structure and activity, they can easily be incorporated into a wide range of skincare routines

DISCLAIMER:

Lab Journals are intended to help educate on specific ingredients and skin care topics. Our articles are written to be informative and informational.
Please note any Naturium products with referenced ingredients are formulated for Cosmetic Use Only and NOT intended as replacements for physician pharmaceutical product recommendations.