When it comes to skincare, many of us find ourselves wondering if there’s a specific order we should follow to get the best results. Whether you’re layering multiple products or just keeping it simple, understanding how your skin absorbs ingredients can help you to maximize their effectiveness.
The Science of Skincare Absorption
The skin is a structured, layered barrier designed to protect your body. It’s neither a sponge nor a surface for skincare products to sit on. The outermost layer, called the stratum corneum, acts as the primary gatekeeper, only allowing certain molecules to pass.
Most skincare ingredients enter through tiny gaps between skin cells or by dissolving in water or fats already in the skin. But not all ingredients get a free pass. According to a 2018 editorial in Pharmaceutics, small and moderately lipophilic (fat-attracting) molecules under 500 Daltons with a log P between 1–4 are most likely to penetrate the skin effectively
- Daltons: This is a unit of molecular weight (mass). One Dalton is roughly equivalent to the mass of a hydrogen atom. When we say a molecule is “under 500 Daltons,” it means the molecule is relatively small in size, which makes it more likely to penetrate the skin.
- log P: This is a measure of how much a molecule likes fat versus water. It tells us how well a molecule will dissolve in fat (lipid) or water-based environments.
- A low log P means the molecule prefers water and may struggle to pass through the skin’s fatty layers.
- A high log P means the molecule prefers fat, which helps it pass through the skin’s lipid barrier but might make it harder to interact with the water-rich layers deeper in the skin.
- Moderate log P values (between 1-4) strike a balance, helping molecules pass through the skin barrier more effectively.
So, size and solubility (water- or fat-based) play a major role in how deep an ingredient goes. Larger or highly polar molecules may stay on the surface unless helped by penetration enhancers—ingredients like alcohols, oils (oleic acid), or nanoemulsions that boost delivery.
The Deeper Layers of Skin & Penetration Limits
Beneath the stratum corneum, the epidermis and dermis contain water, proteins, and cells. Water-soluble (polar) ingredients can move through these layers via hydrogen bonding. However, lipid layers in the stratum corneum act as barriers to water-soluble ingredients, only allowing smaller or appropriately soluble molecules to pass.
Product penetration generally stops when molecules can’t pass through due to size or solubility. Larger or highly polar molecules may remain in the superficial epidermis, while smaller molecules or those with the right solubility can reach deeper layers like the dermis, where they might target hair follicles or sweat glands. For example, minoxidil (a treatment for hair loss) is formulated with nanoemulsions to target hair follicles directly, ensuring it doesn’t penetrate too deeply. Sunscreens are also designed to stay on the surface and combat UV rays.
A Note on Compromised Skin Barrier
When the skin barrier is compromised—due to dryness, irritation, or conditions like eczema—the lipid layer in the stratum corneum is damaged. This allows larger or unwanted molecules to pass through, increasing both the absorption of active ingredients and the risk of irritants or allergens entering the skin, making the skin more vulnerable to external stressors.
Urea: A Great Example of Penetration
A perfect example of an ingredient that effectively penetrates the skin is urea. With a molecular weight of about 60 Daltons, it is extremely small, allowing it to travel deeper into the skin compared to many other ingredients. Its polar nature helps it interact with the water content in the skin, hydrating and softening it while also breaking down the buildup of dead skin cells. Urea’s ability to bind water and follow water pathways means it can penetrate deeper into the skin. The size of the molecule determines how far it can travel—small molecules, like urea, can move deeper into the skin than larger ones.
The Right Order for Your Skincare
The general rule of thumb is to apply products from lightest to heaviest, starting with the thinnest formulations and finishing with the thickest. Here’s a simple breakdown:
- Cleanser: Start with a cleanser to remove dirt, oil, and makeup from the skin.
- Toner (optional): Apply a toner to balance the skin and prepare it for better absorption of subsequent products.
- Serum: Next, use a serum, which is typically more concentrated with active ingredients like vitamin C, hyalauronic acid, or niacinamide.
- Moisturizer: After the serum, use a moisturizer to lock in hydration and keep your skin soft.
- Oil (optional): If you use face oils, apply them after your moisturizer for added hydration.
- Sunscreen: In the morning, always finish with sunscreen to protect your skin from harmful UV rays (also reapply throughout the day)
What About AHAs, BHAs, and Retinoids?
Exfoliants and retinoids are among the most effective active ingredients in skincare—but they also require some strategy when it comes to application. Alpha hydroxy acids (AHAs) like glycolic and lactic acid are water-soluble and work on the skin’s surface to dissolve dead skin cells, helping improve tone and texture. Beta hydroxy acids (BHAs) like salicylic acid are oil-soluble, meaning they can penetrate deeper into pores, making them ideal for acne-prone or oily skin.
Retinoids, on the other hand, are vitamin A derivatives that have a moderate log P, making them excellent candidates for deeper penetration into the skin. They work by increasing cell turnover and stimulating collagen production in the dermis.
If you’re using both exfoliants and retinoids, it’s usually recommended to alternate nights or layer with caution, starting with the gentlest formulation. AHA and BHA products should go on after cleansing and before serums or moisturizers, while retinoids are best applied after your skin has dried post-cleansing, followed by a moisturizer to reduce irritation.
Why Order Matters
The order of application matters because it allows your skin to absorb each product at the right depth. Water-soluble ingredients tend to stay closer to the surface, while fat-soluble ingredients can reach deeper layers. For instance, a serum designed for deeper penetration should be applied before a moisturizer, which acts as a barrier to lock in hydration. Sunscreen, however, should always be the last step, as it needs to form a protective layer on the skin.
Final Thoughts
Uunderstanding the science of skin absorption—like how small molecules move through water and fat layers—can help ensure your products are working their hardest. Whether you’re hydrating, exfoliating, or protecting, applying your skincare in the right order can make a noticeable difference. With that, skincare is not one-size-fits-all. There’s room to experiment and find what works best for you.
You’ve reached the end of this article. Thanks for reading! Hope you found this helpful.
The information in this article is for informational purposes and is not intended to be a substitute for professional medical advice. Always seek the advice of your physician regarding a medical condition or treatment.
Sources
Ng K. W. (2018). Penetration Enhancement of Topical Formulations. Pharmaceutics, 10(2), 51. https://doi.org/10.3390/pharmaceutics10020051
Yousef H, Alhajj M, Fakoya AO, et al. Anatomy, Skin (Integument), Epidermis. [Updated 2024 Jun 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470464/