When you look at a cleanser formula and notice three or four different surfactants listed, do you wonder whether that's just "filling", or whether each one genuinely serves a purpose? In an ideal world, each is doing something different, and the combination makes the system work. The surfactant carrying the most weight in a formula is usually called the primary surfactant. Its job is the main cleaning function: removing oil, sebum, and debris from skin. It's typically the one with the highest percentage and is chosen for its cleaning efficiency and lather characteristics (this also means it tends to be anionic) . SCI (sodium cocoyl isethionate) and sodium lauryl sulfoacetate are common primary surfactants in rinse-off bar formats. The secondary surfactants come in to address things the primary does well at but not perfectly. Many effective primary surfactants can be slightly stripping on their own, so an amphoteric like cocamidopropyl betaine is added not necessarily for cleansing power, but to soften the overall action, improve skin feel, stabilise the foam and often to help thicken the formula. Some secondaries also boost the lather volume or quality, making the formula feel more luxurious without affecting the actual cleaning performance. There's also a third function that often gets overlooked: conditioning and mildness. Surfactants like coco-glucoside or even Lamesoft PO65 (refatting) can add a mild conditioning effect to the rinse, leaving skin feeling less tight. Non-ionics are gentler, so they pair very well with stronger anionics. The real skill in building a surfactant system is knowing what each ingredient contributes and in what ratio. A cleanser that's 100% primary surfactant will be harsher than it needs to be. A cleanser that's mostly secondary surfactants won't clean well enough. So the point I am trying to make is: find the right balance. Try this: Look at the INCI list of a commercial cleanser you like. See if you can identify which surfactant is likely the primary and which are secondaries. What does the combination tell you about what the formulator was going for?

Most formulators learn to watch for the obvious signs of a failed formula: separation, off-smell, and visible contamination. These are all signs of instability, but they're signs of one specific type. There are three types of cosmetic instability, each requiring a different testing approach. 🧪Physical instability is the most visible. It covers changes to the product's appearance, texture, and structure: phase separation in emulsions, graininess in butters, colour and texture changes. This is what most people think of when they think about stability testing, and it's the easiest to assess by observation. 🧪Chemical instability is largely invisible. It covers changes to the formula's chemistry over time: pH drift, oxidation of fatty acids or actives, degradation of preservatives or active ingredients, breakdown of specific compounds under heat or light. A formula can look completely unchanged while its pH has drifted far enough to compromise the preservation system. A vitamin C serum can appear clear and unchanged while the active ingredient is oxidising. 🧪Microbial instability is the most consequential and the hardest to see. It covers the failure of the preservation system, leading to microbial growth. A product can maintain perfect physical appearance while supporting active growth in the water phase. Microbial stability cannot be confirmed without challenge testing by a microbiological laboratory, so this cannot be done at home. The important point: these three types can fail independently, even though they are often connected. Good physical stability tells you nothing about chemical or microbial stability. A complete assessment needs to address all three. For those of you making products for personal use, the minimum to build toward is monitoring pH over time alongside your visual checks. For anyone selling, challenge testing is required for the safety assessment process in the EU and the UK. Try this: next time you assess a formula you've been testing, think about which type of stability you're actually evaluating. Is it physical only? Are you measuring pH? What are you doing about microbial stability? 🌼