Hyaluronic acid may receive most of the attention, but it is not the only useful hydration ingredient. Glycerin and sodium PCA are also used to help a formulation hold water. You may already have several of these ingredients in one moisturiser.
Start with the shared job
Humectants help attract and retain water. In skincare, their effect depends on the formulation and the surrounding conditions. A product can combine humectants with emollient and occlusive components, so it is often more useful to assess the whole moisturiser than to search for a single winning molecule. S11
The immediate feeling of a watery product is not a measurement of lasting hydration. Likewise, a slightly tacky finish does not reveal how much useful moisturisation a product provides.
Hyaluronic acid and sodium hyaluronate
Hyaluronic acid is a water-binding molecule. Sodium hyaluronate is its sodium salt and is widely used in cosmetics. Different molecular sizes and formulations are studied for different purposes. A trial of several topical 0.1% hyaluronic acid creams found improved hydration compared with the control formulation. S18
That evidence does not mean that a serum acts like an injectable filler or reaches every skin layer described in a promotional diagram. Claims about an exact depth, a dramatic volume of water or a guaranteed plumping time need evidence appropriate to that product.
Glycerin
Glycerin, also called glycerol, is a familiar humectant with human research behind its use. In a controlled trial of a 20% glycerin cream, hydration measurements improved compared with the base cream. Its effects on other barrier measures were not identical to its effect on hydration. S46
This is why plain-looking ingredients should not be dismissed as fillers. It is also why an ingredient's presence does not justify promising every possible barrier benefit.
Sodium PCA
Sodium PCA is a salt of pyrrolidone carboxylic acid. Cosmetic ingredient safety assessments describe PCA and its salts as skin-conditioning humectants. This establishes their cosmetic role; a safety assessment is not proof of a particular moisturiser's clinical performance. S19
You do not need to memorise the full chemical name. On an ingredient page, the useful explanation is that it can contribute to a product's water-holding function.
Do you need a separate hydrating serum
Not necessarily. If your moisturiser already gives you comfortable hydration, a serum is an optional extra. If you enjoy a serum, follow its instructions and assess it within your routine rather than judging it by the first few seconds after application.
Applying a moisturiser after washing, while a little moisture remains on the skin, can be useful for dry skin. That does not mean every active product should be applied to wet skin; follow the instructions for treatments and sunscreens. S25
If a serum alone leaves you tight later, consider a moisturiser containing complementary ingredients. Constantly misting your face or stacking multiple humectant serums is not a necessary hydration protocol.
A practical way to choose
Compare comfort, finish under sunscreen and whether you need frequent reapplication. Keep a product that works for you even if its headline ingredient is less fashionable.
Hydration is a formulation job. Hyaluronic acid, glycerin and sodium PCA are different options that can work within that job; you do not need a separate step for each one.