The Same But Different
The thing that makes serum so effective is also what makes it so different from a moisturizer. Serum is a lightweight gel or water like substance that has a high amount of active ingredients. Because serum doesn’t include the thicker oils and waxes found in moisturizers, it has a low molecular weight that makes it easy for these actives to penetrate and enrich your skin on a cellular level.
But because serum lacks sealing ingredients like oils and waxes, it doesn't have anything to lock those ingredients in.
Moisturizer on the other hand, is designed to hydrate and and nourish skin. It contains heavier molecular weight emollient ingredients that provide a physical barrier to keep hydration in, and dirt and pollutants out. Because it’s a thicker product, it’s less effective at penetrating deep within skin with active ingredients. That’s not to say it won’t have some wonderful skin benefiting vitamins or botanicals, but those actives are usually a lower percentage than in a serum, and designed to work more on the surface or first layer of skin.
Serum and moisturizer make such a dynamic duo because their strengths compliment each other. While serum can pack skin with incredible ingredients, it benefits from the barrier that moisturizer provides to lock those actives in. Your skin also benefits from that layer of moisturizer to hydrate, nourish, and protect from external elements.
And on the flip side, a moisturizer is too thick to impact skin deep down on the cellular level- so for high tech, age defying ingredients, you want to use a serum under your moisturizer.