Quick refresher course on antioxidants: They prevent or reduce damage to our cells by neutralizing the production of highly reactive molecules called free radicals. In other words, antioxidants help keep our body and our skin cells healthy. This free-radical fighting power isn't just important for helping to keep signs of premature aging at bay. In neutralizing these free radicals, vitamin C may also help protect skin from precancerous changes caused by UV exposure.
So, in the same way that consuming antioxidant-rich foods aids your body in fighting off free radicals, topical antioxidants do the same for your skin by helping to offset daily aggressors like UV damage and exposure to air pollution.
Amazingly, vitamin C's skin-saving benefits aren't limited to its antioxidant status. It has plenty of other skin-healing properties that make it worthy of a permanent place in your medicine cabinet. For one, because it's highly acidic, when vitamin C is used topically the skin is triggered to heal itself by accelerating the production of collagen and elastin. It contains a property that inhibits your skin's melanin production, which is what causes skin discoloration like dark spots.
You'll also notice that Vitamin C serum bottles are dark — and that's on purpose. Vitamin C actually breaks down when it's exposed to light or air, so dark glass bottles help maintain its stability (and therefore, efficacy). As a general rule, serums should be layered underneath your moisturizer.
That said, dermatologists agree that the best vitamin C skin-care product to use is a serum, because serums are more effective than creams or toners. Moreover, adding vitamin E into the mix is like a one-two punch of skin-saving goodness.
Source ref: https://www.allure.com