Someone asked me this questions and I thought it was quite interesting, How Do Oil-Free Moisturizers Work?
An easy way to explain this is that while most feel that regular oil containing moisturizers provide hydration, they’re doing the real work of preventing your skin from losing valuable moisture from deep within. Oils offer a superior method of accomplishing this–not because they “oil up” the skin, but because they create a moisture-sealing barrier for the skin. There are many types of moisturizers that can do with without the use of oils. For instance products with Hyaluronic Acids are great for people who want to avoid the oils. The ability of Hyaluronic Acid to bind water makes it a great humectant and moisturizing ingredient for skin care products; it has the ability to pull moisture from the atmosphere and trap it on the surface of the skin, and it has been shown to deeply penetrate the stratum corneum and enter the epidermis.