In the What do you want to know? post, Nicole asks: When I first started making anhydrous products, my opinion was that they were the best possible thing for my dry chapped winter hands and dry feet, and that lotions would be better for me for summer. My logic was that if some oil is good then more oil is better, and I viewed water as a filler or a means of diluting the oil if you don’t need that much of it. After reading a lot if your website, I'm starting to wonder if there isn't more to it than that. I know that commercial companies want to save money so they reduce the oil and add thickeners back in, but does water have a special role that I’m overlooking?
The inspiration for this question is that I was pondering humectants. I understand that humectants draw water from the air into your skin. So do they work better if there is water in the product to draw into your skin right away? Does a humectant in a lotion work better than in an anhydrous product (I’m thinking olive oil here, with natural humectant properties) What is the full role of water in a lotion?
Let's take a moment to learn a little about our skin. (From this post - the chemistry of our skin (updated) I really recommend reading it as this is a quick summary.) The top layer of our skin is called the stratum corneum, and it's a dead layer of cells called corneocytes that contain something called the natural moisturizing factor, (NMF) a bunch of awesome water soluble chemicals that help prevent transepidermal water loss (TEWL) or water loss from our skin. The natural moisturizing factor needs to be kept moist to keep our skin feeling elastic and to reduce skin scaling, cracking, or flaking.
We can reduce scaling, cracking, and flaking of our skin and reduce transepidermal water loss by using products we make. Anhydrous or non-water containing products are great for trapping in the moisture you have on your skin, but if you don't have much, you can't trap in much. This is one of the reasons we use products like lotions that contain water - they can bring water to our skin and the oils can trap it against our skin, so we remain moisturized. Anhydrous products might be great for trapping in the water we have, but water-in-oil products are great for bringing water to our skin and replenishing the supply in the NMF.
So water isn't just a filler, it's a necessity to help our skin remain elastic and reduce transepidermal water loss. If we don't have enough in our skin, using a product that contains water introduce it to our skin to help replenish it.
Where do humectants fit into this whole picture? Humectants are ingredients that draw water from the atmosphere to the surface of our skin. If you can draw water to the skin then trap it, you can increase moisturization. Ideally we would have an occlusive ingredient on our skin to trap in the water that it lures, and studies have shown that humectants need occlusive ingredients in low humidity environments to work effectively. (Click here for that post...)
The only humectant we can use in an anhydrous product would be olive oil, and it's not nearly as good as something you'd find in something like a lotion, something like glycerin or sodium lactate or honeyquat. It's not because it's in an anhydrous product, but because olive oil just isn't as effective as these other humectants.
So the short answer is that water in our products isn't a filler; it's there to help our skin remain or become more elastic and reduce cracking, flaking, and scaling and other signs of dry skin. And we can add humectants to our products to