Whenever we are asked to formulate an emulsion, we think of and usually add a humectant. A humectant being defined as “a substance that promotes retention of moisture.” It is derived from Latin “hu-mecta-ns, hu-mecta-nt-, present participle of hu-mecta-re, to moisten, from hu-mectus, moist, from hu-me-re, to be moist.” But these versatile materials deserve our respect. They have functions that go far beyond this simplistic definition!
Let’s start with their chemistry. All humectants have something in common: hydroxyl groups. These groups allow them to partake in the association process known as hydrogen bonding, which occurs when an atom of hydrogen (as part of the hydroxyl group) is attracted by rather strong forces to, for example, a molecule of water. This affinity to bind to and attract water is the reason why humectants are called moisturizers. In fact good humectants have several hydroxyl groups.
Hydrogen bonding is very important and actually plays a key role in emulsification and many other processes that we take for granted. Hydrogen bonding is the reason water is a liquid. Just think about it. Carbon dioxide is a gas and yet it has a molecular weight greater than water. But the water, because of its ability to have internal (intermolecular) hydrogen bonding, becomes a liquid.