We all know what a moisturiser does (it moisturises, duh!) - but what exactly goes in one, and how does it work?
Skin dryness is, quite simply, when your skin loses too much water (the technical term is transepidermal water loss, or TEWL). Moisturisers are used to slow water evaporating, and restore the original soft texture and look of your skin. There are three basic types of ingredients commonly found in moisturisers:
Occlusives slow down evaporation of water from skin by forming a thin, oily layer over the skin (remember from the face washing guide how oils repel water?). These work best when applied to damp skin, such as after a shower. Sometimes these feel too greasy to be comfortable.
Examples: petroleum jelly/Vaseline (super effective, 98% reduction in water loss in some tests!), lanolin, mineral oil, silicones (e.g. dimethicone), vegetable oils, cetyl alcohol, stearyl alcohol, stearic acid, lecithin, cholesterol, candelilla wax, carnauba wax, beeswax
Here are the chemical structures of some occlusives:
As you can see, they have long zigzags without letters on them. This is important because each corner on a structure represents a carbon atom with a bunch of hydrogens. When you have these long zigzag carbon chains, the chemical tends to be "oily" and won't dissolve in water well, and won't evaporate easily, which is why they make good occlusives.
Humectants grab onto water from the surrounding environment and draw it to your skin, and keep it there. It also draws some water from deeper layers of your skin (dermis) into the surface layers (epidermis), which can be counterproductive as it'll speed up water evaporation into the environment. This is why humectants are almost always combined with an occlusive.
Examples: glycerol, urea, hyaluronic acid, ethylene glycol, propylene glycol, sorbitol, pathenol, honey
These are the structures of some humectants. As you can see, they have lots of OH or NH2 groups attached - these are very good at grabbing onto water.
In our lab, it's very annoying to make things with lots of OH or NH2 groups, since it's depressing watching your lovely white crystals turn into sludge before your eyes on a humid day!
Emollients soften skin, making it more flexible and smooth. They act by filling in gaps in the surface skin, and assisting in skin repair. They won't necessarily affect the moisture content of skin and therefore aren't "true" moisturisers, but since they are usually lipids and oils, and some emollients are also occlusive.
Examples: cyclomethicone, dimethicone, isopropyl palmitate, isopropyl isostearate, castor oil, glyceryl stearate, jojoba oil, propylene glycol, vitamin E, cholesterol
Generally in a moisturiser, you'll want some of each type of ingredient, but the relative amount will vary according to skin type and climate, e.g. occlusives might not be necessary for someone with oily skin, and may in fact result in pimples, whereas some humectants can be too irritating for people with dry or sensitive skin.
There are of course lots of other things that might be in your face cream, such as water, specific active ingredients, colours, fragrances, preservatives and UV-blocking sunscreens - but these are the three that make a moisturiser, a moisturiser!
J. N. Kraft and C. W. Lynde, Moisturizers: What They Are and a Practical Approach to Product Selection. Skin Therapy Letter 2005, 10, 1-8.
Z. D. Draelos, Therapeutic moisturizers. Dermatologic Clinics 2000, 18, 597-607.