Surfactants are a necessary part of getting dirt and pollution (in other words, normal gunk as opposed to stubborn silicones and clingy oils such as mineral oil) out of hair. They lift the dirt and separate it from our hair (when we rinse, that is). Hence the image I've used here. A little conceptual, I know, but I get tired of using only hair-related images. Sometimes life isn't only about hair. Sometimes it's about boobs, too. But there I go digressing again.
In layman's terms, here's how surfactants work, per Tonya, our favorite Curl Chemist:
At sufficiently high concentrations in water, surfactant molecules group together to form three-dimensional structures known as “micelles”. These structures are clusters of molecules with an oily center made up of the non-polar tail, surrounded by a shell formed by the polar portion of the molecule. These micelles absorb oils from your skin, hair or clothes, and trap them inside until they are removed from the surface by the rinsing phase of the process. Another very important property of surfactants is their ability to produce significant foaming effects, an attribute considered to be desirable by many product developers.
Tonya, it turns out, just this month created an informative article about the nature of surfactants. (This is point where a well-placed "Great minds think alike" comment gets inserted and I act a little smug.) To summarize what she says, surfactants are chemically composed in such a way that they interact well with oils and water. Because of this chemical composition, surfactants get bubbly when you use them. (This is a desirable quality in hair products. Bubbles are not only fun, apparently, but they make users believe the product is doing something.)
Surfactants come in a variety of strengths, in terms of their effect on hair and skin. One category is the sulfates, which the Curly Girl philosophy urges us to avoid. Sodium laureth sulfate, ammonium lauryl sulfate, ammonium laureth sulfate, and sodium myristyl sulfate and sodium myreth sulfate seem to be the harshest, with sodium lauryl sulfate, sodium myreth sulfate, sodium coco sulfate and sodium C14-16 olefin sulfonate being slight less harsh. Sodium coco or cocoyl sulfate, despite claims that they are gentler than its petrochemical cousins, is not any gentler. Just different because of the coconut oil it's derived from.
Less drying surfactants are the nonionic surfactants, such as sorbitol, decyl glucoside, laureth 4-20, and decyl polyglucose. Tonya's article ends by telling us that many of the cleansing products out there today use co-surfactants, such as fatty alcohols, nonionic surfactants, amphoteric surfactants (cocamidropropyl betaine), and even some oils to reduce the drying tendencies.
For a great list of surfactants, check out this page. If you do not shampoo, the anionic surfactants probably will be irrelevant to you because they are primarily found in traditional shampoos. If you like to shampoo every so often, however, check out the amphoteric surfactant list -- these are the ingredients typically found in milder shampoos.
The cationic surfactant list was useful to me because I've been noticing that conditioners with Behentrimonium Methosulfate (EO Rose and Chamomile, Activate Hydrating, and Robert Craig, to be specific) really do well with my hair. Perhaps, then, it is the mild cleansing they provide -- my hair always feels a bit lighter and bouncier after I use those conditioners.
But what I really want to say here is that if you've been trying to go "natural" with your co-washing, you might be unhappy with the results. Why? Because the surfactant content is low, if not nonexistent. Take Karen's Body Beautiful, for instance. The ingredient list for the Hair Milk is:
Purified Water, Jojoba and Avocado Oils, Vegetable Wax, Vitamin E, Panthenol, Arnica and Calendula Flowers, Echinacea, Nettle and Horsetail Extracts, Vegetable Glycerin, Wheat Germ Oil, Fragrance Oil and Natural Enzymes
I don't see anything there that would do anything but condition your hair. Conditioning is great but for co-washing, you want something that will get rid of dirt and oil. Aside from the fact that the Suave Naturals and VO5 lines are wonderfully affordable, they are also chock full of surfactants. Use 'em, girls. Use 'em.