Recently, I had a question come into my box I found very interesting. The person was curious why the shampoo she was using still generated lather, despite the fact that it did not contain sodium lauryl sulfate or sodium laureth sulfate. I immediately thought “Oh! What a wonderful topic for an article this month!” I thought we could discuss the nature of bubbles, lather, and foam, what causes them to occur, and what ingredients are likely to enhance or minimize their formation.
Imagine my surprise when I started doing a little research to refresh my understanding of bubble formation and found this topic to be an area of much intense research, most of which seemed to be far over my head! Many physicists, astronomers and chemists devote their entire lives attempting to develop a fundamental understanding of bubbles, foam, and lather. So, while feeling a little daunted, I decided to carry on and do my own rudimentary analysis of the subject.
What is a bubble?
A bubble is a ball of gas encapsulated by a microscopically thin film of liquid. The formation of a bubble is made possible by the fact that the surface of a liquid behaves much like a skin. This behavior is due to intermolecular forces.
Water molecules possess a high degree of polarity, due to the presence of two small electropositive hydrogen atoms on one end of the molecule and a single, larger electronegative oxygen atom at the other end. These polar molecules tend to be attracted to one another, and arrange themselves so that there is plenty of dipole-dipole interaction, allowing the hydrogen atoms to align with oxygen atoms, and vice versa.
This phenomenon is known as hydrogen bonding, and while it is technically a physical interaction, the strength of these interactions can be quite high. Hydrogen bonding is responsible for many of the unique properties of water such as its high boiling point, relative to its molecular weight, its relatively high density when in liquid form, and the very interesting fact that it is less dense as a solid than as a liquid.
Within the bulk of the liquid, molecules are in constant motion and are continually encountering one another and then moving into another portion of the liquid. This constant motion means that the dipole-dipole forces exerted on each molecule come from all directions, so the forces are evened out. However, at the air-water interface of a solution, there are not significant levels of intermolecular forces coming from the direction of the air phase. For this reason, the water molecules at the surface experience a much stronger pull in the side-to-side and underneath directions. This has the effect of creating stronger resultant forces (cohesive forces) between water molecules at the surface, which creates a phenomenon known as surface tension. Surface tension is what creates skin-like behavior at the surface of the water.
Figure 1. This is a snapshot of a computer simulation of H2O molecules in liquid water. The dashed blue lines represent hydrogen bonding between the molecules as they flow through the liquid. (Image courtesy of Wikipedia)
So what does all this have to do with bubbles? This surface tension is what makes the formation of bubbles possible. When air (or another gas) is forced into water (via agitation, blowing, rapid flow from a faucet, or injection via any other means) and bubbles form, surface tension in the thin layer of liquid that forms the skin of the bubbles draws the bubble tightly into the shape, which has the least surface area to the highest volume (which is a sphere), and this is called a minimal surface structure.
Pure water will form bubbles, which I am sure you have noticed when filling a glass of water from the faucet. However, you have probably also noticed that these bubbles don’t stick around for long. This is actually a result of the extremely high surface tension of water, due to all those hydrogen bonding forces. The addition of soap or surfactant molecules to an aqueous solution acts to bring down the surface tension of the water (a phenomenon discussed briefly in other articles I have written for this site). By lowering the energy at the air-water contact point, soap molecules act to stabilize this interface. That is why the presence of soap or surfactant molecules in water will act to stabilize bubbles that form.
Soap, Lather and Foam
When there is sufficient soap present to stabilize bubbles, multiple bubbles can begin to meet and come together to form a complex structure called foam.
When three or more bubbles join together, the walls always meet at angles of exactly 120°. Nature can be amazingly precise! Eventually this becomes a complex structure comprised of hexagonal cells, which looks a lot like a beehive. (This type of hexagonal structure is one of very low energy, a state highly favored in nature). This conglomerate of bubbles is called foam, which is really all that lather is.
Certain surfactants, due to the simplicity of their structure, very readily form and stabilize bubbles and foams. For this reason, in the formulating world, they are often referred to as foaming agents. They typically produce lots of lather when they are present in a shampoo or skin cleaner and are frequently included in products in order to boost this effect, as it is considered to be desirable by many consumers.
Sodium lauryl sulfate and sodium lauryl ether sulfate (SLS and SLES) are both well-known for producing lots of lather when used in shampoos. These two ingredients are also classified as harsh cleansers, especially for those of us prone to damage done by the effects of stripping oils away from the hair. For this reason, it has become a commonly held belief that foam and lather necessarily correlate with potentially damaging effects.
However, many of the gentler cleansers, such as cocamidopropyl betaine are also used as foam boosters, and will create lather when a product containing them is used to cleanse the hair. While it is true that some of the extremely mild, larger molecule cleansers will only foam minimally or not at all (a fact that is disconcerting to many consumers), most surfactants will cause lather formation to some extent. Other ingredients in the shampoo, as well the hardness of water being used will affect foam formation also (most often adversely).
It is important when evaluating a shampoo to remember that some foaming and lather formation are actually not indicative of a cleanser being harsh. It is best to avoid products that contain SLS and SLES if your hair is dry and delicate. However, if you choose a shampoo that has an apparently gentle ingredients list, but you still get some lather, fear not! Conversely, if you enjoy a little lather when washing your hair but your gentle shampoo doesn’t do this for you, don’t worry that your hair won’t be clean. Foaming and lather formation are not the main phenomenon by which one should gauge the effectiveness or gentleness of their product.
Sidney Perkowitz, professor of physics at Emory University and the author of "Universal Foam," describes foams as examples of soft matter: They don't flow freely like a true liquid, but neither are they a crystalline solid like a diamond. "We're very good at explaining hard matter like crystals; the entire semiconductor industry is based on them," he says. "Soft matter seems to tell us a lot more about nature and biology."