Cleansing ingredients found in shampoos belong to a category of molecules called surfactants (surface active agents). These materials are comprised of both polar and non-polar segments. The polar segments are water soluble and are referred to as hydrophilic, which means water loving, while the non-polar segments are only very slightly soluble in water due to a lack of sites for favorable interactions with water molecules. The terms hydrophobic (water fearing) and lipophilic (oil loving) are used interchangeably to describe the non-polar portions. Since surfactant molecules possess both lipophilic and hydrophilic qualities, they are referred to as amphiphilic materials.
Frequently, the hydrophilic portion of the surfactant exists on a terminal end of the molecule, and for this reason, is often referred to as the head group. The hydrophobic segment of linear surfactant molecules is typically an alkyl- or aryl-containing chain, and is referred to as the tail group. (Figure 1.) Other types of surfactants, such as bulkier small molecule surfactants with multiple or branched tails, amphiphilic polymers, and biological materials have a more complex molecular architecture that gives them various geometric shapes. Many of these may not possess a distinct head group and tail group, yet they do have definite hydrophilic and hydrophobic segments.
When surfactants are dispersed into water, they cluster together first at the surface of the water, and then at higher concentrations, they cluster together in the bulk of the water and form aggregates known as micelles. In these clusters, the non-polar portions of the molecule all huddle together in the middle of the micelle, while the polar portions form a sort of shield on the outer rim of the micelle, creating a hydration shell and segregating the hydrophobic portions of the surfactant from most of the water. These types of micelles (oil-in-water) can absorb oil into their core and hold it there, while the hydration shell sort of holds the whole thing together.
Most surfactants used for cleansing hair and skin have a negatively charged ion at their head group (example: sulfate) and a positively charged counterion (example: sodium). These are called anionic surfactants. Positively charged (cationic) surfactants are typically used as emulsifiers to facilitate mixing of oils such as silicones and other polymers. Cationic surfactants are also attracted to the negatively charged surfaces of skin and hair, and so they can also be used as mild conditioning agents. Zwitterionic surfactants have both a positive and a negative charge. Some of these, particularly cocamidopropyl betaine, can be quite useful in shampoos and are oftentimes milder than some of the typical anionic surfactants. Nonionic surfactants generally have polar (but not charged) segments with multiple oxygen containing moieties (such as ethylene glycol) and can be quite gentle and effective as cleansing agents.
Representation of a linear surfactant molecule
When determining the relative strength of detergency (oil-stripping ability) of a surfactant, one must consider a variety of things. The size and structure of the tail group is important, as is the size and structure of the polar head group, as well as the size and structure of the counterion (sodium, ammonium, etc.). These are complex properties described by concepts such as self-assembly, critical micelle concentration, packing parameter and the principle of opposing forces.
Essentially, the more compact the molecule and its head group, the more efficiently it can pack into micelles of larger size and capacity. Micelles that are larger can pack more oil into them—thus more efficient at removing oil from the scalp and hair (and other surfaces).