Occasionally discussions arise in the hair product-focused community concerning several cationic surfactants, such as cetrimonium chloride, behentrimonium methosulfate and stearamidopropyl dimethylamine.
For good reason, the INCI (International Nomenclature of Cosmetics Ingredients) names of these molecules are often confused with silicones, sulfate detergents and preservatives. It can be surprising to hear that a “sulfate” is a desirable conditioning agent for many people. A brief review of the different types of surfactants and their various applications, as well as a closer examination of cationic surfactants in particular seems useful to bring some clarity to the confusion.
What are Surfactants?
Surfactants are molecules that possess dual-polarity, having both hydrophobic (water hating) and hydrophilic (water loving) segments. Although hydrophobic is the term most often used to describe non-polar substances, the term lipophilic (oil loving) is perhaps a more accurate descriptor, because they do not so much hate water as they prefer oils. The unique quality of being both lipophilic and hydrophilic is described as being amphiphilic. Molecules that are amphiphilic characteristically possess a long segment that is a non-polar hydrocarbon chain and a polar head group that can be ionically-charged or not.
Due to the uniquely polar properties of water, immersion of an amphiphilic molecule in an aqueous solution results in very interesting behavior. Water molecules have strong intermolecular hydrogen bonding that creates a predictable geometric structure within the bulk of the liquid. This hydrogen bonding forms a tightly stretched molecular film at the interface between air and the liquid. This gives water its characteristically high surface tension. Placing a molecule with hydrophobic properties into that environment disturbs that local structure, so the water excludes the hydrophobic molecule from the solution by pushing it to the surface.
You have certainly witnessed this yourself when you have seen the rainbow display of oil on the surface of a water puddle. What happens when the molecule has both a hydrophilic portion and a hydrophobic portion is that the polar head group is solvated into the solution just below the air-water interface, while the hydrophobic tail group juts out above the surface. This disrupts the hydrogen bonding at the surface of the water, which substantially decreases the surface tension. For this reason, amphiphilic molecules are said to be “surface active agents” and are called surfactants.
Eventually, if the concentration of amphiphilic material is increased, the surface of the solution becomes saturated, and an intriguing phenomenon occurs. In order to preserve as much of the polar structure of water as possible, the amphiphile molecules aggregate together into tiny spheres in the bulk of the solution, with the hydrophobic portions clustered in the center of the sphere, which is surrounded by a hydration shell comprised of the polar head group. These amazing aggregates are called micelles, and are the foundation for many biological functions. They are also useful for many functions such as cleansing, emulsion stabilization, targeted and/or controlled release drug delivery, formation of liquid crystalline structures and many other applications.
Types of Surfactants
These types of surfactants, such as cocamidopropyl betaine, are valued for being mild both to skin and hair. They also provide foam-boosting to shampoos that contain anionic surfactants, which enhances lather, a property preferred by many consumers.
Surfactant molecules are classified according to the ionic charge of the hydrophilic head group. These classes consist of anionic, cationic, nonionic, and zwitterionic surfactants. The ionic surfactants are available in salt form with an appropriate alkali metal or ammonium counterion. Nonionic surfactants typically possess several repeat units of ethylene oxide as the hydrophilic moiety. Dual-charged (zwitterionic) surfactants also have utility in hair care products. Many polymers have amphiphilic qualities as well, and they are quite interesting in their properties. However, in this discussion, for the sake of brevity and focus, we shall confine our examination of surfactants to those which are small molecules, and exclude discussion of polymeric surfactants.
Anionic surfactants, such as sodium lauryl sulfate, ammonium laureth sulfate and sodium cocoyl isethionate carry a negative charge on the polar head group. These materials are typically incorporated into shampoo formulations for their detergency properties. They are highly effective at removing dirt and oil from the hair and scalp. Many consumers with delicate, curly hair find these to be too drying and damaging for frequent use, and they either seek shampoos with different types of surfactants or opt to use cleansing conditioners.
Nonionic surfactants are those such as decyl glucoside and PEG-10 laurate, which have no residual electric charge at all. These surfactants can perform a variety of functions in a formula, such as emulsion stabilization, mild detergency and viscosity modification.
Zwitterionic (or amphoteric) surfactants are dual-charged (have both a positive and negative charge on the molecule). Many display pH-dependent charge behavior, having one charge at lower pH’s and the opposite charge at higher pH’s. These types of surfactants, such as cocamidopropyl betaine, are valued for being mild both to skin and hair. They also provide foam-boosting to shampoos that contain anionic surfactants, which enhances lather, a property preferred by many consumers.
Cationic surfactants such as cetrimonium chloride and stearamidopropyl dimethylamine, have a positive charge on their head group. The composition of the molecules can vary, but is typically a fatty, acid-derived, hydrophobic tail with a nitrogen-containing head group. The nitrogen-containing group can be either a tertiary or quaternary amine. Typically these surfactants are either alkyl amine salts or alkyl quaternary ammonium salts.