Surfactants form the ‘heart’ of most shampoo formulations and perform many different roles in these systems [1-4]. Their primary function is to remove soils, such as sebum and solid particulates, from the hair, but they also are important for foaming, building product viscosity, suspending actives and the solubilization of fragrances. Surfactants also play a key role in the performance of cationic polymer-based deposition systems used to deliver actives onto the hair and scalp. In addition to this, they have to be selected and blended to be as mild to the skin, hair and eyes as possible.
To anybody first entering the field of shampoo surfactants, the large number of materials that are available to choose from, and the many claims made about their efficacy can be daunting. This review provides a broad overview of what is known about shampoo surfactants to help formulators build the most effective products for their target consumers. The first sections of this review analyse the five key benefits associated with shampoo surfactants: cleaning, foaming, rheology control, skin mildness and polymer deposition. For each area, the laboratory screening methods are described that can be used to select the best performing surfactants. To help with the logical choice of surfactants for different benefits, structure–activity relationships, where they have been defined, are also examined. In the next section of this review, the steps that need to be taken to select the most appropriate blend of surfactants for any given shampoo are described. This is followed by detailed descriptions of widely used primary and secondary surfactants, and by a review of specialized surfactants. The final sections of this review cover the new developments in ‘greener’ surfactants, ‘sulphate-free’ approaches to shampoo formulation and structured liquid phases for novel sensory properties.
A detailed explanation of the interfacial and colloid science underlying shampoo surfactant properties and effects is outside of the scope of this review and can be found in textbooks [5, 6]. A comprehensive description of all the surfactants available to formulators is also covered elsewhere [7-10].
Cleansing properties of surfactants
As already mentioned, the key benefits associated with shampoo surfactants are as follows: cleaning, foaming, rheology control, skin mildness and polymer deposition. Of these, the cleaning of hair is, undoubtedly, the most important. Indeed, cleaning of the hair is the main purpose behind using any shampoo. Most modern shampoos, based on alkyl ether sulphates, clean the hair very effectively. As a result, formulators often pay little regard to shampoo cleansing properties. However, consumer demands for new milder and ‘greener’ products mean that this area needs to be addressed again for a new set of technologies. The cleansing action of shampoo surfactants has been reviewed by a number of authors [4, 11-14]. The detergent effects of surfactants are different for different types of hair soils. To describe these different mechanisms, hair soils are broken down into four groups: (1) sebum, (2) skin cell debris, (3) solid air pollutants and (4) hair product soils.
For the removal of sebum, surfactants are proposed to work through four mechanisms: (a) roll-up, (b) spontaneous emulsification, (c) penetration and (d) solubilization. Although there is good evidence for each of these mechanisms, it remains unclear which ones are most important. In reality, it is probably a combination of all the mechanisms that enable sebum removal. The first mechanism, the roll-up mechanism, was first proposed by Adam in 1937 [15, 16]. Here, the driving force causing the oil separation from the solid surface is the reduction in the interfacial tension at the sebum/water and hair/water interfaces created by the surfactants. This allows the surface area of both interfaces to increase and drives sebum lipids to roll up into round droplets and detach from the hair surface. Once removed, lipid soils will tend to stay in solution, as the wet hair, wetted with surfactants, is no longer an attractive surface for oily materials to adhere to. Whilst attractive in theory, the roll-up mechanism relies on the soil to be a free-flowing liquid. In reality, sebum is more viscous and waxy, especially with ageing . Lochhead  also points out that the roll-up mechanism is probably best suited to damaged hair that has a more hydrophilic surface.
The second mechanism, spontaneous emulsification, is an extension of the roll-up mechanism. In this case, it is argued that the reduction in the lipid/water surface tension makes it possible for the surface area of the interface to expand and for buds of lipid soil to be formed from larger soil deposits . These buds spontaneously form emulsified lipid droplets that can be easily removed. The emulsification mechanism is better suited to explain the emulsification of oils from large areas of lipid soil, which are too big to just roll up. However, this mechanism still relies on the soil being mobile and fluid.
The third mechanism, the penetration mechanism of detergency, was first proposed by Lawrence in 1959 [17, 18] after he observed that many soaps and surfactants can penetrate into insoluble lipid soils and produce liquid–crystalline phases at the soil–water interface. Agitation of the system is believed to pull away the loosened material, revealing a fresh layer of oily soil underneath, and so on.
Finally, the fourth mechanism, the micelle mechanism of soil removal involves the transfer of lipid soil molecules from the surface of the soil into micelles adhering to the water/oil interface. This mechanism relies on the kinetics of micelle adsorption to the hair surface, lipid transfer into the micelle and, finally, detachment of the filled micelle back into the bulk solution . The micelle mechanism is the only mechanism that can easily account for the selective removal of lipid soils, as it allows for the removal of lipid soils at a molecular, not a bulk, level.
Laboratory tests for shampoo detergency involve the dosing of hair switches with synthetic sebum. Thompson et al.  describe useful protocols for the artificial soiling of hair, various cleaning processes and the analysis of the lipids remaining on the hair by gas chromatography. Using these protocols, Clarke et al.  have compared the detergency of three surfactants, sodium laureth-2 sulphate, ammonium lauryl sulphate and sodium octeth-1/deceth-1 sulphate. Their work suggests that sodium laureth-2 sulphate is the most effective at removing sebum after one and ten wash cycles. The study also showed that the ammonium lauryl sulphate and sodium octeth-1/deceth-1 sulphate selectively removed different sebum components from the hair. The authors argue that the effectiveness of sodium laureth-2 sulphate is related to its superior detergency, which, in turn, is driven by its lower critical micelle concentration (CMC). The selective removal of different sebum components points to the micelle mechanism as being important for sebum removal. In a closely related study, Clarke et al.  also show that sebum removal is more selective at higher washing temperatures. No explanation for this effect was offered by the authors.
The relative solubilizing power of different surfactants with a given hydrophobic tail usually follows the order non-ionics > cationics > anionics . Nonionic surfactants, such as the alkyl polyglucosides, with their low CMCs, are well known to be very effective detergents for skin and sebum lipids. Unfortunately, inclusion of alkyl polyglucosides, as the primary surfactants in shampoos, can make hair feel stripped and dry. Although they are very mild to the skin, they can also extract lipids and increase skin dryness .
The removal of skin cell debris and solid air pollutants from the hair by detergents is widely believed to occur through surfactant spreading forces, which force water into the soil/water interface and also through the formation of stable dispersions which prevent the re-deposition of soils once they have been removed . It is well known that anionic surfactants increase the negative potential of the electrical double layers on the soil particles and hair, and so increase repulsive forces between the surfaces. This tends to stabilize the dispersion during washing and prevent re-deposition of the soil particles. Cationic surfactants are much less effective at cleaning solid soils than anionic surfactants as they cause, what is known as, an inversion of the cleansing action, that is a cleansing action less than that of pure water .
Hydrodynamic forces are very important in the removal of solid soil particles. These are most effective at removing larger soil particles. However, as streaming velocities reduce closer to the hair surface, there comes a point when Van der Waals forces of attraction between soil particles and the hair surface outweigh the displacement forces from the water. Soil particles smaller than about 0.1 μm cannot be easily removed from textile materials or hair by detergents [13, 14]. In textiles, this leads to irreversible greying of fabrics. In hair, it suggests that soils such as fine sand (90 μm in diameter) may be easy to remove from the hair, but that ultra-fine airborne particulate pollution from, for example, combustion engines (<2.5 μm, known as PM2.5) may be harder to remove. The lack of published work suggests further work should be carried out on the effectiveness of shampoos in removing solid soils of different sizes, and particularly pollution particles.
The mechanisms through which shampoos remove hair product soils are not very well defined, and there do not seem to be any rules governing surfactant choice. However, it is understood that, unlike sebum, the conditioning surfactants, polymers and silicones used in hair products may sometimes not be easily removed by washing with a standard anionic surfactant-based shampoo. Robbins et al. , for example, show that monofunctional cationic surfactants are not completely removed by washing with anionic surfactants. Hannah et al.  have shown that coacervates formed between polyquaternium polymers and anionic surfactants during shampoo use can resist removal from the hair from subsequent shampoo washes. This can cause build-up problems if shampoo products are not carefully formulated. Haake et al.  have shown that the silicones deposited by typical 2-in-1 shampoos are only removed gradually after a number of washes in aqueous solutions of sodium lauryl ether sulphate. In some cases, only 50% of the deposited silicone was removed after three washes. It is widely understood that amodimethicones and silicone quats have the greatest substantivity to the hair (in comparison with non-polar dimethicones) . As a consequence, there is a greater risk of build-up with these ingredients and formulators need to use them with great care.