Take two bottles into the shower?
Shampoo and conditioner in the same bottle is taken for granted by today's silky-haired population. Maria Burke discovers the chemistry behind this clever invention
- Shampoo and conditioner in the same formulation may be widely used, but has only been made possible thanks to chemists
- Conditioners are oils, whereas surfactants in shampoos are designed to remove oil, making their combination problematic
- As conditioners have become the norm, the next challenge is to develop products that do not accumulate on the hair
Everyone uses a shampoo, but not everyone realises the clever chemical tricks that go into combining all the ingredients. Early shampoo formulations were only designed to clean hair, not to condition it. The manufacturers thought it was impossible to mix cleansing and conditioning materials in a single bottle.
This all changed with the advent of '2-in-1' technology and the first of these shampoos reached the shelves in the 1980s. Now, all the major manufacturers produce 2-in-1 formulations, and over 20 per cent of all shampoos sold are 2-in-1.
'In the 1980s when 2-in-1 shampoos were introduced, not many people used conditioners,' says Steve Shiel, senior scientist at Procter & Gamble (P&G) Beauty. 'Now, thanks in part to 2-in-1 shampoos, most people want their hair to be conditioned. From a technological point of view, the majority of shampoos have some elements of the 2-in-1 technology in them; that is, most have some conditioning properties.'
But how do the manufacturers manage to combine a cleanser with a conditioner?
'At first, 2-in-1 shampoos don't make any sense chemically,' says Tony Ryan, ICI professor of physical chemistry at the University of Sheffield. 'The conditioner is an oil, yet shampoos are designed to get rid of anything oily. You'd expect the conditioner to get washed away.' But polymer scientists have come up with a way of getting conditioner onto the hair after the shampoo has cleaned it.
In shampoos, the cleansing job is carried out by a class of chemicals called surfactants. These reduce the surface tension between the water and the greasy matter on the hair, making it easier to remove dirt. They produce a foam, or lather, which contains the bits of dirt, and stops them from being redeposited on the hair. They also stabilise the shampoo mixture, and hold the other ingredients in solution.
Surfactants have both hydrophilic and lipophilic segments. The hydrophilic part is soluble in water, while the lipophilic part is attracted to oils. The water-soluble portion is typically a polar group, which ionises in solution to yield charged species. It can be one of many functional groups and determines the surfactant's nature and a lot of its properties, explains Helen Moore, president of the Trichological Society, a group specialising in hair sciences based in the US. These include sulphate, ethoxy sulphate, succinates, polyhydroxylates, quarternerised groups and many more.
The lipophilic portion usually contains a hydrocarbon of variable length. Common chain lengths are between eight and 18 carbon atoms, with C12 being the most common in cosmetics formulations, according to Moore. 'This strikes the balance between mildness and the ability to remove grease from the hair. Shorter chain lengths have stronger grease-removing properties, while longer chain lengths have greater mildness but less lathering properties.'
Surfactants can be divided into four groups, according to their ionic nature - anionics, cationics, nonionics and amphoterics. Anionics carry a negative charge when ionised. They provide a lot of the lather and detergency. The most commonly used are sodium laureth sulphate and sodium lauryl sulphate. Occasionally ammonium lauryl ether sulphate and ammonium lauryl sulphate are also used.
Cationics carry a positive charge when ionised and are used mostly in conditioners. They cling to hair by static attraction so consequently they are not easily removed during the rinsing process and form the basis of conditioning. Cetrimonium chloride is one of the most common cationic conditioners.
Nonionics have no charge and are good degreasing agents but they provide little foam. They are frequently used to solubilise fragrances and other oily materials. Examples include polysorbate compounds and fatty alcohol ethers such as PPG-10 cetyl ether, laureth-3 or -4 or coco glucosides.
Amphoterics crop up particularly in baby shampoos because they are so mild, but they provide less foam. They are zwitterionic, meaning that they acquire a positive charge in acidic environments and a negative charge in alkaline solution. Examples are cocamido propyl betaine, or occasionally cocamido betaine.
While shampoos remove the dirt from hair, they also remove the oils that protect hair. A conditioner replaces these oils by coating the hair with a protective material such as silicone. 'The silicone coating makes the hair feel softer and means it is less likely to tangle because the silicone coating separates each hair from another,' explains Ryan. 'Coating the hair also fills in all in the bumps and ruts where the hair is damaged. The hair becomes smooth, and as light bounces off smooth objects much more than rough objects, the hair begins to look shiny.'
What's more, the molecules in most conditioning agents carry small positive electrical charges, while the damaged sites within hair are thought to carry a negative charge. The positively charged, cationic conditioning agents serve to eliminate static flyaway from hair by serving as a conductor to dissipate localised charges.
There are two basic approaches to gaining 2-in-1 functionality, according to Mort Westman, president of Illinois-based R&D consultancy Westman Associates. Both approaches employ anionic surfactants because they provide the best lather and cleansing, he says. 'One approach is to mix cationic polymers in anionic shampoo. This was developed and commercialised in the late 1960s and early 1970s, and is still being done. The second approach is a micro-suspension of silicone in anionic shampoo. This is more effective than the cationic polymer approach. This has become the standard art of 2-in-1 shampoos.'
All the companies selling 2-in-1 shampoos use a variation of the same technology, but generally they use different key ingredients. When a 2-in-1 shampoo sits in the bottle, the micro-droplets of silicone are held suspended in the product and remain out of action throughout lathering until the hair is rinsed.
The conditioners are held in suspension by polymers, which bind to the surfactant molecules and hold on to the conditioning molecules. 'The way the surfactant and polymer are bound is sensitive to water,' Ryan explains. 'So when you rinse your hair the surfactant separates from the polymer and conditioner. The polymer and conditioner are not soluble in water, and they precipitate and coat all the hairs.'
Westman believes that the 2-in-1 technology is one of three major innovations in the world of shampoos. 'Incorporating functional silicone in a shampoo sent a technological shockwave through the industry and produced a flurry of patents,' he recalls. (The other two innovations were replacing soap with synthetic detergents in the 1950s and using cationic polymers as conditioners in the 1960s.)
P&G spent many years in the 1980s trying to prevent conditioning ingredients from complexing with surfactants, says P&G's Shiel. 'It was like the curdling you get when you add lemon juice to milk. In the end, we found a way of getting an electrically neutral silicone - which wouldn't complex out - in a form where it could deposit. It deposits very specifically on roughened parts of the hair, or the hair-ends.'
P&G relies on a dispersed crystalline material, such as ethylene glycol di-stearate (EDGS). 'The crystalline matrix surrounds the silicone and holds it in place until rinsing breaks it down and releases the silicone to the hair,' says Shiel. This suspension system is used together with a high-molecular-weight silicone blend, such as dimethicone or polydimethylsiloxane. 'We optimise the particle size of the silicone to prevent overloading of the hair,' says Shiel.
The third part of the formulation is an anionic shampoo base made up of a blend of surfactants to give the right lathering effects and mildness. 'But we use different ingredients to produce different effects for a range of consumer needs,' says Shiel. 'For example, we can add fatty alcohols to give a softer feel.'
The formulators at Unilever rely mainly on a cationic polymer called Jaguar, which is a modified guar gum (guar hydroxypropyltrimonium chloride). The system makes use of the strong electrostatic interaction between the cationic polymer and the anionic surfactant in the shampoo. The resulting polymer-surfactant complex binds to the silicone blend.
Ezat Khoshdel of Unilever Research, Port Sunlight, UK, explains: 'The complexes are soluble when there is a large amount of surfactant - like in a shampoo formulation. When the shampoo is diluted during the wash, the surfactant levels decrease and the complex precipitates out, delivering the polymer and the silicone to the hair.' The guar gum provides extra conditioning, and overall the two together have a better conditioning effect than silicone alone.
Developing the right polymer took a long time, says Khoshdel. He stresses how the choice of polymer as well as its formulation is crucial to the product's performance. For example, the molecular weight, cationic charge density and the polymer architecture play a major role. He explains that the polymer needs to be reasonably big to circle the silicone and just the right weight: if its molecular weight is too low, it will not dissolve in the excess surfactant. The polymer also has to have the right amount of charge. If it has too much, it will be too soluble; too little and it won't be soluble enough.
It also took many years to optimise the molecular weight and architecture of the silicone. Unilever usually uses a blend of different silicones and it is something of an art to get the right mix, says Khosdel. 'We need to get the right amount of silicone to give smoothness but not heaviness to the hair. The distribution and thickness of the deposited layer and the ability to remove it by detergents are important factors in determining the efficacy of the product. It has also been shown by many workers that the nature of the ionic charges on the silicone oil droplet, the droplet size, the viscosity of the silicone oil and the presence of competing adsorbents all play a key role in the performance of the product.'
Getting the quantities right is crucial, agrees the Trichological Society's Moore. She points out that the use of dimethicone can be problematic since it can accumulate on the hair if used constantly in a 2-in-1 formula. 'It needs a plain washing shampoo to be used periodically to prevent such a build up from happening,' she says.
This criticism of 2-in-1 shampoos has proved hard to shake off. A haircare review by the company Information Resources in 2001 found that the 2-in-1 sector had declined by 22 per cent over three years, despite an increasing trend throughout other categories for products that offer convenience. The report found that consumers, especially women, did not like the heavy feel of their hair after continued use, and this adverse publicity had affected sales.
'The silicones are a double-edged sword,' says Westman, 'in that they are so good at conditioning that they could provide too much. The 2-in-1s have the "one size fits all" problem in that the user can't control the amount of conditioning they get. Their hair can feel over-conditioned, limp and oily. Historically, men have been willing to live with this for the convenience of using one product for both conditioning and cleaning, and, in many cases, the psychological benefit of telling themselves they are not using a "sissy conditioner".'
Westman agrees that sales of 2-in-1s have declined because of these problems, but he suspects that the technology has just gone underground in most cases. 'Many marketers are using a lower level of the same technology and calling their products good conditioning shampoos, rather than 2-in-1s.'
Shiel agrees that most people now want their hair to be conditioned, but demand more control over the process and want to use separate products. However, he points out that technology has moved on and the industry has developed silicones with a much better feel. '[P&G] upgraded [its products] at the end of the 1990s producing a silicone blend with finer particle size so it would leave less residue,' he reports.
There is a common drive within the industry to design formulations that leave the hair feeling natural and not heavy. P&G is still working at producing products with better deposition and conditioning properties. 'For the future,' says Shiel, 'we are looking at modifying conditioning polymers, developing different types and properties of silicones, and using different types for different applications; as well as modifying the chemical structures of cationic polymers.'
At Unilever, too, Khosdel reports they constantly strive to develop better silicones, better polymers and better combinations of ingredients.
Ingredient suppliers, such as surfactant manufacturing company Uniqema, are also working on this problem. One of Uniqema's latest ranges is a class of phospholipids that gets around some of the problems associated with delivering silicone on to the hair. Dimethicone is an excellent conditioner and appears in lots of top-selling shampoos, often as dimethicone copolyols, says a spokesman for Uniqema. He goes on: 'But traditionally, its use has been problematic because of the difficulties it presents in product formulation and depositing on hair. We have solved these problems by combining it with a cationic phospholipid material.'
The biomimetic phospholipids are derived from vegetable oils, such as safflower, a natural oil rich in linoleic acid. Uniqema uses patented pyrrolidone and phospholipid chemistry to increase the molecules' polarity so that they remain soluble in water. They are organosilicones and can contain up to 70 per cent of dimethicone by weight.
Uniqema says they deliver high levels of dimethicone - more than twice the amount of dimethicone copolyols. What's more, the spokesman points out this ingredient's 'unusual properties' allow it to provide conditioning effects in clear formulations. Usually, he says, silicone-delivering products have to be used in pearlised or opaque shampoos.
And so to the future. What innovations are left for the chemists to work on? ' Structured surfactants are special because here we have one ingredient that cleans and conditions,' says Westman. This means that manufacturers would not have to use the three main ingredients: surfactant, conditioner and polymer.
The term structured surfactant is used to refer to pourable, fluid compositions, that can hold, and potentially deliver, solid, liquid and gaseous particles in suspension, explains Westman. While these, and similar systems, are already used in liquid cleanser formulations, such as hand and body lotions, they have yet to leave their mark in shampoos. But, such particles could incorporate silicones, or other functional or active ingredients such as anti-dandruff agents.
US chemical company Huntsman has recently advanced the potential application of this technology to shampoos by developing optically clear structured systems that can be made using surfactant, carbohydrate and water. 'This technology provides polymer-free, transparent formulations having excellent suspending power, low freezing point and a wide thermal stability range (-5°C to +50°C),' says Westman. 'Further, Huntsman reports these systems are easy to formulate and manufacture, using readily available raw materials, and are surprisingly self-preserved.'
Rhodia is also working on this technology, but a spokesman said the company was not yet ready to 'communicate broadly' this technology.
Maria Burke is a freelance science writer from St Albans, UK.