BETAINE WHAT IS?
What is... A BETAINE?
Innospec’s Alice Miles sheds some light on this commonly- used amphoteric surfactant
Alice Miles,
Senior Applications Chemist for Personal Care, EMEA, Innospec
Amphoteric surfactants are widely used across many industries, including personal care. They offer benefits to cleansing formulations such as increased mildness, foam boosting, rheology modification and solubilisation of oils. Amphoteric surfactants have both positive and negative charges present in the hydrophilic head group. The balance between the two depends on the pH. At high pH the molecule will be negatively charged (anionic) and at low pH the molecule will be positively charged (cationic). The pH at which the molecule is completely balanced – known as a zwitterion – is called the isoelectric point. The most common amphoteric surfactant used in personal care is cocamidopropyl betaine. Although it is, strictly speaking, not a true amphoteric material, as the structure means it cannot exist in the anionic state. This has a long history of safe use, is economical, has a wide pH stability range of pH 3-11 and is a clear, low viscosity liquid. Cocamidopropyl betaine has good compatibility with a wide range of ingredients. It is often combined with primary anionic surfactants in formulations such as shampoos and shower gels. Cocamidopropyl betaine reduces skin and eye irritation that can be caused by the primary surfactant, and it can also give foam boosting benefits to formulations.
Another key benefit of cocamidopropyl betaine is that it can enhance the formation of rod-like micelles upon the addition of electrolyte, such as sodium chloride, to the formulation. This affords a convenient and cost effective means of rheology
control (ie, thickening) of the formulation. Cocamidopropyl betaine can be used in combination with sulfate-based surfactants, such as sodium coco-sulfate or sodium lauryl ether sulfate; or ‘sulfate-free’ surfactants, such as sodium lauroyl methyl isethionate or sodium methyl cocoyl taurate.
Cocamidopropyl betaine can be used in formulations where it is the main surfactant and this is commonly the case where extra-mild systems are required in which the co-surfactants include alkyl polyglucosides, sodium lauroyl/cocoyl glutamate, etc, but not typical mainstream anionic surfactants, such as sulfates or sulfonates. Such systems are not capable of being thickened with electrolyte and thus rheology modifiers such as xanthan gum or acrylate polymers are needed to provide the desired viscosity.
There are two main methods of producing cocamidopropyl betaine. Triglyceride from coconut oil or palm kernel oil can be reacted with dimethylaminopropylamine (DMAPA), followed by carboxymethylation with sodium monochloroacetate. This is a lower cost method that results in a wide range of carbon chain lengths and glycerol as by-product. An alternative method is to begin with stripped fatty acid, which results in a narrower range of higher carbon chain lengths and no glycerol by-product. This material can achieve more efficient thickening when combined with sodium lauroyl methyl isethionate or sodium methyl cocoyl taurate compared with the triglyceride based material.
Other triglyceride oils, such as castor oil, soy oil and shea butter oil, are also used instead of coconut oil or palm kernel oil to make the corresponding alkyl amidopropyl betaines. However, these are much less commonly used in formulations than cocamidopropyl betaine. Other betaines used in personal care include lauryl betaine and coco-betaine. These are made by reaction of the corresponding fatty alkyl dimethyl amine with sodium monochloroacetate. These alkyl betaines generally have a higher level of naturally derived content ~76% compared with ~66% for cocamidopropyl betaine. As the availability of sustainably-derived starting materials increases this level may increase.
Some companies subject their betaine grades to a desalination process to give low-salt versions of cocamidopropyl betaine and lauryl/coco-betaine grades which are of appeal to some formulators for certain applications.
Cocamidopropyl betaine can add foam boosting benefits to formulations
cosmeticsbusiness.com
As always in the personal care industry, consumers can influence which ingredients are popular or unpopular and betaines are an easy target due to their huge usage in the industry. However, as long as high-quality products with low impurity levels are used, betaines are a long-standing and trusted choice for the industry. The many benefits betaines bring for formulators and consumers alike mean that they are likely to remain a popular choice to formulators for many years to come
December 2021 43
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