Nomenclature
Amphoteric surfactants have both a positive (cationic) and a negative (anionic) group. The word “amphoteric” is derived from the Greek “amphi,” meaning both. Sometimes the phrase ampholytic is used. The nomenclature of some amphoterics has been confused in the past, partly due to mistakes in the original chemical structures, and, more recently, due to retention of the word betaine for products that are strictly not amphoteric. The main categories of amphoterics are as follows
N-alkyl aminopropionates or N-alkyl iminodipropionates:
The reaction of amines with chlorpropionic acid or acrylic acid. In the case of the aminopropionate, the NH group is still reactive and will react with a further molecule of acrylic acid giving an iminodipropionate. This does not occur with the glycinate.
N-alkyl betaines:
The word betaine originally referred to the compound trimethyl glycine, (CH3)3N+CH2COOH, which contains a quaternary nitrogen. The word was then extended to N-trialkyl derivatives of amino acids. In the scientific literature, it now means the internal salt of a quaternary ammonium, oxonium or sulfonium ion. They are formed when chloracetic acid reacts with a tertiary nitrogen. Note that a quaternary nitrogen group is formed by this reaction but not formed in the case of the glycinate and aminopropionate. Strictly speaking, the betaines are not amphoterics because they are never anionic. They are more like a quaternary ammonium compound. Nevertheless, by common usage, they have been included in amphoterics.
Example:
Laurylamidopropyldimethyl betaine.
Note the alkyl betaines have sometimes been described as alkyl dimethyl glycinates.
N-alkyl glycinates:
These are specific alkyl amino acids with the alkyl group attached to the nitrogen atom of the amine. They are derivatives of glycine, called glycinates and formed from chloracetic acid and an alkyl amine
Example:
Coco glycinate.
Carboxy glycinates:
Indicates that there are two carboxyl groups present by formation with two molecules of chloracetic acid. It does not define the structure and the products are nearly always mixtures.
Alkyl imidazoline-based products:
If made from an alkyl imidazoline and chloracetic acid they are called: monocarboxylic derivatives or amphoglycinates; dicarboxylic derivatives or amphocarboxy glycinates. The imidazoline amphoterics were the first amphoterics to be commercialized on a large scale and it is these products which have given confusion in naming. The products made from imidazolines and chloracetic acid were once thought to have a ring structure but are now known to have a linear structure as the imidazoline ring breaks down during the formation of the amphoteric. They are derivatives of glycine acid and do not give betaines. At one time, mixtures of imidazoline amphoterics with alkyl sulfates or alkyl ether sulfates were also described as complexes but no evidence for complex formation has been found.
Alkyl polyamino carboxylates or polyamphocarboxy glycinates:
These are products where there is more than one nitrogen capable of reacting with chloracetic acid. The abbreviation APAC is now becoming increasingly used.
Coco APAC reveals that the alkyl group is derived from coconut oil but does not give any idea of the number of nitrogen groups or the number of carboxyl groups. Amine oxides:
Amine oxides show amphoteric properties but in neutral and alkaline conditions they are essentially non-ionic.
General properties
General:
By altering the pH of an aqueous solution the anionic or cationic character of the amphoteric can be changed. At some intermediate pH (not necessarily pH 7) both ionic groups show equal ionization and this pH is called the isoelectric point or (area). This type of molecule is often described as a zwitterion (German for hybrid).
The isoelectric point is not a sharp point but depends upon the nature of the anionic and cationic groups. The most common anionic group is the carboxyl group (COOH) and the most common cationic group is the amine group (NH2). At the isoelcctric point amphoterics generally have minimum surfactant properties; i.e. minimum foam, minimum wetting, minimum detergency. An amphoteric which is soluble at the isoelectric point is soluble across the whole pH range. The ionic nature of an amphoteric is rarely wholly anionic or cationic above and below the isoelectric area. What is more important is that it changes the properties, thus comparing the two surfactants R-CH1CH1CH1COONa and R-NHCH2CH2COONa. The substitution of NH for CH2 improves hard water tolerance.
Solubility:
Excellent solubility in aqueous solution but at a minimum in the isoelectric area.
Compatibility with aqueous ions:
Excellent.
Compatibility with other surfactants:
Amphoterics show excellent compatibility with other surfactants. Mixed micelles are frequently formed, and the mixtures often have functional properties not found in either of the components. For instance, skin irritation can be reduced below the level of either of the ingredients. Biocidal activity sometimes found in amphoterics can be reduced when stable mixed micelles are formed. The effect of adding an amphoteric to an anionic is to reduce the eye and skin irritation, increase viscosity in the presence of electrolyte, improve foam stability and improve detergency. The effect of adding amphoterics to non-ionics is more specific to the amphoteric than in the case of anionics.
Most amphoterics will act as hydrotropes in solubilizing non-ionics in high electrolyte concentrations particularly at high temperatures. This ability to solubilize depends upon the structure of the amphoteric. Amphoterics with polycarboxy groups and non-ionics can show significant synergism in detergency depending upon the ratio of non-ionic to amphoteric.
Functional properties:
Excellent wetting agent in the presence of high electrolyte concentration.
Biocidal activity:
Amphoterics possess biocidal activity but this is weak for the simpler amphoterics and only becomes pronounced when the number of amine groups (particularly secondary) increases. Some amphoterics show synergism with betaines. Amphoterics have the advantage over quaternary compounds of being unaffected by hard water and alcohol.
Imidazoline base amphoterics give the following properties: good lime soap dispersion properties; mild, detoxify anionics, non-stinging to the eyes; improve foaming properties of AES in the presence of sebum; viscosity adjustment is difficult because dialkanolamides are ineffective; PEG (150) distearate usually used as thickener; slight bacteriostatic effect.
Amino propionates, isoelectric area, pH 3.5 imino propionates (two moles of acrylic acid), isoelectric area, 1.75-3.5, i.e. lower because of the two COOH groups; Because of the lower isoelectric point, the imino derivatives will have better detergency and solubility at acid and neutral pH. The wetting properties are inferior.
Applications
Amphoterics are used in amphoteric/anionic mixtures in shampoos, foam baths, shower gels, liquid soaps, hand cleaners, hand laundry and hand dish washing. The main products used are: coco amido propyl betaine; coco amphocarboxy glycinate (imidazoline base); coco dimethyl betaine; tallow polyamino carboxylates. Amphoterics will detoxify anionics in shampoo formulations due to the formation of mixed micelles. However, these mixed micelles may inhibit biocidal behaviour. Amphoterics are used in amphoteric/non-ionic mixtures in liquid and powdered laundry detergents.
Amphoterics tend to be less deactivated by protein than are quaternary ammonium compounds and this should make them suitable for use in dairy cleaning. They are also relatively easily removed from metals, whereas quaternaries are more tenacious and not so easily removed.
References:
Amphoterics International (1987) European Patent 0, 214, 868.
Scher Chemicals (1983). The chemistry and applications of amido-amines, presented to the Society of Cosmetic Chemists Annual Seminar, Cincinnati, OH.
Sykes, G. (1965) Disinfection and Sterilization, 2nd edn., pp. 377-378.
Verdiccho, RJ. and Walts, J.M. (1976) US Patent 3,950,417 to Johnson & Johnson.
