The term amphoteric is used to describe a class of compounds that have two or more different groups, oftentimes an amino and a carboxyl. Depending upon the pH, amphoteric compounds can have a positive charge, a negative charge or no charge at all. Therefore, amphoteric compounds can be anionic (-), cationic (+) or zwitterionic (- and +), as shown in Figure 1.
The pH of the formulation has a dramatic effect upon the foam of propionates. At neutral and alkaline pH values, the compounds foam well. At their isoelectric range, the foam is essentially zero and below the isoelectric range, conditioning properties are observed. Below their isoelectric range, the compounds are protonated and exist as cationic surfactants. This explains their substantivity.
Ampholytes have a fully quaternized nitrogen and consequently cannot lose their positive charge. As a result, they exist as either zwitterionic (- and +) or cationic (+) compounds, depending upon the pH of the formulation. They cannot exist in an anionic (-) state, since the positive charge on the nitrogen is always present, as shown in Figure 2.
Cocamidopropyl betaine is an important surfactant used in the personal care market. It is a mild, moderate foaming product used extensively in shampoos, bubble bath products and other cleansing products. Cocamidopropyl betaine is a detergent that complexes with anionic surfactants, providing viscosity and conditioning effects when formulated properly.