Components of cocamidopropyl betaine: Surface activity and self-assembly of pure alkyl amidopropyl betaines

Cocamidopropyl betaine (CAPB) is widely used in personal care and industrial products, due to its mildness, simple synthesis, valuable surface active properties and synergism with other surfactants. CAPB is a mixture of amidopropyl betaines of different tail lengths, typically dominated by C12 (lauryl) amidopropyl betaine in commercial formulations. The feedstock used to synthesise this surfactant mixture dictates the specific individual amidopropyl betaine components present, their blend, and ultimately the properties of CAPB. Here, we investigate the surface activity and self-assembly of synthesised pure amidopropyl betaines that are typically found within CAPB. Small-angle neutron scattering, bubble pressure tensiometry, pendant drop tensiometry, foaming studies, and polarising light microscopy are employed to determine each molecule’s behaviour. It is evident that an increase in alkyl tail length controls properties within this class of molecules, leading to greater surface activity and the formation of different micelle geometries in solution, and modulating adsorption dynamics and foaming capabilities. These results indicate potential for optimisation of CAPB feedstocks along with new and facile approaches to tuning the properties of mixed surfactant systems, using carefully selected tailgroup mixtures.