Balancing detergent power and foam formation is important for developing dishwashing detergents. Many ingredients that improve foaming have been studied, but the mechanism of action in the formulation system is not clear. The purpose of this study is to clarify the components that improve foam formation while maintaining detergent power as well as their mechanisms of action. In this study, the maximum rate of decrease in dynamic surface tension ((dγt/dt)max) was used as an indicator of foam formation. Rapid bubble generation promotes fine foam forming. The index of bubble formation is how much energy required to expand the area of foam can be reduced. (dγt/dt)max represents the point at which the adsorption rate changes from surface adsorption dominated to surface diffusion dominated. The effects of pH, (dγt/dt)max, and molecular mobility within micelles were evaluated to understand the foaming mechanism. The foam-boosting effect of fatty acid monoethanol amide (MEA) and fatty alcohol polyoxyethylene ether (AE) on aqueous solutions of polyoxyethylene alkyl ether sulfate (AES) and N,N-dimethyldodecylamine N-oxide (DDAO), which are the essential components of dishwashing detergents, was evaluated. When polyoxyethylene n-dodecyl monoethanolamide (C12MEA(2)) was added to the AES/DDAO solution, (dγt/dt)max increased at pH values above 6, suggesting enhanced DDAO deprotonation compared with that in the AES/DDAO aqueous solution without MEA. When polyoxyethylene n-dodecyl ether (C12AE(20)) was added to the AES/DDAO solution, (dγt/dt)max did not change with increasing pH. However, a comparison of molecular mobility within micelles revealed that AES/DDAO/C12AE(20) had higher molecular mobility than AES/DDAO/C12MEA(2). The present study indicates that a nonionic surfactant that can promote DDAO deprotonation with increasing pH without weakening the intermolecular interactions of AES and DDAO could effectively promote adsorption at the air-water interface and foam formation in aqueous AES/DDAO solutions.
Keywords: N,N-dimethyldodecylamine N-oxide; dishwashing detergents; dynamic surface tension; foam booster; polyoxyethylene alkyl ether sulfate.