Amphoteric Surfactant-Chelating Agent Interactions: Impact on Bulk and Surface Properties

Recent findings indicate that there are interactions between amphoteric surfactants and chelating agents in solution, which influence the capacity of the system to solubilize non-water-soluble nonionic ethoxylated surfactants. We hypothesize that these interactions are primarily driven by the chemical affinity between the head groups of the amphoteric surfactant and the chelating agent. By modulating the strength of this interaction, it may be possible to control the surface properties of the mixed system containing non-water-soluble nonionic ethoxylated surfactants.

To investigate the relationship between chemical structure and the previously reported interactions, we employed a set of amine-containing surfactants and polycarboxylic acid-type chelating agents. The interactions were monitored using NMR spectroscopy, specifically through ¹³C chemical shift and line shape analysis. The impact of these interactions on surface properties was assessed by measuring surface tension and contact angle on hydrophobic surfaces.

The occurrence of interactions between surfactants and chelating agents depends heavily on their structure and is found stronger when both surfactants and chelating agents bear both positive and negative charges. Since the interactions take place in solution, it increases system solubilization power, yet the surface properties of the complexes are not affected by the interactions. However, when a poorly soluble nonionic surfactant is introduced, improved wetting properties are observed.