Surfactant-Chelating Agent Interactions

Licentiate thesis, 2024

The influence of chelating agents on the self-assembly process in surfactant solutions remains largely unexplored and merits further study. Traditionally, chelating agents are seen as salting-out ions that promote surfactant separation. However, the opposite effect has been observed with iminodipropionate-based surfactants, where aminopolycarboxylate chelating agents increase the solubility of nonionic ethoxylated surfactants in mixed micellar systems. In this work, specific interactions between chelating agents and surfactants are hypothesized to influence both the self-assembly process and the macroscopic properties of surfactant formulations.
To investigate these interactions and their relationship to the chemical structures of the surfactant and chelating agent, NMR spectroscopy was employed, focusing on 13C chemical shift and line shape analysis. Moreover, the impact of these interactions on the formulation properties was assessed by measuring cloud point, viscosity, surface tension, and contact angle on hydrophobic surfaces.
The study reveals that interactions between the head group of amphoteric surfactants and chelating agents lead to the formation of oligomeric surfactant analogues with larger hydrophilic moieties, resulting in smaller, more spherical micelles. The strength and likelihood of these interactions depend on several factors, including the presence of oppositely charged moieties, neighboring groups near the interacting atoms, and the volume-to-length ratio of the surfactant. The interaction offers possibilities for tuning the aggregation behavior of systems containing surfactants and chelating agents, and consequently, the macroscopic properties of the formulation.