Cationic and Nonionic Surfactant Micelles in a Halogen-Free Carboxylic Acid-Based Deep Eutectic Solvent

In recent years, it has been shown that deep eutectic solvents (DES) and similar mixtures solvate and allow for self-assembly of surfactants, serving as potential "green" alternatives as solvents for, for example, templating nanomaterials or drug delivery applications. Which surfactants are soluble and how they self-assemble depends strongly on the mixture components and their molar ratio. Here, we present the surfactant behavior in halogen-free citric acid: glycerol-based systems and show how a change in the molar ratio can affect the micellization of cationic surfactants. We also study micellization of nonionic ethylene oxide surfactants, which are insoluble in the most common hydrophilic choline chloride-based DES, such as choline chloride: urea and choline chloride: glycerol, in the absence of water. We find that the cationic C₁₂TANO₃ and C₁₆TANO₃ form spherical micelles with significantly higher intermicellar interactions than in comparable choline chloride-based DES, indicating that less charge screening due to the solvent components takes place. The nonionic Brij L23 (main component C₁₂EO₂₃) is also found to form spherical micelles in 1:2 citric acid: glycerol, while the nonionic Brij L4 (main component C₁₂EO₄) forms less clearly structured phases at similar concentrations.