Rheological modulation of a pH-responsive wormlike micelle driven by charge and cosurfactant

Abstract

Responsive wormlike micelles are intriguing systems capable of transitioning their viscosity from that of water to a gel-like state in response to external stimuli such as pH changes. Referred to as “smart wormlike micelles”, these structures offer the unique ability to adapt their viscosity for specific applications. In this study, we explored the rheological behavior of smart wormlike micelles formed by combining a cationic surfactant, Cetyltrimethylammonium bromide (C16TAB) with phthalic acid. The aromatic nature of phthalic acid, featuring two pKas at 2.9 and 4.5, provides control over the system's pH, enabling adjustments in the fraction of neutral, anionic, or dianionic species and thereby modulating the micellar rheology. The partitioning of phthalic acid between the micelle palisade and bulk solution plays a crucial role in inducing the formation of wormlike micelles by influencing curvature changes of the aggregate. This partitioning behavior is intricately governed by the balance between charge and hydrophobicity of the aromatic molecule. The investigation spanned a wide pH range, encompassing various concentrations of C16TAB and phthalic acid. Cryo-TEM imaging allowed direct characterization of distinct aggregate types formed at different pH levels. Additionally, to minimize electrostatic repulsion between cationic micellar heads in wormlike micelles, long alkyl alcohols were introduced as cosurfactants, enabling the exploration of synergistic effects at the micelle interface.