Molecular interactions and the viscoelasticity of micellar aggregates

Intermolecular interactions are responsible for the macroscopic properties of materials. Self-assembled micelles of ionic surfactants in the presence of salt are a result of the balance between hydrophobic-hydrophilic and ionic forces. For example, sodium salicylate (NaSal) undoubtedly offers a powerful means of increasing the viscoelasticity of hexadecyl trimethylammonium bromide (CTAB) solutions by orders of magnitude, which results from the formation of wormlike micelles (WLMs). The efficiency of this additive relies on its ability to integrate and alter the repulsive interactions governing CTAB micelles. Consequently, small modifications in the molecular structure of NaSal influences the nature of these interactions. Nevertheless, the full potential of formulation for tailoring the system’s viscoelasticity has yet to be unleashed. Herein, we investigate a series of structurally similar molecules varying in terms of geometry and size. The depth and molecular orientation of their insertion into the micellar core were monitored by proton nuclear magnetic resonance (¹H-NMR) and correlated with the corresponding viscoelastic response. After detailed observation of the impact of molecular interactions on zero-shear viscosity η₀, we discuss it in terms of the effective packing parameter (PP_eff). All the investigated additives increased PP_eff, triggering anisotropic micellar growth toward WLMs. The simplicity of our approach is attractive for predicting and controlling the viscoelastic properties of WLM solutions from an intermolecular level.