Are Ellipsoids Feasible Micelle Shapes? An Answer Based on a Molecular-Thermodynamic Model of Nonionic Surfactant Micelles

Abstract

The existence of ellipsoidal micelles in aqueous solution has been debated in the literature. Although a number of experimental studies suggest that certain surfactants form ellipsoidal micelles, many theoretical studies have claimed that micelles with an ellipsoidal shape cannot exist. To shed light on this topic, in this paper, we develop a curvature-corrected, molecular-thermodynamic model for the free energy of micellization of nonionic surfactant biaxial ellipsoidal micelles. We subsequently use this model to evaluate the feasibility of forming ellipsoidal micelles, compared to forming spherical, spherocylindrical, and discoidal micelles, and conclude that ellipsoidal micelles can exist in solution. Utilizing the model developed here, we also establish theoretical limits on the size of the ellipsoidal micelles. These limits depend solely on the chemical structure of the surfactant molecule.

Micellar structure and transformations in sodium alkylbenzenesulfonate (NaLAS) aqueous solutions: effects of concentration, temperature, and salt

“…At NaLAS concentrations of 5 wt% and above, the major axis of the ellipsoidal micelles grows significantly, while the minor axis remains unchanged at about 20 Å in line with observations in other systems. 49,50 We find similar trends at different temperatures (Fig. 2d).…”

Dependence of Micelle Size and Shape on Detergent Alkyl Chain Length and Head Group

“…Beyond this simplified treatment of the head group interactions, Iyer and Blankschtein [37] and Dupuy et al [30] propose models for non-ionic surfactants that predict ellipticity and shape based on the packing and interactions between head groups (and solvent) that are independent of chain length. Their model also predicts that oblate micelles are preferred for small nonionic detergents, but that as the head group size increases or electrostatic repulsion occurs prolate micelles may be the preferred.…”

Molecular Dynamics Simulations of Cetyltrimethylammonium Bromide (CTAB) Micelles and their Interactions with a Gold Surface in Aqueous Solution

“…6 Variations of these factors yield aggregates with different morphologies such as spherical or ellipsoidal micelles, cylindrical or thread-like micelles, disk-like micelle, membranes and vesicles. 7 These self-assembled structures have been characterized by a number of techniques, such as dynamic light scattering (DLS), 8 nuclear magnetic resonance (NMR), 9,10 fluorescence spectroscopy, 11 quasielastic neutron scattering (QENS), 12,13 small-angle X-ray scattering (SAXS), 14,15 and small-angle neutron scattering (SANS). 16,17 Computational simulations have also been employed to explore the structures and dynamical behaviors of micelles for different surfactants.…”