Thermodynamic stability of worm-like micelle solutions and the corresponding ion behavior*

Worm-like micelles are nano-scale self-assemblies widely used for viscosity enhancement, for drug delivery, and in personal care products. The stability of WLMs under variable ionic and surfactant concentrations is important to applications of these fascinating materials. A mixed surfactant system consisting of sodium laureth sulfate and cocoamidopropyl betaine was examined using small-angle neutron scattering. It is demonstrated that structural screening, and the associated virial coefficient, has an increasing impact on scattering with increasing surfactant concentration. A linear relationship between the second virial coefficient, A₂, and the salt to surfactant ratio, Θ_s–s, is derived based on SANS results. The Θ_s–s-dependency is described via association/dissociation kinetics of salt ions between the bulk and an ion cloud surrounding the WLMs. An ion-cloud model for the high ionic strength condition is proposed and verified based on this work. It is also demonstrated that a virial approach can be used to understand and predict WLM stability.
K. Vogtt, et al. Soft Matter, 2017, 13, 6068
H. Fujita. Polymer solutions. Elsevier, Amsterdam, The Netherlands, 1990, pp. 195–197