The Relationship Between Wormlike Micelle Scission Free Energy and Micellar Composition: The Case of Sodium Laurylethersulphate and Cocamidopropyl Betaine

The scission energy is the difference in energy between two hemispherical caps and the cylindrical region of a wormlike micelle. This energy difference is exponentially proportional to the average micelle length, which affects several macroscopic properties such as the viscosity of viscoelastic fluids. Here we use a recently published method by Wang et al (Langmuir 2018 34 1564-1573) to directly calculate the scission energy of micelles composed of monodisperse Sodium Laurylethersulphate (SLESnEO), an anionic surfactant. We perform a systematic study varying the number of ethoxyl groups (n) and salt concentration. The scission energy increases with increasing salt concentration, indicating that the formation of longer micelles is favoured. We attribute this to the increased charge screening that reduces the repulsion between head groups. However, the scission energy decreases with increasing number of ethoxyl groups as the flexibility of the head group increases and the sodium ion becomes less tightly bound to the head group. We then extend to look at the effect of a common co-surfactant, Cocamidopropyl Betaine (CAPB) and find that its addition increases the scission energy, stabilising wormlike micelles at a lower salt concentration.