Uniaxial Extension of Surfactant Micelles: Counterion Mediated Chain Stiffening and a Mechanism of Rupture by Flow-Induced Energy Redistribution

We study the configurational dynamics in uniaxial elongational flow of rodlike and U-shaped cationic surfactant micelles of cetyltrimethylammonium chloride (CTAC) in the presence of sodium salicylate (NaSal) counterions in water using molecular dynamics simulations. Above the critical strain rate, approximately equal to the inverse of the micelle relaxation time, hydrodynamic forces overcome the conformational entropy of the micelle and a configurational transition from a folded to a stretched state occurs. As the accumulated strain exceeds a critical value of O(100), the micelle ruptures through a midplane thinning mechanism facilitated by the advection of the counterions toward the micelle end-caps. The change in the total pair-potential energy as a function of micelle elongation is well described by a Hookean spring model that allowed to estimate the stretching modulus of the micelle. Micelle stiffness depends greatly on the degree of screening of electrostatic repulsion among the CTA⁺ head groups by the Sal^- counterions condensed on the surface. A moderate increase in the counterion concentration makes the molecular assembly tighter and more immune to deformation by hydrodynamic stresses, resulting in an order magnitude enhancement in the stretching modulus.