Enthalpy and Entropy of Scission in Wormlike Micelles

The free scission energy is the thermodynamic parameter that governs the contour length of wormlike micelles (WLMs). It is the contour length and the propensity to coil and entangle that determine the viscoelastic properties of this commercially important substance class. The free scission energy ΔF_sc and the associated change in enthalpy ΔH_sc and entropy ΔS_sc on scission have been determined for a mixed anionic/zwitterionic surfactant system (sodium laureth sulfate and cocamidopropyl betaine) at various salt concentrations (3–5 wt % NaCl). Both enthalpy ΔH_sc and entropy ΔS_sc changes decrease linearly with increasing NaCl concentration. At NaCl concentrations above 4 wt %, ΔS_sc even adopts negative values. The term TΔS_sc decreases more rapidly than ΔH_sc around room temperature and causes the observed elongation of WLMs upon addition of NaCl. It is suggested that ΔS_sc is initially positive due to fewer bound counterions per surfactant molecule at end caps compared to the intact, cylindrical parts before scission, leading to a net release of ions upon scission. Negative values of ΔS_sc are attributed to hydrophobic hydration occurring at the end caps at high salt concentrations. ²³Na NMR measurements indicate the presence of immobilized ions, supporting a previously proposed ion-cloud model based on neutron scattering results.