la8b02930_si_001.pdf (348.83 kB)
Enthalpy and Entropy of Scission in Wormlike Micelles
journal contribution
posted on 2018-10-23, 00:00 authored by Hanqiu Jiang, Karsten Vogtt, Jacqueline B. Thomas, Gregory Beaucage, Andrew MulderigThe
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 ΔFsc and
the associated change in enthalpy ΔHsc and entropy ΔSsc 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 ΔHsc and entropy ΔSsc changes decrease linearly with increasing NaCl concentration.
At NaCl concentrations above 4 wt %, ΔSsc even adopts negative values. The term TΔSsc decreases more rapidly than
ΔHsc around room temperature and
causes the observed elongation of WLMs upon addition of NaCl. It is
suggested that ΔSsc 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
ΔSsc 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.