Enthalpy and Entropy of Scission in Wormlike Micelles

Published on 2018-11-08T20:16:32Z (GMT) by
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 Δ<i>F</i><sub>sc</sub> and the associated change in enthalpy Δ<i>H</i><sub>sc</sub> and entropy Δ<i>S</i><sub>sc</sub> 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 Δ<i>H</i><sub>sc</sub> and entropy Δ<i>S</i><sub>sc</sub> changes decrease linearly with increasing NaCl concentration. At NaCl concentrations above 4 wt %, Δ<i>S</i><sub>sc</sub> even adopts negative values. The term <i>T</i>Δ<i>S</i><sub>sc</sub> decreases more rapidly than Δ<i>H</i><sub>sc</sub> around room temperature and causes the observed elongation of WLMs upon addition of NaCl. It is suggested that Δ<i>S</i><sub>sc</sub> 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 Δ<i>S</i><sub>sc</sub> are attributed to hydrophobic hydration occurring at the end caps at high salt concentrations. <sup>23</sup>Na NMR measurements indicate the presence of immobilized ions, supporting a previously proposed ion-cloud model based on neutron scattering results.

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Jiang, Hanqiu; Vogtt, Karsten; Thomas, Jacqueline B.; Beaucage, Gregory; Mulderig, Andrew (2018): Enthalpy and Entropy of Scission in Wormlike Micelles. ACS Publications. Collection.