Kinetics of thermo-induced micelle-to-vesicle transitions in a catanionic surfactant system investigated by stopped-flow temperature jump

Jingyan Zhang and Shiyong Liu
Phys. Chem. Chem. Phys, 2011,13, 12545–12553

Abstract

The kinetics of thermo-induced micelle-to-vesicle transitions in a catanionic surfactant system
consisting of sodium dodecyl sulfate (SDS) and dodecyltriethylammonium bromide (DEAB) were
investigated by the stopped-flow temperature jump technique, which can achieve T-jumps within
B2–3 ms. SDS/DEAB aqueous mixtures ([SDS]/[DEAB] = 2/1, 10 mM) undergo microstructural
transitions from cylindrical micelles to vesicles when heated above 33 1C. Upon T-jumps from
20 1C to final temperatures in the range of 25–31 1C, relaxation processes associated with negative
amplitudes can be ascribed to the dilution-induced structural rearrangement of cylindrical micelles
and to the dissolution of non-equilibrium mixed aggregates. In the final temperature range of
33–43 1C the obtained dynamic traces can be fitted by single exponential functions, revealing one
relaxation time (t) in the range of 82–440 s, which decreases with increasing temperature.
This may be ascribed to the transformation of floppy bilayer structures into precursor vesicles
followed by further growth into final equilibrium vesicles via the exchange and insertion/expulsion
of surfactant monomers. In the final temperature range of 45–55 1C, vesicles are predominant.
Here T-jump relaxations revealed a distinctly different kinetic behavior. All dynamic traces can
only be fitted with double exponential functions, yielding two relaxation times (t1 and t2),
exhibiting a considerable decrease with increasing final temperatures. The fast process
(t1 B 5.2–28.5 s) should be assigned to the formation of non-equilibrium precursor vesicles, and
the slow process (t2 B 188–694 s) should be ascribed to their further growth into final
equilibrium vesicles via the fusion/fission of precursor vesicles. In contrast, the reverse
vesicle-to-micelle transition process induced by a negative T-jump from elevated temperatures to
20 1C occurs quite fast and almost completes within the stopped-flow dead time (B2–3 ms).

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