Thermodynamics and kinetics of vesicle↔mixed micelle transitions of sodium tridecyl-6-benzene sulfonate/sodium dodecyl sulfate surfactant systems

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Abstract
When aqueous salt solutions ([NaCl]0.02 mol dm-3) of the double-chain surfactant sodium tridecyl-6-benzene sulfonate (STBS) are titrated with solutions of sodium dodecyl sulfate (SDS) at low concentrations (≈mM) of the two surfactants, mixed micelles of STBS/SDS are formed at the expense of STBS bilayer vesicles. The extent and the kinetics of the interconversion are readily followed by the change in light scattering (turbidity) of the system at λ=300 nm. The data reflecting the surfactant self-assembly/disassembly processes can be analysed in terms of phase-transition-like conversions between vesicles and mixed micelles. The conversions are characterised by the critical free concentration [Dc] of the single-chain surfactant required to effect STBS vesicle dissolution. For example, for addition of SDS at [NaCl]=0.05 M, the characteristic lyotropic transition constant is [Dc]=0.48±0.03 mM, and for ethanol [Dc]=0.60±0.05 M at 293 K (20°C). The thermodynamic analysis also yields the critical ratio Rc of bound single-chain surfactant for vesicle dissolution (RV c). A similar value (RM c) can be obtained for the mixed micelle. For instance, when SDS is used, the values are RV c=0.10 and RM c=0.71. This means that within the transition region one bound SDS molecule per 10 STBS molecules effects vesicle dissolution and the mixed micelles which are formed following dissolution contain ca. 70 SDS molecules per 100 STBS molecules. The kinetics of surfactant-induced vesicle dissolution are associated with a highly cooperative surfactant/vesicle interaction which provides a lag phase followed by an exponential decay phase associated with vesicle disintegration/mixed micelle formation and characterised by a Hill coefficient nH of ca. 8. The overall rate constants for micelle formation from vesicles are dramatically dependent on the SDS concentration, decreasing from kexp=0.090 s-1 at [SDST]=0.65 mM to kexp=0.023 s-1 at [SDST]=0.55 mM.

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