Vesicle to micelle transition in the ternary mixture of L121/SDS/D₂O: NMR, EPR and SANS studies

Subtle changes in the microstructure and dynamics of the triblock copolymer L121, (ethylene oxide)₅ (propylene oxide)₆₈ (ethylene oxide)₅i.e., E₅P₆₈E₅, and sodium dodecylsulfate (SDS) system in aqueous medium were investigated using high-resolution nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) and small-angle neutron scattering (SANS) methods. NMR self-diffusion measurements helped us to understand the nature of binding of SDS with L121, and the formation of their mixed aggregates. These results showed that even at low [SDS] (∼2 mM), the addition of L121 stabilized the dynamics of SDS. Furthermore, the increase in [SDS] resulted in progressive changes in the diffusion behavior of both SDS and L121. ¹³C chemical shift analysis revealed that preferential binding of L121 occurred on the SDS micelle surface. Deuterium (²H) NMR spin-relaxation data evidenced that the formed mixed aggregates were non-spherical in terms of relaxation rate changes, and slowed the dynamics. The rotational correlation times of mixed aggregates were estimated from EPR analysis. A SANS study indicated the presence of uni- and multi-lamellar vesicles of L121 at low [SDS]. The vesicles transformed to mixed L121-SDS micelles in the presence of a higher [SDS]. This was supported by the measurements of ²H NMR spin-relaxation and EPR rotational correlation times.