Salt effect on microstructures in cationic gemini surfactant solutions as studied by dynamic light scattering

Area of Science:

• Colloid and Surface Chemistry
• Physical Chemistry
• Materials Science

Background:

• Gemini surfactants, characterized by two hydrophilic heads and two hydrophobic tails linked by a spacer, exhibit unique aggregation properties compared to single-chained surfactants.
• Understanding surfactant behavior in aqueous solutions is crucial for applications in detergents, drug delivery, and nanotechnology.
• The influence of salt concentration on surfactant aggregation and micelle formation is a key area of research in colloid science.

Purpose of the Study:

• To investigate the aggregation behavior of a cationic gemini surfactant, dodecanediyl-1,12-bis(dodecyldiethylammonium bromide) (C12C12C12(Et)), in aqueous solutions.
• To compare the behavior of the gemini surfactant with its single-chained counterpart, dodecyl triethylammonium bromide (DTEAB).
• To elucidate the effect of varying sodium bromide (NaBr) concentrations on the micellar structures and dynamics of the gemini surfactant.

Main Methods:

• Dynamic Light Scattering (DLS) was employed to study the hydrodynamic properties of surfactant aggregates.
• Experiments were conducted in aqueous solutions with systematically varied NaBr concentrations.
• The behavior of the gemini surfactant was contrasted with that of a single-chained surfactant (DTEAB) under identical conditions.

Main Results:

• The cationic gemini surfactant (C12C12C12(Et)) displayed an "ordinary-to-extraordinary (o-e) transition" as salt concentration decreased, similar to polyelectrolytes.
• At high salt concentrations, DLS revealed a single relaxation mode attributed to regular micelle diffusion.
• In the "extraordinary regime" (low salt), two distinct relaxation modes were observed, differing by over two orders of magnitude in diffusion coefficient.
• The fast mode aligns with coupled-mode theories and electrostatic repulsion, while the slow mode is attributed to multimacroion domains.

Conclusions:

• The study demonstrates the complex aggregation behavior of cationic gemini surfactants in response to salt concentration.
• The observed "o-e transition" highlights the transition from conventional micellar structures to more complex, potentially charged, aggregates at low salt.
• The presence of two relaxation modes in the "extraordinary regime" provides insights into the formation and dynamics of multimacroion domains, influenced by electrostatic interactions.