Measurement of Mixed Micelle Size by Dynamic Light Scattering

Importance of Mixed Micelle Size Analysis

Surfactant mixtures, particularly combinations of ionic and nonionic types, are prevalent in applications ranging from detergents to drug delivery systems. Measuring the size and aggregation number of mixed micelles is critical because it influences solubilization capacity, solution viscosity, and overall performance of formulations.

Objectives and Study Overview

The study aimed to quantify the hydrodynamic radius of mixed micelles formed by sodium dodecyl sulfate (SDS) and Triton X-100. The work examined how varying the mole fraction of SDS and the ionic strength of the medium affected micelle dimensions.

Methodology and Instrumentation

Dynamic light scattering using the Cumulants analysis was applied to determine micelle size. Key experimental parameters:

• Total surfactant concentration set at 20 mM, well above the critical micelle concentration.
• SDS mole fraction (Y) varied from 0 to 1 to cover pure and mixed systems.
• Ionic strength adjusted to 100, 400, and 600 mM with NaCl to probe electrostatic screening effects.
• Measurements conducted at controlled temperature conditions using the DynaPro-801 instrument.

Key Results and Discussion

Micelle size exhibited distinct trends depending on salt concentration:

• At low ionic strength (100 mM NaCl): Micelle radius decreased steadily with increasing SDS fraction due to strong repulsion between ionic head groups.
• At intermediate ionic strength (400 mM NaCl): Micelle size remained nearly constant for SDS fractions between 0.3 and 0.8, indicating balanced head group interactions.
• At high ionic strength (600 mM NaCl): Size increased with SDS content up to a maximum near Y = 0.9, followed by a reduction as electrostatic contributions reemerged, reflecting diminished screening.

These observations align with the concept of effective head group area, where ionic repulsion and salt screening govern micelle aggregation number.

Benefits and Practical Applications

• Optimizing mixed surfactant composition can tailor solubilization efficiency in pharmaceuticals, cosmetics, and cleaning products.
• Control over micelle size allows fine-tuning of solution viscosity for process and consumer performance.
• Understanding head group interactions aids in designing formulations resistant to destabilization by electrolytes or temperature changes.

Future Trends and Applications

Advances may include:

• Integration of complementary techniques such as small-angle neutron scattering for detailed shape and structure analysis.
• Exploration of ternary or more complex surfactant systems to achieve multifunctional properties.
• Development of predictive models linking molecular-level interactions to macroscopic formulation behavior.

Conclusion

Dynamic light scattering reveals that mixed micelle size is a non-linear function of surfactant composition and ionic environment. By adjusting SDS fraction and salt concentration, one can strategically control micelle dimensions, optimizing product performance.

Reference

None provided.