Gels from surfactant solutions with densely packed multilamellar vesicles

Area of Science:

• Colloid and surface science
• Materials science
• Rheology

Background:

• Surfactant gels form spontaneously from specific phase structures (L alpha, L3) of alkyldimethylaminoxide and cosurfactant.
• Charging these structures with ionic surfactant or HCl initiates gel formation.
• Multilamellar vesicles are key components within these surfactant gels.

Purpose of the Study:

• To investigate the rheological behavior of surfactant gels with densely packed multilamellar vesicles.
• To understand the influence of salinity, charge density, and surfactant chain length on gel properties.
• To validate theoretical models describing the gel's mechanical response.

Main Methods:

• Dynamic rheological measurements to assess storage modulus (G') and yield stress (sigma y).
• Preparation of gels using varying salt concentrations and water-glycerol mixtures.
• Interference contrast microscopy and conductivity measurements for vesicle characterization.
• Application of theoretical models (van der Linden, Lekkerkerker) for data interpretation.

Main Results:

• Both storage modulus and yield stress decrease with increasing salinity due to reduced electrostatic contributions and vesicle compression modulus.
• G' and sigma y increase with charge density, plateauing based on surfactant chain length.
• Gel moduli and yield stress are independent of solvent viscosity.
• Microscopy revealed larger vesicles than expected; conductivity data estimated 5-6 shells per vesicle.

Conclusions:

• Salinity and charge density significantly impact the rheological properties of these surfactant gels.
• Theoretical models effectively explain the observed experimental data, linking macroscopic properties to microscopic structure and interactions.
• The findings provide insights into the structure-property relationships of complex surfactant systems.