Microemulsions with Didodecyldimethylammonium Bromide Studied by Neutron Contrast Variation

Area of Science:

• Colloid and Surface Science
• Materials Science
• Physical Chemistry

Background:

• Water-in-oil microemulsions are complex systems with applications in various industries.
• Understanding the structure and properties of the surfactant film is crucial for controlling microemulsion behavior.
• Didodecyldimethylammonium bromide (DDAB) is a common surfactant used in microemulsion formulations.

Purpose of the Study:

• To investigate the structure of water-in-oil microemulsion droplets stabilized by DDAB.
• To obtain detailed information about the curved surfactant film using selective deuteration.
• To analyze the film properties in relation to droplet size and interfacial tension.

Main Methods:

• Small-angle neutron scattering (SANS) with selective deuteration of water, DDAB, and cyclohexane.
• Analysis of neutron contrast data using a Schultz distribution of core-shell particles.
• Measurement of cyclohexane-water interfacial tensions using surface light scattering.

Main Results:

• The surfactant film was found to be liquid-like with a density of 0.80 g cm⁻³ and no solvent penetration.
• The film thickness was determined to be 11-12 Å, approximately 70% of the C12 chain length.
• The area per head group and alkyl chains were quantified, and the film rigidity was found to be approximately 1.0kBT.

Conclusions:

• The DDAB surfactant film exhibits liquid-like properties and a defined thickness.
• Droplet polydispersity is a natural consequence of the film's rigidity, not an independent parameter.
• The film bending energy model effectively describes the behavior of DDAB microemulsions.