Surface shear inviscidity of soluble surfactants

Area of Science:

• Colloid and Surface Science
• Rheology
• Materials Science

Background:

• Foam and emulsion stability are traditionally linked to the surface shear viscosity of stabilizing surfactants.
• Interpreting surface shear viscosity measurements presents challenges, and correlation does not imply causation.

Purpose of the Study:

• To conduct the most sensitive and precise measurements of surface shear viscosity for various soluble surfactants, with a focus on sodium dodecyl sulfate (SDS).
• To investigate the relationship between surface shear rheology and the stability of foams and emulsions.

Main Methods:

• Utilized a sensitive technique to excite purely surface shear deformations.
• Performed control and complementary measurements, including direct visualization of monolayer deformation.
• Tested a variety of soluble surfactants: SDS, polymeric, ionic, and nonionic surfactants with high- and low-foaming characteristics.

Main Results:

• The surface shear viscosity of SDS was found to be below the technique's sensitivity limit, with an upper bound of approximately 0.01 μN·s/m.
• This result contradicts previous studies reporting significantly higher values (10^3-10^4 times greater).
• No measurable surface shear viscosity was detected for any tested soluble, small-molecule surfactant.

Conclusions:

• The findings seriously undermine the widely accepted correlation between foam stability and the surface shear rheology of soluble surfactants.
• The study suggests that other factors likely dominate foam and emulsion stability for soluble surfactants.
• Re-evaluation of the role of surface shear viscosity in interfacial phenomena is warranted.