Oscillatory structural forces due to nonionic surfactant micelles: data by colloidal-probe AFM vs theory

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Materials Science

Background:

• Nonionic surfactants form micelles in solution, influencing fluid properties.
• Understanding micellar behavior under confinement is crucial for various applications.
• Colloidal-probe atomic-force microscopy (CP-AFM) is a powerful tool for probing interactions at the nanoscale.

Purpose of the Study:

• To investigate the behavior of Brij 35 and Tween 20 surfactant micelles confined between surfaces using CP-AFM.
• To compare experimental force measurements with theoretical predictions for micellar fluids.
• To determine the stability and interaction dynamics of different micellar structures.

Main Methods:

• Confining micellar solutions of Brij 35 and Tween 20 between surfaces in a CP-AFM.
• Measuring oscillatory forces during layer-by-layer micelle expulsion.
• Comparing experimental data with hard-sphere fluid theoretical models.
• Analyzing force-distance curves to determine micelle aggregation numbers and interaction ranges.

Main Results:

• Experimental oscillatory forces agreed well with hard-sphere fluid predictions, allowing full curve reconstruction.
• Brij 35 micelles (aggregation number ~70) behaved as rigid particles, with spherical micelles showing size-dependent oscillation periods.
• Elongated Brij 35 micelles exhibited thickness-dependent periods, while Tween 20 micelles were largely destroyed by shear, indicating labile structures.
• Non-equilibrium portions in force curves indicated jumps between stable and unstable states.

Conclusions:

• Surfactant micelle behavior under confinement varies significantly, with some acting as rigid particles and others being disrupted by CP-AFM shear.
• Micelle stability, influenced by surfactant concentration and relaxation times, dictates their response to confinement and shear forces.
• The study highlights the importance of considering micellar lability and rigidity in interfacial phenomena and nanotechnology.