Neutron Reflectometry of an Anionic Surfactant at the Solid-Liquid Interface under Shear

Area of Science:

• Surface science and colloid chemistry
• Materials science
• Rheology

Background:

• Understanding interfacial surfactant behavior is crucial for applications like enhanced oil recovery, detergency, and nanoparticle stabilization.
• Anionic surfactants, such as sodium bis(2-ethylhexyl) sulfosuccinate (AOT), form complex structures at interfaces, influencing interfacial properties.
• The mechanical response of adsorbed surfactant layers under shear is not fully understood, particularly the transition from simple to complex structures.

Purpose of the Study:

• To investigate the shear response of adsorbed anionic surfactant layers at the alumina-water interface using combined neutron reflectometry and in situ rheology.
• To differentiate the structural stability of a single surfactant bilayer versus a multilamellar phase under varying shear conditions (steady and oscillatory).
• To elucidate the influence of strain amplitude on the structural integrity of adsorbed surfactant layers.

Main Methods:

• Utilized neutron reflectometry to probe the structure of adsorbed sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at the alumina-water interface.
• Integrated in situ rheology to apply controlled shear (steady and oscillatory) to the interfacial surfactant layers during neutron reflectometry measurements.
• Analyzed structural changes in response to varying surfactant concentrations and applied shear parameters (shear rate, strain amplitude).

Main Results:

• At low AOT concentration, a stable single bilayer adsorbed at the interface, showing no structural changes under steady or oscillatory shear.
• At higher AOT concentration, a multilamellar surfactant structure formed, which exhibited a significant loss of structural order under both steady and oscillatory shear.
• The disruption of the multilamellar phase was dependent on strain amplitude, with distinct differences observed between steady and oscillatory shear responses.

Conclusions:

• The mechanical stability of adsorbed surfactant layers is highly dependent on their structural organization.
• Multilamellar surfactant phases are susceptible to shear-induced structural breakdown, unlike stable single bilayers.
• Neutron reflectometry coupled with in situ rheology provides a powerful tool for characterizing the dynamic response of interfacial surfactant structures.