Direct observation of critical adsorption on colloidal particles

Area of Science:

• Colloid and Surface Science
• Soft Matter Physics
• Neutron Scattering Techniques

Background:

• Adsorbed solvent layers are critical for Casimir forces between colloidal particles.
• The precise structure of these layers, especially around curved surfaces, remains largely undetermined.
• Understanding these structures is key to predicting colloidal behavior in critical fluids.

Purpose of the Study:

• To directly determine the structure of adsorbed solvent layers on spherical silica particles.
• To investigate the excess adsorption (Γ) and surface concentration profiles as a function of temperature and concentration.
• To elucidate the contributions of adsorption, including profile shape, and their dependence on curvature and temperature.

Main Methods:

• Small-angle neutron scattering (SANS) was employed to measure critical adsorption.
• Experiments were conducted on silica nanoparticles in a lutidine-water binary mixture.
• Measurements were performed at critical and off-critical concentrations across varying temperatures.

Main Results:

• The study successfully differentiated three contributions to excess adsorption, including the profile shape.
• Adsorption associated with the profile shape increased with 2,6-lutidine concentration.
• A scaling relation Γ∼(T-Tc)/Tc (-0.52) was observed, indicating curvature-induced corrections to planar adsorption.

Conclusions:

• The structure of adsorbed solvent layers on curved surfaces was directly measured for the first time.
• Curvature effects significantly influence adsorption, leading to a distinct scaling behavior compared to planar systems.
• These findings provide crucial insights into the role of solvent layers in critical Casimir forces and colloidal interactions.