Binary Colloidal Nanoparticles with a Large Size Ratio in Analytical Ultracentrifugation

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Materials Science

Background:

• Sedimentation of small colloidal particles in mixtures with larger particles can exhibit unexpected buoyancy effects.
• Understanding these effects is crucial for characterizing complex colloidal systems.

Purpose of the Study:

• To investigate the in situ buoyancy of colloidal particles within a concentrated binary suspension under a centrifugal field.
• To validate the concept of effective buoyancy in mixtures of colloids with a large size ratio.

Main Methods:

• Utilized a multi-wavelength analytical ultracentrifuge to measure in situ concentration gradients of two types of silica nanoparticles (90 nm and 30 nm).
• Employed fluorescence labeling for distinct particle tracking.
• Calculated effective solvent density using the concentration gradient of larger nanoparticles.

Main Results:

• The concentration gradient of smaller (30 nm) silica nanoparticles was accurately predicted using the exponential Boltzmann equation with a locally varying effective solvent density.
• Experimental results confirmed the theoretical predictions, demonstrating the validity of effective buoyancy.

Conclusions:

• Effective buoyancy is a valid phenomenon in colloidal mixtures.
• The developed model provides a robust framework for studying sedimenting polydisperse colloids.