Competition between entropy and electrostatic interactions in a binary colloidal mixture of spheres and platelets

Area of Science:

• Colloid and Surface Science
• Materials Science
• Soft Matter Physics

Background:

• Stable aqueous mixtures of laponite and maghemite nanoparticles are challenging due to surface chemistry incompatibilities.
• Silica coating of maghemite nanoparticles improves their compatibility with laponite.
• Understanding phase behavior is crucial for designing functional nanomaterial systems.

Purpose of the Study:

• To investigate the phase behavior of aqueous mixtures of laponite and silica-coated maghemite nanoparticles.
• To establish a fluid-solid state diagram for these nanoparticle mixtures.
• To explore the effect of magnetic fields on the mixture's structure and stability.

Main Methods:

• Preparation of silica-coated maghemite nanoparticles.
• Characterization of nanoparticle mixtures using small-angle neutron scattering (SANS).
• Optical microscopy to observe intermediate-scale structures.
• Application of external magnetic field gradients to study transitions.

Main Results:

• A "fluid-solid" state diagram was established, showing transitions dependent on laponite and maghemite volume fractions.
• Increased maghemite content shifts the transition to lower laponite concentrations.
• Magnetic field gradients induce a solid-to-liquid transition above a critical laponite volume fraction.
• Microscopic phase separation occurs, forming laponite domains and liquid maghemite pockets that grow over time.

Conclusions:

• The system exhibits a tunable fluid-solid transition influenced by nanoparticle concentrations and magnetic fields.
• Microscopic phase separation into laponite networks and maghemite-rich liquid pockets drives sample instability under magnetic fields.
• The interplay of entropic effects and electrostatic repulsions governs the observed phase separation and freezing behavior.