Phase behavior of hard colloidal platelets using free energy calculations

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Physics

Background:

• Understanding the phase behavior of anisotropic particles is crucial for designing advanced materials.
• Oblate hard spherocylinders (OHSC) serve as a model for colloidal hard platelets and rigid discotic molecules.
• Previous models, like hard cut spheres, do not fully capture experimental observations for disc-like molecules.

Purpose of the Study:

• To map the phase diagram of oblate hard spherocylinders (OHSC).
• To compare the phase behavior of OHSC with that of hard cut spheres.
• To identify the most suitable model particle for colloidal platelets and discotic molecules.

Main Methods:

• Free energy calculations using Monte Carlo simulations.
• Systematic variation of the aspect ratio (L/D) of OHSC particles.
• Comparative analysis with existing hard cut sphere phase diagrams.

Main Results:

• A detailed phase diagram for OHSC was generated, including isotropic, nematic, columnar, tilted crystal, and aligned crystal phases.
• A tilted crystal phase, common in experiments with disc-like molecules, was identified for OHSC but not for hard cut spheres.
• The cubatic phase was found to be unstable in OHSC, contrasting with ongoing debates about its stability in hard cut spheres.

Conclusions:

• Oblate hard spherocylinders (OHSC) exhibit a richer phase behavior than hard cut spheres, particularly with the presence of a tilted crystal phase.
• The subtle shape differences between OHSC and hard cut spheres lead to significant variations in phase boundaries.
• OHSC are a more accurate model for simulating the phase behavior of experimental colloidal platelets and discotic molecules.