Phase behavior of a binary fluid mixture of quadrupolar molecules

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Binary mixtures can exhibit complex phase behavior, including closed-loop coexistence regions.
• Understanding microscopic properties is key to predicting macroscopic phase diagrams.
• Anisotropic molecular interactions significantly influence fluid phase behavior.

Purpose of the Study:

• To propose and investigate a model molecule for studying binary mixtures with closed-loop coexistence.
• To elucidate the role of anisotropic interactions, specifically quadrupoles, in phase behavior.
• To characterize the microscopic structures formed in different phase regions.

Main Methods:

• Utilizing Gibbs ensemble Monte Carlo simulations to study phase equilibria.
• Employing molecular dynamics simulations to observe aggregate formation.
• Designing a model molecule combining Lennard-Jones interactions with a uniaxial quadrupole.

Main Results:

• Demonstrated closed-loop immiscibility in binary fluids with oppositely signed quadrupoles.
• Observed shrinkage of the coexistence region with increasing quadrupole magnitude.
• Identified temperature-dependent aggregate structures: strings below the lower critical solution temperature and branched aggregates above the upper critical solution temperature.

Conclusions:

• Anisotropic quadrupole interactions are critical for controlling the phase behavior of binary mixtures.
• The model molecule effectively captures essential features of closed-loop immiscibility.
• Microscopic aggregate structures provide insights into the thermodynamic driving forces of phase separation.