Phase-behaviour of Janus Dumbbell Liquids and Assemblies by Monte Carlo Techniques

Abstract

This thesis summarises a theoretical investigation into the phase behaviour of model ‘Janus’ dumbbell systems composed of two tangent beads, one possessing a hard-core potential and the other a square-well. The relationship between shape and interaction anisotropy is explored via varying the particle properties over two orthogonal pathways. We elucidate the effects of varying interaction and shape anisotropy on the fluid properties of these particles. A parameter, α is developed to control the size ratio between the interaction sites. The model allows particles to be tuned over the full range of diameter ratios. Variegated phase behaviour is observed with varying α. Gas-liquid critical phenomena is observed, in general, for the more isotropic particle descriptions where the particle possesses an attractive tail. As the interaction profile becomes more anisotropic the system properties change drastically. Phase diagrams are computed via collation of gas-liquid phase separation data, calculated via Successive Umbrella Sampling with histogram re-weighting, along with Simulated Annealing simulations in the Canonical ensemble, both of which employ an implementation of the Aggregation-Volume-Bias Monte Carlo intra-box swap move algorithm. Self-assembled phases are observed to perturb gas-liquid critical phenomena, where the critical point becomes metastable to bilayer structures before the Janus limit (α = 1). Bilayer structures in this region are observed to tolerate significant curvature until the two interaction sites are equal in size (α = 1) where only flat lamellar structures are found. Where the attractive site is smaller than the repulsive site, the systems are observed to display up to seven structurally differentiable behaviours controlled by α and the state-point, including: a monomer gas, micelles, platelets, strips, strip networks, sponges, and lamellar structures. Arguments are developed to account for this variation from first principles.