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

Faculty of Science School of Chemistry & Department of Molecular Sciences and Nanosystems Doctor of Philosophy Phase-behaviour of Janus Dumbbell Liquids and Assemblies by Monte Carlo Techniques by Patrick Ignatius O’Toole This thesis summarises a theoretical investigation into the phase behaviour of model ‘Janus’ dumbbell systems composed of two tangent spherical sub-particles, one possessing a hard-sphere potential and the other a square-well potential. The relationship between shape and interaction anisotropy is explored via varying the particle properties over two orthogonal pathways from reference systems to the target system to elucidate the effects of increasing particle interaction and shape anisotropy on the fluid properties at a range of state-points. Three different simulation techniques are employed to variously study the gas-liquid properties, self-assembled structures, and compression behaviour. The reference systems involved are the hard-sphere model, the square-well model, and the square-well dumbbell, each forming an extremum of the parameter space. A parameter, α is developed to control the size ratio between the interaction sites. The parameterisation developed herein allows the particles to be tuned over the full range of diameter ratios to explore from a pure hard-sphere system at α = 0, to a pure square-well system at α = 2, with the region between 0 < α < 2 characterising the heterogeneous Janus dumbbell. Variegated phase behaviour is observed over the range of α. Gas-liquid critical phenomena is observed, in general, for the more isotropic particle descriptions where the particle possesses an attractive portion. 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 in the Canonical ensemble, both of which employ an implementation of the AggregationVolume-Bias Monte Carlo intra-box swap move algorithm. The development of selfassembled phases that perturb gas-liquid critical phenomena is observed until the critical point becomes metastable with respect to the formation of bilayer structures where the diameter of the purely repulsive interaction site is similar to the attractive site. On the scale of the simulations bilayer structures in this region are observed to tolerate significant curvature until the hard-cores of the two interaction sites are equal in size (α = 1). In the intervening regime (1 < α < 1.3) several topologically different structures are observed including: vesicles, tubes, gyroids and other bi-continuous structures. Where the diameters are equal only micelles are observed at low density which eventually form lamellar sheets at higher densities. 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. The location of these structures is identified on phase diagrams at each α studied.