How patchy can one get and still condense?

Tavares, J.M.¹ Teixeira, P.¹ and Telo da Gama, M.¹

¹ Centro de Física Teórica e Computacional da Universidade de Lisboa, Portugal

We investigate the influence of strong directional, or bonding, interactions on the phase diagram of complex fluids, and in particular on the liquid-vapour critical point. To this end we revisit a simple model and theory for associating fluids which consists of spherical particles having a hard-core repulsion, complemented by three short-ranged attractive sites on the surface (sticky spots), two of type A and one of type B.  Results are interpreted in terms of the equilibrium cluster structures of the coexisting phases. In systems where there is no AB interaction, the critical point exists for all values of the BB interaction strength. By contrast, when there is no BB interaction, there is no critical point below a certain finite value of the AB interaction strength. These somewhat surprising results are rationalised in terms of the different network structures of the two systems: two long AA chains are linked by one BB bond (X-junction) in the former case, and by one AB bond (Y-junction) in the latter. The vapour-liquid transition may then be viewed as the condensation of these junctions and, we find that X-junctions condense for any fraction of BB bonds, whereas condensation of the Y-junctions requires a finite fraction of AB bonds.