Abstract

We investigate the liquid-vapour interface of a model of patchy colloids. This model consists of hard spheres decorated with short-ranged attractive patches of different types on their surfaces. We focus on a one-component fluid with two patches of type A and nine patches of type B, which has been found to exhibit reentrant liquid-vapour coexistence curves and very low-density liquid phases. We have used the density-functional theory form of Wertheim’s first-order perturbation theory of association to calculate the surface tension, and the density and degree of association profiles, at the liquid-vapour interface of our model. In re-entrant systems, where AB bonds dominate, an unusual thickening of the interface is observed at low temperatures. Furthermore, the surface tension versus temperature curve reaches a maximum. If BB attractions are also present, competition between AB and BB bonds gradually restores the monotonic temperature dependence of the surface tension. Lastly, the interface contains a region where the average chain length is close to that in the bulk liquid,but where the density is that of the vapour. Sufficiently strong BB attractions remove these features. We next show that the non-monotonic temperature dependence of the surface tension and interfacethickness in this model can be qualitatively reproduced by a local, square-gradient type DFT , with a density-dependent prefactor. This simpler theory is then applied to colloids decorated with two patches of type A and ten patches of type B interactions chosen so as to exhibit a closed-loop phase diagram when only AA and AB bonds are present. As BB attractions are gradually switched on, the lower critical point disappears and the surface tension vs temperature curve acquires a very deep minimum. On further increasing the strength of the BB attractions, the surface tension becomes a monotonically decreasing function of the temperature, as in atomic fluids. The very low surface tension at the minimum for weak BB attractions, combined with the ability to form physical gels, suggests that patch colloids might be exploited as temperature-controlled surfactants and foam stabilizers.