Very low surface tension liquid–vapour interfaces of patchy colloids

The liquid–vapour interface of a model of one-component patchy colloids is re-visited. The model consists of hard spheres decorated with short-ranged attractive sites (‘patches’) of different types on their surfaces. In an earlier paper [A. Oleksy and P.I.C. Teixeira, Phys. Rev. E 91 (2015) 012301], the density-functional theory (DFT) form of Wertheim's first-order perturbation theory (TPT1) was used to study a realisation of this model with two patches of type A and nine patches of type B (2A9B colloids), which exhibit re-entrant liquid–vapour coexistence curves and very low-density liquid phases. Here, it is shown that the non-monotonic temperature dependence of the surface tension and interface thickness in this model can be qualitatively reproduced by a local DFT, where the spatial non-uniformity is introduced through the square gradient of the density multiplied by 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 (2A10B colloids), and interactions chosen so as to exhibit a closed-loop phase diagram when only AA and AB bonds are present [N.G. Almarza et al., J. Chem. Phys. 137 (2012) 244902]. In this case, the surface tension goes to zero at both upper and lower critical points, with a maximum at some intermediate temperature. As BB attractions are gradually switched on, the lower critical point disappears and the surface tension vs temperature curve acquires a very deep minimum at a temperature lower than that of the maximum. On further increasing the strength of the BB attractions, both the minimum and the maximum disappear and 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 2AnB colloids might be exploited as temperature-controlled surfactants and foam stabilizers.