Phase behavior and interfacial properties of nonadditive mixtures of Onsager rods

Abstract

Within a second virial theory, we study bulk phase diagrams as well as the free planar isotropic-nematic interface of binary mixtures of nonadditive thin and thick hard rods. For species of the same type the excluded volume is determined only by the dimensions of the particles, whereas for dissimilar ones it is taken to be larger or smaller than that, giving rise to a nonadditivity that can be positive or negative. We argue that such a nonadditivity can result from modelling of soft interactions as effective hard-core interactions. The nonadditivity enhances or reduces the fractionation at isotropic-nematic ($IN$) coexistence and may induce or suppress a demixing of the high-density nematic phase into two nematic phases of different composition ($N_1$ and $N_2$), depending on whether the nonadditivity is positive or negative. The interfacial tension between co-existing isotropic and nematic phases show an increase with increasing fractionation at the $IN$ interface, and complete wetting of the $IN_2$ interface by the $N_1$ phase upon approach of the triple point coexistence. In all explored cases bulk and interfacial properties of the nonadditive mixtures exhibit a striking and quite unexpected similarity with the properties of additive mixtures of different diameter ratio.