The importance of correlations in free-volume models for depletion phenomena

Two different levels of the free-volume approximation for describing one-dimensional mixtures of hard rods of two different lengths are analysed and tested against the exact solution. The mean-field level free-volume approach, where only the statistically pre-averaged free volume of the larger rods is accounted for, yields a spurious phase transition. Taking correlations into account not only gives better quantitative results, it also shows qualitative right physics, i.e. no phase transition.