Phase behavior of binary mixtures of thick and thin hard rods

Using a straightforward extension of the Onsager-theory for hard rods, we c onsider the thermodynamic stability of the isotropic (I) and nematic (N) ph ase of binary mixtures of thick and thin hard rods of the same length. We s how that such mixtures not only exhibit the expected I-N ordering transitio n and the previously predicted depletion driven I-I demixing transition, bu t also a N-N demixing transition driven by the orientation entropy of the t hinner rods. For various values of the diameter ratio of the two species we present the phase diagrams, which exhibit I-N, I-I and N-N coexistence, I- N-N and I-I-N triple points, and I-I and N-N critical points. We also prese nt the results of computer simulations of the I-I and I-N coexistence for d iameter ratio 1:10, which are in good agreement with the theory. (C) 1998 E lsevier Science B.V. All rights reserved.