The fundamental nature of the nematic-nematic (N-N) phase separation in binary mixtures of rigid hard rods is analyzed within the Onsager second-virial theory and the extension of Parsons and Lee which includes a treatment of the higher-body contributions. The particles of each component are modeled as hard spherocylinders of different diameter (D1 D2), but equal length (L1=L2=L). In the case of a system which is restricted to be fully aligned (parallel rods), we provide an analytical solution for the spinodal boundary for the limit of stability of N-N demixing; only a single region of N-N coexistence bounded at lower pressures (densities) by a N-N critical point is possible for such a system. The full numerical solution with the Parsons-Lee extension also indicates that, depending on the length of the particles, there is a range of values of the diameter ratio (d=D2 D1) where the N-N phase coexistence is closed off by a critical point at lower pressure. A second region of N-N coexistence can be found at even lower pressures for certain values of the parameters; this region is bounded by an "upper" N-N critical point. The two N-N coexistence regions can also merge to give a single region of N-N coexistence extending to very high pressure without a critical point. By including the higher-order contributions to the excluded volume (end effects) in the Onsager theory, we prove analytically that the existence of the N-N lower critical point is a direct consequence of the finite size of the particles. A new analytical equation of state is derived for the nematic phase using the Gaussian approximation. In the case of Onsager limit (infinite aspect ratio), we show that the N-N phase behavior obtained using the Parsons-Lee approach substantially deviates from that with the Onsager theory for the N-N transition due to the nonvanishing third and higher order virial coefficients. We also provide a detailed discussion of the N-N phase behavior of recent experimental results for mixtures of thin and thick rods of the same length, for which the Onsager and Parsons-Lee theories can provide a qualitative description.
|Journal||Physical Review E - Statistical, Nonlinear, and Soft Matter Physics|
|Publication status||Published - Nov 1 2005|
ASJC Scopus subject areas
- Statistical and Nonlinear Physics
- Statistics and Probability
- Condensed Matter Physics