The phase behavior of an aqueous suspension of rodlike tobacco mosaic viruses is investigated theoretically as a function of the virus density and the concentration of added salt. The total free energy involves ''volume terms'' from the microscopic counter-and co-ions and an effective pair interaction between the colloidal rods described by a Yukawa-segment model according to linear screening theory. Within a thermodynamic perturbation approach, the short-range repulsion between the rods is mapped onto a reference system of effective hard spherocylinders. The free energy of the spherocylinder system is gained from combining a cell model with scaled particle theory, which yields a reasonable phase diagram. The remaining long-range interaction is treated within a mean-field approximation. As a result we find stable fluid, nematic, and smectic phases as well as AAA-and ABC-stacked crystals. For increasing salt concentration at fixed rod concentration, there is a nematic reentrant transition. We finally discuss our results in view of experimental data.

We investigate the magnitude of density jumps across phase transitions in soft-matter systems composed of macromolecular particles, like star polymers or colloidal suspensions. The standard route to predict phase transformations is to start from an effective interaction potential between these macroparticles and map the phase diagram onto that of the corresponding effective one-component system. Using density-functional perturbation theory, we demonstrate that this procedure leads to wrong density jumps if the number of microscopic degrees of freedom is coupled to the number of macroparticles. In particular, the microscopic degrees of freedom can drive a macroparticle phase transition to be isochoric, i.e., to occur without any jump in the macroparticle density. ͓S1063-651X͑98͒04005-7͔

The phase behaviour of hard spherocylinders is calculated using the classical density functional theory of freezing. In particular, we construct a modified weighted-density approximation and calculate the bulk phase diagram. While the stability regime of the isotropic, nematic and smectic-A phases is in reasonable agreement with computer simulation data, the theory fails to describe the correct stability of the crystalline phases. Furthermore, we investigate the phase behaviour in an external field coupling to the orientational degrees of freedom. As a result, bulk phases which have the same translational symmetry but a different orientational symmetry can be transformed continuously into each other above a critical strength of the external field.

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