Phase diagram of tobacco mosaic virus solutions

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 $\mathrm{AAA}$- and $\mathrm{ABC}$-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.