  We study the accuracy of the theory of Stroobants, Lekkerkerker and Odijk,
called SLO theory (Macromolecules 19 (1986) 2232-2238), to describe the
thermodynamic properties of an isotropic fluid of charged rods. By
incorporation of the effective diameter of the rods according to SLO theory
into scaled particle theory (SPT) we obtain an expression for the rod
concentration-dependent free volume fraction and the osmotic pressure of a
collection of charged hard spherocylinders. The results are compared to Monte
Carlo simulations. We find close agreement between the simulation results and
the SLO-SPT predictions for not too large values of the Debye length and for
high rod charge densities. The deviations increase with rod density,
particularly at concentrations above which isotropic-nematic phase transitions
are expected.
