Second virial coefficients of dipolar hard spheres

An asymptotic formula is reported for the second virial coefficient B2 of a dipolar hard-sphere (DHS) fluid, in zero external field, for strongly coupled dipolar interactions. This simple formula, together with the one for the weak-coupling B2, provides an accurate prediction of the second virial coefficient for a wide range of dipole moments, including those that are experimentally accessible in magnetite ferrofluids. The weak-coupling B2 also yields an estimate of the magnetic moment minimally needed for isotropic gas–liquid phase-separation, if any, in the DHS fluid.