- Albert P Philipse: Van't Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, Debye Institute for Nanomaterials Science, Padualaan 8, 3584 CH Utrecht, The Netherlands.
An asymptotic formula is reported for the second virial coefficient B(2) 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 B(2), 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 B(2) also yields an estimate of the magnetic moment minimally needed for isotropic gas-liquid phase-separation, if any, in the DHS fluid.