Isotropic fluid phases of dipolar hard spheres

Monte Carlo simulations are used to calculate the equation of state and free energy of dipolar hard sphere fluids at low temperatures and densities. Evidence for the existence of isotropic-fluid-isotropic-fluid phase transitions is presented and discussed. Condensation in the dipolar hard sphere fluid is unusual in that it is not accompanied by large energy or entropy changes. An explanation of this behavior is put forward.