  The interfacial free energy is a central quantity in crystallization from the
meta-stable melt. In suspensions of charged colloidal spheres, nucleation and
growth kinetics can be accurately measured from optical experiments. In
previous work, from this data effective non-equilibrium values for the
interfacial free energy between the emerging bcc-nuclei and the adjacent melt
in dependence on the chemical potential difference between melt phase and
crystal phase were derived using classical nucleation theory. A strictly linear
increase of the interfacial free energy was observed as a function of increased
meta-stability. Here, we further analyze this data for five aqueous suspensions
of charged spheres and one binary mixture. We utilize a simple extrapolation
scheme and interpret our findings in view of Turnbull's empirical rule. Our
first estimates for the reduced interfacial free energy, $\sigma_{0,bcc}$,
between coexisting equilibrium uid and bcc-crystal phases are on the order of a
few $k_BT$. Their values are not correlated to any of the electrostatic
interaction parameters but rather show a systematic decrease with increasing
size polydispersity and a lower value for the mixture as compared to the pure
components. At the same time, $\sigma_0$ also shows an approximately linear
correlation to the entropy of freezing. The equilibrium interfacial free energy
of strictly monodisperse charged spheres may therefore be still greater.
