Electrical conductivity of binary mixtures near the critical point

The electrical conductivity σ of several liquid mixtures was measured near their respective critical points. The systems studied were pure phenol–water, KCl-doped phenol–water, and isobutyric acid–water. The measurements spanned the reduced temperature range 10−5≲ε≲10−2, where ε≡ (T-Tc)/Tc. All systems clearly showed the existence of a singular contribution to the conductivity. A least-squares fit of the data to the equation, (σ-σc)/σc∝εϑ+ background terms, showed that the singularity could be characterized by a critical exponent ϑ=0.70+0.15−0.10. A possible explanation of this singularity is offered which uses percolation theory in the mean field limit. This percolation approach yields ϑ=2β, in accord with our own observations. The exponent β (?1/3) characterizes the shape of the coexistence curve.