Critical behavior of the refractive index in a binary fluid

We have performed very accurate interferometer measurements of the refractive index n (precision ±6×10−7) in the critical mixture nitroethane–iso-octane. n depends on the density ρ and local field effects F. Even though this mixture is expected to show a very small anomaly in Δρ/ρ (exponent α) so that local field effects should have been observed, in fact we never saw a contribution due to F and the critical behavior of n is governed by the density anomaly alone. Using the matching of refractive index components and of thermal derivatives, the regular part of n can be evaluated and the value α=0.10±0.02 is found, in agreement with the group renormalization value 0.110. The corresponding amplitude is compatible with the value obtained from ξ0 and (∂Tc/∂p) V,xc measurements using universal amplitude relations.