Refractive index near the critical point and pressure dependence of the critical temperature of a water–sodium di-2-ethylhexylsulfosuccinate–decane microemulsion

Experimental data are presented for the refractive-index behavior in the homogeneous phase below the cloud-point temperature of a critical microemulsion composed of sodium di-2-ethylhexylsulfosuccinate (AOT), water, and decane. The pressure dependence of the critical temperature (${\mathrm{dT}}_c$/dP) is also reported. Fitting the refractive-index data as a function of temperature, we find a (1-α) power-law singularity with an α value consistent with the Ising value ${\alpha }_I$=0.11. It is shown that the refractive-index anomaly observed in this microemulsion is an intrinsic effect, most likely related to the electric field dependence of the critical temperature (${\mathrm{dT}}_c$/${\mathrm{dE}}^2$). Further, using two-scale-factor universality and experimental values for ${\mathrm{dT}}_c$/dP and the correlation-length critical amplitude ${\xi }_0$, we also found that the density contribution to the refractive-index anomaly is negligibly small.