Pressure dependence of the critical temperature of microemulsions near a critical end point

Experimental data for the pressure dependence of the critical temperature (dTc/dP) are reported for several microemulsions of n-dodecane, water, n-pentanol, and sodium dodecylsulfate, with critical points along a critical line ending at a critical end point (CEP). Negative dTc/dP values, substantially decreasing near the CEP, are obtained for all the microemulsions investigated. Using two-scale-factor universality and combining dTc/dP with correlation-length critical-amplitude ξ₀ values, we also find that a density anomaly is very likely unobservable experimentally. From the negative sign of dTc/dP it can also be concluded that if there is any critical contribution of the density anomaly to the refractive index n, it must be opposite to the anomaly recently reported for n by Rebbouh and Lalanne [J. Chem. Phys. 90, 1175 (1989)].