Liquid-liquid coexistence curve under gravity

Simultaneous studies of the height and temperature dependences of the refractive index, n(z, T), and the refractive index gradient, dn/dz(z, T), of an inhomogeneous binary methanolhexane solution under gravity near the critical consolute temperature have been carried out in the work by using complex refractometry technique. It has been found out, that the value of the refractive index, n(T), is the nonmonotonic temperature function in the lower phase of the methanol-hexane solution for the liquid-liquid coexistence curve. This result is explained by a volume expansion of solution with system temperature increasing and by a solution concentration change in both phases. The obtained data were used for calculating the coexistence curve equation for the investigated solution in the terms of different order parameters on the basis of the fluctuation theory of phase transitions and the Van der Waals model of fluctuation gas. (C) 2003 Elsevier Science B.V. All rights reserved