Experimental study of the renormalization of beta near a double critical point: The 2-butanol and water system.

Abstract

The phase-separation temperature has been measured as a function of pressure and composition for the 2-butanol and water system. From the experimental data, the coexistence curves at different pressures have been interpolated. Fitting the data to simple-scaling equations, the critical-point line has been obtained, and found to be parabolic near the double critical point. The critical exponent \ensuremath{\beta} has been found to renormalize from the single-critical-point Ising value, 0.325, to twice that value near the double critical point, following the same curve previously found for the critical exponent of the electrical resistance. A value \ensuremath{\beta}=0.325 allows the description of the whole set of data if, instead of the usual reduced temperature, a thermodynamic field that follows naturally from a geometrical picture as well as the Landau-Ginzburg theory, is used.