DOUBLE-CRITICAL-POINT PHENOMENA IN 3-COMPONENT LIQUID-MIXTURES - LIGHT-SCATTERING INVESTIGATIONS

Measurements of osmotic compressibility (near the lower consolute point, T(L)) in two reentrant liquid mixtures [3-methylpyridine (MP)+water (W)+heavy water (HW) and MP+W+NaCl] are presented. The closest approach to the double critical point (DCP) was marked by a sample of loop size (DELTAT) = 250 mK. Analyzing the data by means of the conventional field variable t[=\(T(c)-T)/T(c)\] yielded an exact doubling of the critical exponent (CE) gamma for a DELTAT=250 mK. The approach to double criticality (for intermediate DELTAT) is described by a crossover of the CE from the doubled to its single limit as t --> 0. Recourse to a more appropriate field variable, t(UL)[=\(T(U)-T)(T(L)-T)/T(U)T(L)\], restores the Ising value of gamma(=1.24) for any DELTAT. The salt-doped mixtures permitted us to observe a doubling of the extended scaling exponent (DELTA) and also to scrutinize the ionic critical phenomena. The range of simple scaling in MP+W+HW was found to be extremely large. Switching to a modified variable t(UL)'[=\(T(U)-T)(T(L)-T)/T2\] led to a remarkable enhancement in the weight of the extended scaling term in both the mixtures-in apparent disagreement with the earlier findings that reported a widening of the asymptotic region. Non-phase-separating samples of MP+W+HW showed the expected saturating divergence preceded by a region of doubled gamma as T(D) (DCP temperature) was neared. Most of the facets of our investigations can be comprehended in terms of the geometrical picture of phase transitions as well as the Landau-Ginzburg theory as applied to the reentrant phase transitions.