Double-critical-point phenomena in three-component liquid mixtures: Light-scattering investigations

Measurements of osmotic compressibility (near the lower consolute point, TL) 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 ({Δ}T)=250 mK. Analyzing the data by means of the conventional field variable t[=|(Tc-T)/Tc|] yielded an exact doubling of the critical exponent (CE) {γ} for a {Δ}T=250 mK. The approach to double criticality (for intermediate {Δ}T) 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, tUL[=|(TU-T)(TL-T)/TU{T}L$|], restores the Ising value of {γ}(=1.24) for any {Δ}T. The salt-doped mixtures permitted us to observe a doubling of the extended scaling exponent ({Δ}) 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 tUL^'[=|(TU-T)(TL-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 {γ} as TD (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.