New insight into 3-picoline - deuterium oxide (D$_2$O) mixtures of limited miscibility with the lower critical consolute temperature

TL;DR

This study investigates the phase behavior of 3-picoline and D2O mixtures near their critical point, revealing violations of classical laws and providing new methods for analyzing critical phenomena with high precision.

Contribution

It introduces a novel high-resolution method for analyzing coexistence curves and critical parameters in limited miscibility mixtures, challenging classical rectilinear diameter law assumptions.

Findings

Violation of Cailletet-Mathew law in LCT mixtures

Critical exponent beta ≈ 0.326 near Tc

New simple method for determining critical concentration

Abstract

Coexistence curves in mixtures of limited miscibility with the lower critical consolute temperature (LCT), of 3-picoline with deuterium oxide (D$_2$O), and D2O/H2O have been determined. They were tested with respect to the order parameter, the diameter of the binodal, and coordinates of the critical consolute point (critical temperature Tc and critical concentration fic) Studies were carried out using the innovative method based on the analysis of relative volumes occupied by coexisting phases, yielding high-resolution data. The clear violation of the Cailletet-Mathew law of rectilinear diameter for the LCT mixtures of limited miscibility is evidenced. For the order parameter, the new distortion-sensitive analysis method yielded the evidence for the model-value of the order parameter critical exponent beta = 0.326.up to ca. 1 K from Tc. Finally, the simple & easy method for determining the critical concentration by testing relative volumes of coexisting phases (or alternatively fractional meniscus heights, h) is presented. The significance of the invariant value hc(Tc, fic) = 1/2 is highlighted. The appearance of the milky & bluish critical opalescence is also shown