ABSTRACT
The effect of near-criticality upon the properties of dilute solutions of attractive solutes has been previously studied only using pure solvents close to the vapor-liquid critical point. The experimental difficulties that plague this thermodynamic region have somewhat obscured the interpretations of the results. Consequently, the coupling of long-range critical fluctuations with short-range intermolecular interactions is still a matter of debate. We developed a new strategy consisting of studying the changes in the solvation shell of probe molecules dissolved in a binary solvent mixture close to its lower consolute critical point. The study is based on UV-vis absorption and fluorescent emission measurements of the solvatochromic and thermochromic effects of two dyes, Reichardt's Dye and Nile Red, dissolved in mixtures of lutidine-water. The results show unambiguously the existence of a subtle change in the composition of the near-critical solvent surrounding the probe molecules with respect to the bulk composition, thus we conclude that the coupling of long-range fluctuations with short-range interactions is now firmly established.
ABSTRACT
The effect of cosolvents on the solubility equilibria near the critical end-point of binary mixtures is analyzed. The problem has received recent attention for the particular case of ionic cosolvents (Mazo, R. M. J. Phys. Chem. B 2007, 111, 7288-7290), where a pronounced salting-out effect was predicted. Here we make a thermodynamic analysis to view the problem from a different perspective. Our conclusions are that, at the critical end-point, the Setchenov constant, which reflects the effect of cosolvents on solubility equilibria, diverges following the behavior of the osmotic susceptibility and that the sign of this divergence is encoded in the critical end-line of the ternary system.
ABSTRACT
Refractive indices n and salt concentrations ms of coexisting phases of the ternary system 1,4-dioxane + water + potassium chloride were measured along the liquid-liquid-solid coexistence curve near the liquid-liquid critical end point. Refractive index measurements were carried out in the range 0.689 x 10-3 < t = (T - Tc)/Tc < 0.118 while salt concentrations were determined for the temperature range 1.84 x 10-3 < t < 8.07 x 10-2. From these experimental results, compositions fD (mass fraction of dioxane on a salt-free basis) and densities rho of coexisting phases were obtained. The shape of the coexistence curve was analyzed using alternatively n, ms, fD, and rho as order parameters. In all cases, the obtained coexistence curve displays, asymptotically, Ising behavior. Outside the asymptotic critical domain, n, ms, and rho show significant deviations of the effective critical exponent from its Ising value, while the concentration variable fD requires no corrections to simple scaling. On the basis of the present results, we conclude that this system shows no indication of multicritical behavior.
