Critical Effects on Attractive Solutes in Binary Liquid Mixtures Close to Their Consolute Point: A New Experimental Strategy

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.