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.