Does a Binary Phase Diagram Exist for a Solvent Containing a Single Solute Molecule? Case of a Neutral Solute Molecule in Ethanol

Binary phase (BP) diagrams have been a cornerstone in both the industrial and the academic research. Traditionally, the two-component BP diagrams always consider a finite amount of solute in a finite solvent host. Here, we consider a special situation: a single solute molecule in a structurally different solvent host and pose the question “Can the traditional BP diagram account for the behavior of a single solute in a finite solvent host?” To date, this aspect remains unexplored because of both practical difficulties in probing such dilute solutions and conceptual difficulties in formulating a BP diagram for such a single solute molecule case. In this work, we have overcome the experimental barrier and produced such a system in ethanolic solutions of a neutral solute molecule by exploiting the ethanol’s property of crystallization from a translationally rigid plastic crystalline state. We studied the freezing behavior of dilute solutions of nitroxyl spin probes, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), 4-hydroxy-TEMPO, 4-oxo-TEMPO, and 4-amino-TEMPO, in ethanol by electron paramagnetic resonance (EPR) spectroscopy. Computer simulations based on Stochastic Liouville theory were used to analyze the EPR spectra. The EPR results reveal that liquid domains (LDs) containing a single solute molecule are formed in ethanol ice and are subjected to strong confinement because of its own frozen solvent phase. No evidence of crystallization of these LDs is observed at any temperature. The observed behavior cannot be rationalized by the bulk BP diagram but could be understood by a self-confined BP diagram, where the freezing point curve is not bounded at lower temperatures. Interestingly, the size of the LD was estimated to be 2.0 nm, which is similar to the lower limit for crystallizing water.