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

Abstract

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)...