Structural hysteresis of dilute fluorine salt aqueous solutions

In contrast with the generally accepted picture for most of the dilute salt aqueous solutions, which defaults that only one structural state/form exists at the given values of system/thermodynamic parameters, in this work, combining characterization of 19F nuclear magnetic resonance, ultraviolet-visible spectroscopies with theoretical calculations, we found that the F−-bearing aqueous solutions with identical component, but prepared by both different pathways, one of which is by mixing subsaturated MF2 solutions and Na//ClO4−(SO42−) solutions (path 1) and another is by mixing concentrated M2+//ClO4−(SO42−) solutions (M = Zn, Cd, Co and Cu) and NaF solution (path 2), can long-termly exist spectrographic distinguishable different thermodynamic steady states, in which the F− in the former preferentially exists in the form of more M2+-F− contact ion pairs (CIP), whereas in the form of more, even all M2+-F− solvent-separated/shared ion pairs (SIP) for the latter. This results from the different solution initial states, where path 1 solutions contain certain amounts of CIPs, whereas there is no CIPs initially in path 2 solutions, the high energy barrier of conversion between SIP and CIP, coupling with the low concentration gradient and thereby small conversion driving force. This makes the solutions exhibit an unusual “apparent” structural hysteresis. Due to the universality of the causes, present findings can expand to other sparingly soluble salts and enhance the significance of solution preparation bias on the thermodynamic studies in solution chemistry domain of sparingly soluble salts.