Spin Probe ESR Signature of Freezing in Water: Is it Global or Local?

TL;DR

This study uses spin probe ESR to investigate water freezing, revealing local interactions and concentration-dependent effects of TEMPOL and TEMPO probes, with implications for understanding ice formation at the microscopic level.

Contribution

First systematic ESR analysis of water freezing with TEMPOL and TEMPO, highlighting local probe-water interactions and concentration effects on freezing behavior.

Findings

Spin probe signature collapse at low temperatures indicates freezing.

Freezing and melting points depend on probe concentration.

Different behaviors observed for TEMPOL and TEMPO probes.

Abstract

First systematic spin probe ESR study of water freezing has been conducted using TEMPOL and TEMPO as the probes. The spin probe signature of the water freezing has been described in terms of the collapse of narrow triplet spectrum into a single broad line. This spin probe signature of freezing has been observed at an anomalously low temperature when a milimoler solution of TEMPOL is slowly cooled from room temperature. A systematic observation has revealed a spin probe concentration dependence of these freezing and respective melting points. These results can be explained in terms of localization of spin probe and liquid water, most probably in the interstices of ice grains, in an ice matrix. The lowering of spin probe freezing point, along with the secondary evidences, like spin probe concentration dependence of peak-to-peak width in frozen limit signal, indicates a possible size dependence of these localizations/entrapments with spin probe concentration. A weak concentration dependence of spin probe assisted freezing and melting points, which has been observed for TEMPO in comparison to TEMPOL, indicates different natures of interactions with water of these two probes. This view is also supported by the relaxation behavior of the two probes.