EPR Evidence of Liquid Water in Ice: An Intrinsic Property of Water or a Self-Confinement Effect?

Abstract

Liquid water (LW) existence in pure ice below 273 K has been a controversial aspect primarily because of the lack of experimental evidence. Recently, electron paramagnetic resonance (EPR) has been used to study deeply supercooled water in a rapidly frozen polycrystalline ice. The same technique can also be used to probe the presence of LW in polycrystalline ice that has formed through a more conventional, slow cooling one. In this context, the present study aims to emphasize that in case of an external probe involving techniques such as EPR, the results are influenced by the binary phase (BP) diagram of the probe-water system, which also predicts the existence of LW domains in ice, up to the eutectic point. Here we report the results of our such EPR spin-probe studies on water, which demonstrate that smaller the concentration of the probe stronger is the EPR evidence of liquid domains in polycrystalline ice. We used computer simulations based on stochastic Liouville theory to analyze the lineshapes of the EPR spectra. We show that the presence of the spin probe modifies the BP diagram of water, at very low concentrations of the spin probe. The spin probe thus acts, not like a passive reporter of the behavior of the solvent and its environment, but as an active impurity to influence the solvent. We show that there exists a lower critical concentration, below which BP diagram needs to be modified, by incorporating the effect of confinement of the spin probe. With this approach, we demonstrate that the observed EPR evidence of LW domains in ice can be accounted for by the modified BP diagram of the probe-water system. The present work highlights the importance of taking cognizance of the possibility of spin probes affecting the host systems, when interpreting the EPR (or any other probe based spectroscopic) results of phase transitions of host, as its ignorance may lead to serious misinterpretations.

Vitrification and increase of basicity in between ice Ih crystals in rapidly frozen dilute NaCl aqueous solutions

“…32 The freeze concentrated solution (FCS) threading the ice crystals creates a specific and hitherto not fully described medium whose behavior is of utmost importance in various fields, including geological, atmospheric, life, and pharmaceutical sciences. To date, FCSs have been examined via microscopy [33][34][35][36] and also characterized spectroscopically [37][38][39][40] and calorimetrically, [41][42][43] applying numerous systems at various temperatures within diverse fields. The FCS was found to behave distinctly from the impurities deposited from the gaseous phase.…”

Effect of Sodium Chloride on Internal Quasi-Liquid Layers in Ice Ih

“…For pristine ice, atomistic simulations indicate that the mobility of water molecules in such layers is significantly lower than in bulk supercooled liquid water at the same temperature, displaying subdiffusive behavior on time scales of ∼100 ns. , Similar behavior has also been observed at ice/clay nanocomposite interfaces . This enhanced sluggishness has been linked to the confined nature of these quasi-liquid regions, with the restraining presence of adjacent crystal grains restricting molecular motion. ,− However, whether or not this sluggishness is affected when sodium chloride is dissolved is an open question and it is directly related to, for instance, mass transport along the GB regions and mechanical deformation mechanisms based on grain sliding processes.…”

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

“…Recently, we observed a novel type of confinement in the electron paramagnetic resonance (EPR) studies of very dilute frozen aqueous solutions of a paramagnetic solute molecule, TEMPO, where the frozen solvent matrix itself acts as a confining medium for the unfrozen solution. 5 Importantly, no external porous medium is needed to confine the solution. The origin of self-confinement lies in an important aspect of the bulk BP diagram, which is that the size of the liquid domains (LDs) that coexist with the frozen solvent is predicted to depend on the initial concentration of the solute (Supporting Information, Figure S1).…”

“…9 A home written program, using the programming language C, was used, details of which can be found elsewhere. 5 The computer program also allows for axially symmetric rotational diffusion along with isotropic rotational motion. Different rotational models were considered, such as Brownian, jump, and free diffusion.…”

“…Thus, confinement of solutions in foreign porous materials is of limited applicability here. Recently, we observed a novel type of confinement in the electron paramagnetic resonance (EPR) studies of very dilute frozen aqueous solutions of a paramagnetic solute molecule, TEMPO, where the frozen solvent matrix itself acts as a confining medium for the unfrozen solution . Importantly, no external porous medium is needed to confine the solution.…”