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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
EPR Evidence of Liquid Water in Ice: An Intrinsic Property of Water or a Self-Confinement Effect? Muthulakshmi Thangswamy, Priya Maheshwari, Dhanadeep Dutta, Vinayak G. Rane, and Pradeep Kumar Pujari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03605 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018
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The Journal of Physical Chemistry
EPR Evidence of Liquid Water in Ice: An Intrinsic Property of Water or a SelfConfinement Effect? Muthulakshmi
Thangswamy†, Priya
Maheshwari†, Dhanadeep
Dutta†,
Vinayak
Rane†,*and Pradeep K. Pujari†,‡ †
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, INDIA ‡
Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, INDIA
*
To whom correspondence should be addressed. Address: Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, INDIA. Telephone No: 25590630, E-mail:
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Abstract Liquid water (LW) existence in pure ice below 273 K has been a controversial aspect primarily because of the lack of experimental evidences. 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, upto 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 solid 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 behaviour 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.
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The Journal of Physical Chemistry
Introduction Water is one of the simplest molecules yet the most complex system to study because of its several dynamic and thermodynamic anomalies.1Unusual behaviour of water under different set of conditions makes it one of the most curious systems to be extensively studied over decades by many researchers. One such aspect is the co-existence of ice and liquid water (LW) below 273 K, its normal freezing point. This observation primarily originated from the researchers working in the field of glaciology.2 The initial interest was in the microstructure of ice-water system existing in equilibrium at 273 K. Based on thermodynamic ground, Nye and Frank predicted that such water would exist in the form of pockets, which are further connected by veins to form a network like structure.2 The exact geometry of such vein-like network was later determined by Mader.3 A breakthrough observation appeared in this direction when the proton nuclear magnetic resonance (NMR) experiments indicated the existence of liquid water even below 273 K.4 The NMR feature of water domains in ice was ascribed to the presence of the intergranular water. This found strong support among the glaciologists who used this aspect to explain the ability of glaciers to remove the trapped impurities from its lattice.5 Furthermore, the LW existence was also linked to the anomalous decrease of activation energy for the ice deformation between 263 and 273 K.5 However, soon the model was questioned, primarily because of the irreproducibility of the observed NMR signal of such unfrozen water. With time, the NMR observation of LW below 273 K was increasingly believed to be an effect of irremovable dissolved impurities and ions that are intrinsically present in water.6A strong argument in this favour was the observation of high conductance of polar ice, where the vein structure was hypothesised, compared to that of pure ice, grown in the laboratory.3 The above scenario changed again in early nineties, when it was shown theoretically that even impurity-free ice can have LW at temperatures much below 273 K.7In this work, a quantitative estimate of the unfrozen water was determined, based on the established hypothesis of the presence of LW in the vein network of ice.2 The theoretically calculated volume fraction of water in the veins is reported to be a very small quantity ~108. It thus becomes evident that because of the limitations and experimental difficulties in probing such small quantities, the evidence in favour of LW existence at sub-zero temperature is scarce. The only support in favour of this is the observation of higher value of experimentally measured heat capacity of pure ice than its theoretically predicted value.7,8 In the above context, it is worth mentioning of a recent electron paramagnetic resonance (EPR) study,9 which probed the super cooled water of rapidly frozen polycrystalline ice. The authors rapidly froze the water from the ambient temperature to the liquid helium temperature, and studied the dynamics of supercooled water present in the interstices of the resulting polycrystalline ice. In a later work,10 the authors also studied strongly confined interstitial water by altering their strategy of ice preparation. Both these studies
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involved observing the dynamics of a paramagnetic spin probe dissolved in the water under investigation, as a function of temperature. The mobility of the spin probes at even below 273 K was rationalized by inclusion of the spin probes in the supercooled water of polycrystalline ice. Inspired by these results, it is quite encouraging to investigate whether EPR can also be used to probe the intrinsic LW of a polycrystalline ice, as predicted by Nye’s model.2 At the same time it should be noted that pioneering measurements on ice from other techniques, such as atomic force microscopy,11 near edge X-ray absorption spectroscopy,12 total internal reflection spectroscopy13 and a very recent advanced optical microscopy,14 have also pointed towards the existence of LW at sub-zero temperature, though LW evidences are assigned to the disordered water on the surface of ice and not to the internal vein structure. The above discussion basically lays the foundation for our present work. To justify our objective better, we would like to bring forth the reason for invoking impurity as a possibility for explaining such LW domains in ice at sub-zero temperature. Basically, the impurity converts the initial system of pure water to a binary system, whose equilibrium behaviour is governed by the binary phase (BP) diagram of the impurity-water system. As a consequence, the presence of water domains becomes inevitable up to the eutectic temperature of the binary system. Since the eutectic temperatures can be much below 273 K, depending on the solute, it is evident that consideration of BP diagrams is of prime importance. Ignoring this possibility could easily lead to the erroneous conclusion of the presence of LW domains in frozen pure ice at sub-zero temperature. Since EPR studies involve dissolution of an external spin probe in water, the above discussion mandates that a consideration of the spin probe-water phase diagram is necessary, before drawing any conclusions about the LW domains in ice at sub-zero temperature. However, this aspect is often overlooked in usual EPR studies. One reason for this disregard could be the belief that the BP diagrams, being a thermodynamic concept, may not be applicable for the very low concentrations of spin probes generally used in these studies. Careful experimental evidence in support of this belief, however, is required. This brings us to the crux of the present work, which broadly aims to answer three questions: 1) Does a lower critical concentration exist, below which the BP diagrams are not applicable? 2) Does EPR spectroscopy gives any evidence of LW in polycrystalline ice at sub-zero temperature and 3) Is the LW in ice evidenced by EPR, a manifestation of the spin probe-water BP diagram or water’s intrinsic property as governed by Nye’s model? Our work demonstrates that there indeed exists a lower critical concentration for the applicability of BP diagram below which they fail to explain the experimental observations. We propose a simple modification of the BP diagram which involves incorporation of the confinement of the liquid domains in ice. We demonstrate that the modified BP diagram accounts for the observed EPR evidence for the mobility of spin probes below 273 K which could be mistaken as the intrinsic LW water of polycrystalline ice (Nye’s model). Experimental and Theoretical Methods
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The samples were prepared using Type II deionised water (Merck Millipore, resistivity=18 M cm). Since deionised water (DI) could have trace organic contamination, we further distilled the DI water twice and repeated some of the experiments. We observed no significant changes in the nature of the EPR spectra obtained from samples prepared with DI or double distilled DI water. TEMPO (2,2,6,6tetramethylpiperidin-1-oxyl) and TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1oxyl)purchased from Aldrich (98 % purity) were used as the spin probe. TEMPO was purified by vacuum sublimation (P=103 mbar, T=293 K). Samples were prepared by adding 10 µL of the required solution in a 3-mm O.D. quartz tube (I.D. =1mm). This resulted in a sample height of 1 cm, over which the temperature drop was minimal (
