We study by means of Nuclear Magnetic Resonance (NMR) spectroscopy the local order of water molecules in solution with lithium chloride at eutectic concentration. In particular, by measuring the proton chemical shift as a function of the temperature in the interval 203K < T < 320K, we observe a net change at about 235 K. We ascribe this result to the increase of the hydrogen bond interaction that on decreasing the temperature favors the formation of the network that characterizes the low density liquid phase of water. Furthermore, the Gaussian deconvolution of the NMR peak allows the investigation of the mutual difference between the chemical shift of water solvating lithium and chlorine individually. The thermal behavior of this quantity confirms previous results about the role of the temperature in the solvation mechanisms down to about 225 K. This temperature coincides with that of the so-called Widom line for water supporting the liquid-liquid transition hypothesis

Cicero, N.

Corsaro, C.

Dugo, G.

Mallamace, D.

Mallamace, F.

Vasi, S.

English

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DOI 10.1393/ncc/i2016-16301-3Colloquia: SWGM 2015IL NUOVO CIMENTO 39 C (2016) 301The local order of supercooled water in solution with LiClstudied by NMR proton chemical shiftC. Corsaro(1)(2)(∗), D. Mallamace(3), S. Vasi(2), N. Cicero(3)(4),G. Dugo(3)(4) and F. Mallamace(1)(2)(5)(1) CNR-IPCF, Istituto per i Processi Chimico-FisiciViale F. Stagno D’Alcontres 37, 98158 Messina, Italy(2) Dipartimento MIFT, Sezione di Fisica, Universita` di MessinaViale F. Stagno D’Alcontres 31, 98166 Messina, Italy(3) Dipartimento di Scienze biomediche, odontoiatriche e delle immagini morfologichee funzionali, Universita` di Messina - Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy(4) Science4Life, spin-off Universita` di MessinaViale F. Stagno DAlcontres 31, 98166 Messina, Italy(5) NSE Department, Massachusetts Institute of Technology - Cambridge MA 02139, USAreceived 19 July 2016Summary. — We study by means of Nuclear Magnetic Resonance (NMR) spec-troscopy the local order of water molecules in solution with lithium chloride ateutectic concentration. In particular, by measuring the proton chemical shift asa function of the temperature in the interval 203K < T < 320K, we observe anet change at about 235K. We ascribe this result to the increase of the hydrogenbond interaction that on decreasing the temperature favors the formation of thenetwork that characterizes the low density liquid phase of water. Furthermore, theGaussian deconvolution of the NMR peak allows the investigation of the mutualdifference between the chemical shift of water solvating lithium and chlorine indi-vidually. The thermal behavior of this quantity confirms previous results about therole of the temperature in the solvation mechanisms down to about 225K. Thistemperature coincides with that of the so-called Widom line for water supportingthe liquid-liquid transition hypothesis.1. – IntroductionThe Larmor frequency of the proton in a Nuclear Magnetic Resonance experimentdepends on the values of the magnetic field that each proton feels. This corresponds tothe vectorial sum of the static field B0 and of the local field [1]. In turn, the local field(∗) Corresponding author. E-mail: ccorsaro@unime.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 C. CORSARO et al.depends on the local arrangement of atoms and molecules and provokes a shift from theproton Larmor frequency of an isolated proton. The presence of ions in the sample altersthe shielding effect causing an additional shift of the resonance frequency. The entity ofthe shift depends on the type of ions and on their concentration [2].The values of the chemical shift are usually referred to that of a standard that can beinternal or external. By using an isolated water molecule in its gas phase as standard, onecan gain information about the interaction of water molecules with their surroundings. Inparticular, the properties of the hydrogen bonding and thus of the water local order canbe investigated by studying the proton chemical shift [3-7]. The variation of the chemicalshift with the temperature accounts for changes in the local order of the studied molecule.This especially holds for systems that possess many orientational degrees of freedom, suchas water and water systems, that are switched on or off by temperature changes. Notethat also the rate of variation of the chemical shift, as well as the local arrangementof water molecules, is not influenced by the choice of the reference and depends on thetype and concentration of ions. In particular, it was shown that the rate of variation ofthe proton chemical shift for pure bulk water is higher with respect to that of water insolution. This is probably due to the breakup of the hydrogen-bonded structure of bulkwater on increasing temperature although water-anion H-bonds in basic anion solutionsare much stronger than water-water H–bonds [2].Aqueous solutions of salts have been studied since many years for their properties tolower the freezing point of water. The precise salt concentration for which the solutionremains liquid up to the lowest temperature is called eutectic. The salt that possessesthe lowest eutectic temperature, that is about 200K [8,9], is lithium chloride (LiCl) andthe eutectic concentration corresponds to a salt molar fraction of 20% (6.76 M). In thiscondition there are 7 water molecules per molecule of LiCl. Lithium chloride is the mostsoluble salt in water with the smallest cation and allows studying the polymorphism ofwater in the supercooled phase [10-12]. In fact, the use of aqueous solutions of lithiumchloride allows to study water properties in a bulk-like condition in the supercooledregime well inside the so-called “No Man’s Land” up to 200K (eutectic temperature). Insuch a way, one can test the validity of the different physical scenarios hypothesized forwater [13,14] that try to explain its many anomalies [15-21]. In particular, the existenceof the Widom line [22] and of the associated liquid-liquid critical point [23] can be verified.Even if the presence of ions in solution modifies the local structure adopted by watermolecules, depending on the salt concentration, water/LiCl thermodynamical propertiesresemble those of bulk water [9,24-29]. The Li and Cl ions perturb in a different way thetetrahedral structure of water and in particular the Li cation is an enhancer of the watertetrahedral structure (structure maker) whereas the Cl anion tends to disrupt waterstructure (structure breaker) [30]. In fact, MD studies showed that for this system theLi-O radial distance is about 0.19 nm and the Cl-O radial distance is about 0.31 nm thatmust be compared with the O-O distance in pure water and in solution with LiCl whichis about 0.27 nm [31]. In fig. 1 we report just the results of the cited MD study for LiClaqueous solution at 14% of salt molar fraction with a corresponding picture of the watertetrahedron in the three different conditions. Note that from the top to the bottom thetetrahedral structure is more open and less rigid [31].The presence of tetrahedral structures confirms that the water/LiCl solution allowsthe study of water properties in the supercooled regime in bulk-like conditions.In this paper we extend our previous preliminary [32] and consolidated studies [33] byconsidering the proton chemical shift of the different local structures that water moleculesadopt in the studied solution. We will shed more light on the open debate about theTHE LOCAL ORDER OF SUPERCOOLED WATER IN SOLUTION ETC. 3Fig. 1. – The pair correlation functions of the water oxygen atom in the three configurationsobtained by MD simulations for 14% of salt molar fraction by Aragones et al. [31]. A snapshotof the tetrahedral symmetry that water molecules can adopt is depicted in each panel. Fromthe top to the bottom the tetrahedral structure is more open and less rigid [31,33].existence of a dynamical crossover in water solutions with LiCl [34,35]. The contrastingexperimental observations are caused by the coexistence of different local structures andhence different relaxations that were probed by different techniques [36,37].2. – Materials and methodsThe methods describing the sample preparation and experimental procedures arereported elsewhere [33]. We must mention that we used as an external reference thetetramethylsilane signal calibrated at 0.00 ppm. In this study we acquired the NMRspectra by cooling down the solution from 320K to 205K in steps of 5K, then we coolto 203K and heat up to 318K in steps of 5K.For water systems, the proton chemical shift δ provides information on the localhydrogen bond (HB) geometry being a function of the number of HBs and also of theintermolecular distances and angles [5]. Therefore the behavior of δ(T ) gives details of thethermal evolution of the water local structure especially in the supercooled regime wherehydrogen bond interactions are favored and generate clustering phenomena. Note that4 C. CORSARO et al.Fig. 2. – The proton NMR spectra of water/LiCl solution at eutectic point in a stack plot byvarying the temperature. The arrows indicate the thermal path used for the experiments.although the value depends on the chosen reference, its thermal behaviour does not [2,4].Furthermore, the enhancement of ions mobility in strong magnetic fields provokes thedisruption of the hydrogen bonding [38] that is more evident at high salt concentration.This justifies the upfield shift of the proton resonance frequency and a smaller rate ofchanges with temperature.The chemical shift, being a direct measure of the local order, corresponds essentiallyto the entropy [39] and its derivative with respect to the temperature is proportional tothe configurational specific heat [7].The recorded NMR spectra of aqueous solutions of LICl at eutectic concentrationare shown in fig. 2 in a stack plot at different temperatures. The arrows indicate thethermal path used for the experiments. Note that the process is completely reversibleand that the magnetization shows a maximum at about 225K [33]. The increase of themacroscopic magnetization on lowering the temperature (up to 225K) in the consideredthermal range is due to the corresponding increase of the polarization of water moleculeson decreasing the thermal energy (or disorder).As done before [32,33], we have performed a Gaussian deconvolution with three com-ponents to best fit the spectra and extracted the obtained peaks position. In agreementwith theoretical and simulations results [31, 40, 41], the three Gaussian components aredescribed by three different widths that correspond to three slightly different dynamicalregimes due to the individual rigidity of the local structure [33].3. – Results and discussionsThe measured peaks positions corresponding to the three Gaussian components arereported in fig. 3 as a function of the temperature.As mentioned above, the presence of ions in solution provokes an upfield shift withTHE LOCAL ORDER OF SUPERCOOLED WATER IN SOLUTION ETC. 5Fig. 3. – The thermal evolution of the chemical shift of the three studied contributions extractedby the NMR spectra for water/LiCl solution at eutectic concentration. The value refers to thesignal of the tetramethylsilane at 0.00 ppm. Open and close symbols correspond to cooling andheating paths, respectively.respect to the chemical shift of bulk water due to a deshielding effect provoked by theirmotion [2]. Note that the temperature behavior is the same for all the three componentsand that shows two different linear behaviors above and below T ≈ 235K. This indicatesthat the local order of water molecules and its temperature dependence changes whenthe water correlation length reaches its maximum value that is on approaching the tem-perature of the Widom line. This enhances the micro-segregation of the solution withthe development of regions with higher solute concentration.The increase in δ on lowering the temperature reflects the increase in the local orderof the system due to the formation of high ordered structures and the development ofthe hydrogen bonded network of water. However, from these results no evidences on thedifferent actions exerted by the presence of both Li cations and Cl anions can be inferred.This confirms that the water properties in solution with LiCl are not so altered by thepresence of the salt at this concentration.Although the temperature trend is the same for all the components, by looking atthe mutual differences one can obtain more insight into the different actions exertedon water by the two ions and therefore into the dynamical evolution of the differentlocal structures. In fact, in our previous work we observed that the ions exert on watera different influence by lowering the temperature [33]. Figure 4 reports the differencebetween the peak position of the Gaussian contributions of water around Li and Clions, that is the difference between the chemical shift value of the Gaussian functioncorresponding to water solvating lithium and that solvating chlorine.The quantity reported in fig. 4 shows an interesting behavior as a function of thetemperature. In particular, it decreases from 320K to 280K where it shows a minimum.Successively, on lowering the temperature it increases up to 240K where it shows amaximum and then it decreases again until 225K. Below this important temperature for6 C. CORSARO et al.Fig. 4. – The difference between the chemical shift value of the Gaussian function correspondingto water solvating lithium and that solvating chlorine.water, it increases again reaching values bigger than those at higher temperature. Thisresult confirms that the local structure of water solvating the two ions changes with thetemperature. Furthermore, these changes are different within different thermal regions.In particular, for 320K > T > 280K the difference between the chemical shift valueof water solvating lithium and solvating chlorine decreases: the two Gaussian functionsapproach each other. In this thermal region water molecules belong to the coordinationspheres of both lithium and chlorine with the same weight or probability [33]. ForT < 280K the two Gaussian functions begin to move away reaching the maximumdistance of about 0.0085 ppm at about 240K meaning that the solvation of the ionsoccurs in a different way. This happens up to 225K where the chemical shift differenceshows a relative minimum. In fact, for T < 225K the chemical shift difference increasesagain and more rapidly, reflecting a net change in the ions solvation mechanism due tothe development of an extended hydrogen bond network of water molecules. This isconsistent with the crossing of the Widom line for water, below which Li and Cl ionsconstitute only local defects of the water hydrogen bond network [40,42].4. – ConclusionsIn this work we have studied the proton chemical shift of water in solution with LiCl ateutectic concentration (20% alcohol molar fraction) as a function of the temperature bymeans of cooling and heating paths in the range 203K < T < 320K. A Gaussian decon-volution with three components allowed us to study separately the thermal behavior ofthe different local structures that water adopts within the mixture. In fact, several simu-lations studies have shown the presence of different clusters dominated by water-lithium,water-chlorine or water-water interactions. Although the presence of ions in solutionperturbs the local water structure, the thermodynamical properties of water/LiCl solu-tions (at the considered concentration) are very similar to those for water in a bulk-likeenvironment. The starting temperature of our experiments (320K) coincides with thetemperature at which bulk water thermodynamic properties such as the isothermal com-THE LOCAL ORDER OF SUPERCOOLED WATER IN SOLUTION ETC. 7pressibility and the thermal expansion coefficient show peculiar behavior. In detail, atthis temperature the former shows a minimum also by varying the pressure and the latterhas the same value that is not dependent on pressure. Essentially, above 320K waterproperties resemble those of a simple liquid because the strength and lifetime of hydrogenbonds are not enough to form the characteristic tetrahedral structure [43]. Our mainresult is the observation of a change in the local order of the system at about 235K thatis associated with the full development of the characteristic hydrogen bond network ofwater. Below this temperature the population of the so-called Low Density Liquid (LDL)phase of water dominates over that of the High Density Liquid (HDL) [44]. 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The local order of supercooled water in solution with LiCl

studied by NMR proton chemical shift

http://eprints.bice.rm.cnr.it/19297/1/ncc11196.pdf

The local order of supercooled water in solution with LiCl studied by NMR proton chemical shift

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