Structure of Water in Amorphous Sorbitol - ACS Publications

Jul 5, 2016 - ABSTRACT: The structure of water in 70 wt % sorbitol−30 wt % water ... Water structure in the sorbitol matrix is compared with that of...
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Water in a Soft Confinement: Structure of Water in Amorphous Sorbitol Evgenyi Shalaev*,† and Alan K. Soper*,‡ †

Allergan Plc, Irvine, California 92612, United States ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxon OX11 OQX, United Kingdom

‡

S Supporting Information *

ABSTRACT: The structure of water in 70 wt % sorbitol−30 wt % water mixture is investigated by wide-angle neutron scattering (WANS) as a function of temperature. WANS data are analyzed using empirical potential structure refinement to obtain the site−site radial distribution functions (RDFs). Orientational structure of water is represented using OW−OW−OW triangles distributions and a tetrahedrality parameter, q, while water−water correlation function is used to estimate size of water clusters. Water structure in the sorbitol matrix is compared with that of water confined in nanopores of MCM41. The results indicate the existence of voids in the sorbitol matrix with the length scale of approximately 5 Å, which are filled by water. At 298 K, positional water structure in these voids is similar to that of water in MCM41, whereas there is a difference in the tetrahedral (orientational) arrangement. Cooling to 213 K strengthens tetrahedrality, with the orientational order of water in sorbitol becoming similar to that of confined water in MCM41 at 210 K, whereas further cooling to 100 K does not introduce any additional changes in the tetrahedrality. The results obtained allow us to propose, for the first time, that such confinement of water in a sorbitol matrix is the main reason for the lack of ice formation in this system.

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INTRODUCTION Systems rich in polyhydroxy compounds (PHC), such as sugars and sugar alcohols, are widely represented in pharmaceutical and food systems as well as biological organisms. In the pharmaceutical, biotech, and food industries, sugars and sugar alcohols are commonly used cryoprotectors and lyoprotectors, which are added to products in order to improve their stability during processing (e.g., during freeze-drying) and storage.1−3 PHC have also been shown to be effective in protecting both single-cell and multicellular organisms against environmental stresses such as subzero temperatures or prolonged drought.4−6 The cryo-protective action of PHC is usually related to their ability to interfere with the water-to-ice transition, that is, to keep a portion of water molecules in an amorphous (unfrozen) form during cooling.7 Depending on the solute concentration, freezing (formation of ice) is inhibited either partially or completely. In relatively dilute frozen solutions of PHC, hexagonal ice coexists with the freeze-concentrated solution consisting of the solute and unfrozen water, with the water content in the freeze concentrate ranging typically from approximately 30 to 15 wt %.8,9 On the other hand, in solutions with a high concentration of PHC (typically above 70 wt %), ice does not form at all, and the system remains in a single-phase amorphous state. Considering the fundamental importance of freezing behavior of aqueous solutions, it is essential to understand physical mechanism(s) behind the well-established experimental facts of the existence of unfrozen water in PHC solutions. The inhibition of freezing by various noncrystalline solutes (including PHC) has been considered to be a purely kinetic © 2016 American Chemical Society

phenomenon due to a steep increase in the viscosity leading to the transformation from supercooled solution to glassy state. According to this view (which was also supported by one of the authors of this paper), water diffusion and therefore its crystallization are expected to be prevented below the glass transition temperature, Tg, of the system.10−12 It should be stressed, however, that relations between viscosity, diffusion, and crystallization are not straightforward. Indeed, crystallization was reported to occur below the Tg in both aqueous and nonaqueous systems, albeit at a slower rate.13,14 Furthermore, it has been shown that the translational diffusion of water does not stop on the experimental time scale even well below the Tg.15−17 Therefore, the “Tg” explanation of the inhibited (in concentrated solutions) or incomplete (in dilute solutions) water-to-ice transition is probably oversimplified and not entirely satisfactory. Are there any alternatives to the “Tg” hypothesis about unfrozen water in solutions of PHC? To address this question, we point to another type of system, which shows deep supercooling and even complete hindrance of crystallization, namely, water confined in small pores. When water is absorbed in typical mesoporous materials of small enough pore size, it can be supercooled below the homogeneous ice nucleation temperature; moreover, water crystallization (ice formation) can be completely avoided if the pore size is smaller than a certain threshold value. In porous silica, for example, the dimension of the nonfreezing layer for Received: June 17, 2016 Revised: June 24, 2016 Published: July 5, 2016 7289

DOI: 10.1021/acs.jpcb.6b06157 J. Phys. Chem. B 2016, 120, 7289−7296

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corresponding to approximately 70 wt % sorbitol. WANS patterns were collected at 298, 213, 173, and 100 K. Other experimental details are described in ref 32. The Gudrun suite of programs developed by the ISIS disordered materials group35,36 has been used to correct the experimental data for multiple scattering, absorption, and inelasticity effects, along with subtraction of the scattering from the sample container and data reduction to an absolute scale. To obtain the atomic radial distribution functions, the neutron scattering data are interpreted in terms of a molecular structure model, using empirical potential structure refinement (EPSR).37 This method, which is the analogue of crystal structure refinement, but for disordered materials such as liquids and glasses, builds an atomistic model of the scattering system using standard interaction potentials and models of the individual component molecules and then perturbs those potentials and molecular structures in a manner to bring the simulated radial distribution functions as close as possible to the measured data. It does this while, at the same time, imposing physical constraints on the atomic positions, so as to prevent atomic overlap and to allow hydrogen-bonding between atomic species where appropriate. The details of the EPSR procedure for the sorbitol−water solutions are available from ref 32. Additional analysis of the WANS data is performed in order to characterize the shape of the space occupied by water molecules, using a so-called “void” analysis on the simulation box.38,39 In this method, the simulation box is divided into a large number of volume pixels. In the present case there are approximately 100 pixels along each of the x-, y-, and z-axes, making a total of order 106 pixels, with the pixel step size set to approximately 0.4 Å. Each pixel is assigned a status of either “occupied” or “not occupied”. To be classed as occupied, there must be at least one water molecule within a specified distance from that pixel; otherwise, the pixel is classed as not occupied. At the end, the array of occupied pixels is to a good approximation a representation of the space occupied by the water molecules in this solution. The specified distance to a water molecule is set to 2.15 Å in the present case so that the volume fraction of occupied pixels is close to the volume fraction of water in the solution, namely ∼40%. To assess the shape of this space, an autocorrelation is performed on these occupied pixels, giving a radial distribution function of occupied pixels, otherwise called the “volume distribution function”, gV(r). Various texts from the theory of small-angle scattering have given functional forms for this correlation function depending on the shape of the space involved (see for example ref 40). The cases of a sphere, a long cylinder, and a large flat disk have also been discussed recently.41 In particular, as discussed in those references, the slope of the decay of these correlation functions at r = 0 gives a measure of the “confining length” of the space represented. In the case of unconnected spheres the correlations disappear for radius values larger than the diameter of the sphere, but for long cylinders and flat disks the correlations proceed to much larger distances, in spite of the relatively sharp falloff at short distances. Hence, the detailed shape of the void distribution function is sensitive to the way the pixels that created it are arranged. Wide-angle X-ray scattering (WAXS) data for 70 wt % sorbitol−water solution were obtained at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, on beamline ID-02 between 298 and 93 K. The solution was filled into 1.5 mm borosilicate glass capillary and sealed. The

confined water was estimated to be 8 Å for spherical pores and 17 Å for cylindrical pores.18 In another study, cooling of MCM41 with 0.43 g of water sorbed per 1 g of substrate (approximately 30 wt % water) to 210 K (i.e., well below the homogeneous ice nucleation temperature of approximately 235 K for a bulk water19) resulted in predominantly amorphous sample.20 MCM41 is a highly porous silica substrate with a twodimensional hexagonal array of cylindrical pores with a wide range of diameters down to approximately 15 Å. The water/ice transformation in nanopores was reviewed in ref 21 with a conclusion that freezing and melting of water as a first-order phase transition disappears at a limiting pore diameter near a value of 20 Å. In an agreement with the experimental studies, a computer simulation study22 suggested that water in pores with diameter 20 Å freezing is expected to take place. Note that freezing behavior of water confined in hydrogel was also invoked to compare with freezing behavior of hydrated proteins.23 Furthermore, we point out to two additional similarities between water confined in solid pores and water in concentrated solutions of PHCs. First of all, water in PHC solutions is not distributed homogeneously but forms clusters, as proposed in a number of original studies24−28 and summarized in ref 29. A second related fact is that mobility of water molecules is decoupled from that of the sugar (sucrose30,31 and sorbitol31) matrix below the calorimetric Tg, thus closely resembling the case of mobile water in solid pores such as MCM41. Therefore, based on the three factors described above, i.e., prevention of freezing, inhomogeneity of water distribution (water clustering), and decoupling of mobility of water molecules from that of sugar, it seems logical to suggest that water in concentrated PHC−water mixtures resembles water confined in small solid pores, such as water in MCM41. In this study, the hypothesis is being explored by comparing water structure in a concentrated PHC solution (i.e., 70 wt % sorbitol−water mixture) with water absorbed onto MCM-41 pores. The present report is an extension on an earlier smallangle and wide-angle neutron scattering (WANS) study of the sorbitol−water solution at different temperatures.32 The refined analysis of the WANS data allows us to confirm that water molecules are not distributed homogeneously but rather clustered in “pores” formed by sorbitol matrix and also to determine the pore size to be approximately 5 Å. Furthermore, the orientational structure of water is determined in the wide temperature range of 100−298 K, covering regions both above and below the calorimetric glass transition temperature. The tetrahedral character of water clusters is significantly enhanced upon cooling from 298 and 213 K but remains essentially constant between 213 and 100 K. Finally, we observe that water structure in sorbitol matrix at 213 K and lower temperatures resembles that of water confined in MCM41 at 210 K.

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EXPERIMENTAL AND THEORETICAL METHODS The variable temperature WANS experiments were carried out on the SANDALS diffractometer33 at the ISIS Facility in the U.K.34 The experiments were performed on samples with different combinations of isotopic H/D substitution, i.e., sorbitol(H14)−H2O mixture, sorbitol(H14)−D2O mixture, and sorbitol(D8)−D2O, to extract information on the selected site−site radial distribution functions, g(r). The mole fraction of sorbitol in each of the samples was kept at around 0.19, 7290

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The Journal of Physical Chemistry B radiation wavelength λ was 0.996 Å (silicon channel-cut monochromator), and the beam cross section was 0.2 mm × 0.2 mm. The exposure time was adjusted to using the maximum dynamic of the detector and was less than 1 s in the majority of cases. The 2-D images were subsequently normalized to an absolute intensity scale after performing the standard detector corrections and azimuthally averaged to obtain the corresponding 1-dimensional X-ray diffraction curves. The WAXS Q-scale was calibrated with p-bromobenzoic acid powder. Sample-todetector distance was 115 mm for WAXS. The Linkam cooling stage was used to control the sample temperature. Temperature was changed in steps, and the sample was allowed to equilibrate for 1 to 10 min at each temperature. DSC analysis for sorbitol−water solution with sorbitol concentration of 70.2 wt % was performed with a Q2000 DSC system (TA Instruments). Nitrogen was used as purge gas, and calibration was performed using indium as the standard. Approximately 20 μL of the solution was sealed in Tzero aluminum pan with hermetic lid, cooled to 193 K, and heated to 293 K with the scanning rate of 5 K/min.

elements, i.e., water and sorbitol liquidus curves, the diagram also presents the composition-dependent glass transition temperature, water dipole relaxation temperatures (marked as Tgw), and a transition detected in 70 wt % sorbitol by terahertz spectroscopy. The star symbols mark temperatures of the WANS study for 70 wt % sorbitol, and the numbers correspond to the viscosity values in Pa·s. The diagram shows that while single amorphous phase represents an equilibrium state of the 70 wt % sorbitol at 298 K, it is metastable with respect to twophase system of hexagonal ice and crystalline sorbitol at three other temperatures used in the neutron diffraction study, i.e., 213, 173, and 100 K. However, neither water nor sorbitol crystallizes, and the sample remains amorphous under typical experimental conditions, as demonstrated in DSC and WAXS tests. DSC and synchrotron wide-angle X-ray scattering results for the 70 wt % sorbitol solution are shown in Figure 1B,C. DSC cooling/warming curves do not show any exothermic/ endothermic peaks due to crystallization/melting events, indicative of the absence of water and sorbitol crystallization. An endothermic step corresponding to the glass transition is observed on the DSC warming scan at approximately 200 K. In the agreement with the DSC data, WAXS patterns have an amorphous halo only, without sharp peaks of a crystalline phase. The observation that neither water nor sorbitol crystallization is expected during cooling/warming of 70 wt % sorbitol solution is also consistent with the previous DSC study.42 Neutron Diffraction. A partial analysis of WANS data of 70 wt % sorbitol−water mixture was reported in ref 32. The results showed an increase in the intensity of the first peak of water− water radial distribution functions (RDFs) upon cooling from 298 to 100 K, indicative of an increase in the local (nanometerscale) ordering of water. In addition, the system was shown to be heterogeneous, consisting of water-filled voids in sorbitol matrix. The simulation box is shown in Figure 2b to visualize

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RESULTS AND DISCUSSION Thermal Behavior of Sorbitol−Water Mixtures. In order to represent the physical states of the 70 wt % sorbitol solution at different temperatures in relation to the thermodynamically stable phases of the system, a supplemented temperature−composition phase diagram of the sorbitol−water system is shown in Figure 1A. In addition to equilibrium

Figure 1. (a) Supplemented phase diagram of the water−sorbitol system. Water liquidus (thin black curve), sorbitol liquidus (green curve), and the eutectic temperature (horizontal green line) are based on data from ref 43. The broken lines represent graphical extrapolation of the liquidus curves. The red vertical stick shows a transition observed in terahertz spectroscopy study for 70 wt % sorbitol.44 The Tg line is constructed by fitting the calorimetric Tg (onset) data31,42 to the Gordon−Taylor equation.45 Onset temperature for water dipole relaxation, Tgw (thick blue curve) is from the thermally stimulated current study.31 The thin blue curve represents the homogeneous ice nucleation.46 Star symbols map temperatures of the WANS study. Numbers next to the stars correspond to viscosity, Pa·s, at 298 K,47 and the estimate at 213 K from48 (see Supporting Information). Elements of the phase diagram that include crystalline sorbitol (which crystallization is not observed in this study) are shown in green. The phase diagram is not comprehensive; it does not reflect several polymorphic forms of sorbitol and sorbitol hydrate. (b) WAXS patterns for 70 wt % sorbitol−water sample at different temperatures. (c) DSC cooling/heating curves of 70 wt % sorbitol−water solution.

Figure 2. (a) Water volume distribution function for 70 wt % sorbitol−water solution. (b) Simulation box showing sorbitol molecules only. (c) Water volume surface contour plot at 100 K showing the regions occupied by water molecules in the sorbitol− water solution. 7291

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Figure 3. (a−c) Water−water radial distribution functions for 70 wt % sorbitol−water solution (this study, black), pure bulk water (green),41 and water confined in MCM4141 (red) at 298 K. (d) Temperature dependence of the full width at half-maximum for the first peak in Ow−Ow g(r) in different systems.

the presence of pores (voids) in sorbitol matrix, which are filled by water (water molecules are not shown). In the present paper, the WANS results are analyzed using a refined approach in order to extract details on dimensions of the voids, orientational order of water, and also to compare structure of water in sorbitol matrix with water confined in solid pores of MCM41 matrix. Figure 2a shows the volume distribution function obtained for water clusters in 70 wt % sorbitol at two temperatures, 100 and 298 K. The asymptotic behavior at large r of these functions represents the volume fraction of the water in the solution, which is about 40% at this concentration. The low r behavior of this function is representative of the “confining distance” of the water, which in this case is quite small, approximately 5.0 Å. Note that this distance, although expressed as a diameter, cannot be interpreted as implying the pores are necessarily spherical in shape. The volume distribution functions at 298 and 100 K are essentially identical, indicating that the major change in the dynamics of the system, which is liquid at 298 K and an amorphous solid (glass) at 100 K, does not have any noticeable impact on the water volume distribution function. We should add also that the observation of pores (voids) in the sorbitol matrix raises a broader question about structure of amorphous materials in general, and future studies should explore if such voids might also be present in a single-component glass-forming systems, such as pure sorbitol without water. To visualize regions of the solution occupied by water, Figure 2c shows a surface contour of the water density in the sorbitol− water solution. This plot is generated by assigning a density to

each point in the simulation according to its distance from the water oxygen atoms, using a Gaussian broadening function: ρ (r ) =

1 (σ 2π )3

2 2 OW ) /2σ )

∑ e−((r− r OW

where the sum is over all the water molecules and rOW is the displacement of each water molecule, in the simulation box. The width parameter, σ, is set to 1.4 Å (the approximate radius of a water molecule), and the contour level is set to 0.01 atoms/ Å3, corresponding to ∼1/3 the oxygen atom density in bulk water. The surface contour of the water density illustrates that water occupies a predominantly percolating volume in this solution, with only occasional isolated water molecules. RDFs of water in sorbitol are compared with water in MCM41, as well as ordinary bulk water, at 298 K in Figure 3. The first peak and the minimum after the first peak represent the first coordination sphere, the second peak is due to the second coordination sphere, and so on. As discussed previously,41 there are two major differences between bulk water and water confined in MCM41, i.e., larger amplitude of the first peak, and the oscillation of the distribution function at about 2 at low r values (vs 1 for bulk water). The latter observation is due to the higher local density of water (by a factor of ∼2) confined in MCM-41 compared to the density of water averaged over the full MCM41 unit cell volume. This is known as an excluded volume effect and will occur even when the pores are 100% filled. In MCM-41 the confining distance, which is the diameter of the pores, is approximately 25 Å. The same effect should occur if water is “confined” in a sorbitol matrix but is much less obvious due to the much lower confining distance, ∼5 Å. 7292

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Figure 4. (a) Oxygen−oxygen water−water g(r) for 70 wt % sorbitol−water solution at four temperatures. (b) Magnified portions of the g(r)s around the second peak.

Figure 5. (a) Ow−Ow−Ow triangles distribution for 70 wt % sorbitol−water solution at four temperatures. To obtain the distribution, the cutoff OW−OW distance is set to 3.20 Å, which corresponds to the position of the first minimum in the OW−OW g(r).41 (b) Temperature dependence of the water tetrahedrality parameter q in 70 wt % sorbitol−water (black squares, this study), MCM41−water (blue rhombes41), and bulk water (green circle41). Vertical red dotted lines show various characteristic temperatures as follows: Tg of the 70 wt % sorbitol (202 K); terahertz absorption transition (approximately 230 K);44 estimate of the break in the Stokes−Einstein relationship for water diffusion (see text).

Hence, the higher intensity of the first peaks in these radial distribution functions compared to bulk water probably reflects these excluded volume effects in the sorbitol solutions and does not necessarily imply stronger interactions. But it does reinforce the idea that the water is not uniformly distributed in the sorbitol matrix. Furthermore, in order to compare the degree of sharpness (ordering) of the first coordination shell for water in different systems, the width of the first g(r) peak is determined. The full width at half-maximum (FWHM) for the Ow−Ow g(r) is shown in Figure 3d, demonstrating the similarity in between water in MCM41 and in sorbitol. The lower FWHM values for both MCM41 and sorbitol matrixes correspond to a narrower distribution of the distances in the first shell in these systems than in a bulk water at 298 K. On the other hand, there is a difference in the region around the second peak in the Ow−Ow g(r), with water in sorbitol showing two poorly resolved peaks, in contrast to water in MCM41 with a single peak. Taking into account that the second Ow−Ow g(r) peak reflects tetrahedral arrangement of water molecules, the difference in the second peak points to a different orientational structure of water in MCM41 vs water in sorbitol, at least at 298 K. Note that water orientational

structure in sorbitol solution changes with temperature and approach that in MCM41, as described below. Ow−Ow RDFs for water−sorbitol at four temperatures are shown in Figure 4a. While a general appearance of the patterns is similar at all four temperature, as indeed might be expected because the sample does not crystallize and remains amorphous, the intensity of the first peak growth at lower temperatures with the simultaneous decrease in the peak width (Figure 3d), indicative of increase in the order of the first coordination sphere. Moreover, one important detail should be noted. There is a noticeable change in the shape of the second peak between 298 and 213 K, as highlighted in Figure 4b showing magnified portion of the g(r) region around the second peak. While the 298 K pattern has two poorly resolved peaks at r values 2−5 Å, cooling from 298 to 213 K produces a single peak with a shoulder, indicative of a change in the tetrahedral arrangement of water molecules. It should also be noted that further cooling to 173 K and finally to 100 K does not introduce any additional changes in the g(r) patterns. In order to elaborate on the structural rearrangements corresponding to the change in the second peak in the Ow− Ow g(r) between 298 and 213 K, the orientational structure of water is represented with the help of OW−OW−OW triangles 7293

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ferrocene−methanol diffusion in sucrose−water solutions,52 at 1.5Tg for glycerol and 1.53Tg for ferrocine−methanol diffusion in water−glycerol,53 and at 1.67Tg for fluorescein diffusion in trehalose−water.54 For diffusion of water in a PHC−water solution, in particular, the break in the SE relationship was reported at 1.25Tg.55 If a similar pattern corresponds to different PHC−water mixtures, the break in the SE relationship for water diffusion should occur at approximately 250 K in the 70 wt % sorbitol−water mixture (the Tg ∼ 200 K). Separately, a transition in the relevant temperature interval was reported for the same system in a terahertz (THz) spectroscopy study, in which a deviation from the linear dependence of the absorption coefficient was observed at approximately 230 K.44 Considering that THz absorption is sensitive to water structure, it is possible that this THz-detected transition could correspond to the change in the tetrahedral arrangement of water molecules.

distributions as well as a tetrahedrality parameter, q. The q order parameter was introduced by Errington and Debenedetty and is based on the angle (θ) between triplets of neighboring water molecules.49 The q parameter was redefined for such systems as water in confinement and concentrated solutions41 as follows: π

q=1−

9 ∫0 p(θ )(cos θ + 4

1 2 sin 3

)

θ dθ

π

∫0 p(θ) sin θ dθ

where the factor of 9/4 is required to ensure q goes from 0 (P(θ) = constant) to 1 (P(θ) = δ(cos θ − 1/3). In this case, instead of averaging q, it is P(θ) that is averaged over the simulation box and over molecular configurations. This average distribution, P(θ), is then used to calculate q. The OW−OW−OW triangle distributions at four temperatures are represented Figure 5a, and the corresponding values of q are provided in Figure 5b. All four triangle distributions (Figure 5a) show a broad peak at approximately 100° corresponding to loosely tetrahedrally bonded arrangements of triplets of neighboring water oxygen atoms, plus a peak near 60° corresponding to triplets for which at least one pair of the hydrogen bond is heavily distorted or broken. There is an obvious difference in the tetrahedral peak between 298 K and at lower temperatures of 213 to 100 K, with the lowertemperature patterns showing a stronger tetrahedral arrangement. At 213 K, the peak is centered at approximately 109.5°, closely corresponding to the ideal tetrahedral angle of 109.47°, whereas at 298 K a flat hump (rather than a peak) is observed between approximately 80° and 120°. Accordingly, the temperature dependence of the q parameter shows a marked increase upon cooling from 298 to 213 K, whereas the q is essentially constant between 213 and 100 K (Figure 5b). Furthermore, the tetrahedrality of water in the sorbitol matrix is compared with the q parameter in bulk water and water confined in MCM41. The q parameter at 298 K is different between all three systems, probably due to the influence of the surrounding (i.e., bulk water vs water in solid pores of MCM41 vs water in the mobile matrix of sorbitol). Upon cooling to 210 K, the q value for water confined in the MCM41 approaches the value for water in sorbitol. In considering the temperature dependence of q for 70 wt % sorbitol−water system (Figure 5b), first of all let us note that the calorimetric glass transition temperature of this system is approximately 200 K, i.e., below the temperature interval in which the increase in the tetrahedral structure of water is observed (between 298 and 213 K). Therefore, the strengthening of the tetrahedral orientational order in water clusters takes place in the liquid state and not directly related to the glass transition. Even above the Tg, however, cooling of this solution from 298 to 213 K results in a major (approximately 10 orders of magnitude) increase in viscosity. At 213 K, viscosity of 70 wt % sorbitol solution is estimated to be approximately 109 Pa·s, based on the literature data for amorphous sorbitol48 (see Supporting Information), whereas viscosity at 298 K is approximately 0.15 Pa·s.47 For such highly viscous liquids, molecular diffusion coefficients do not follow the Stokes−Einstein (SE) relationship, with the break reported to occur above their calorimetric glass transition temperatures.50 In PHC−water mixtures, in particular, the break in the SE relationship was observed at 1.16Tg for diffusion of fluorescein in water−sucrose mixtures,51 at 1.32Tg for

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CONCLUSION The analysis of the WANS data shows that 70 wt % sorbitol− water solution consists of a sorbitol matrix with percolating voids, which are filled with water. Water molecules in these voids are clustered with the length scale of approximately 5 Å. Orientational ordering of water is described using the tetrahedrality parameter, which reveals that water in sorbitol possesses a stronger tetrahedral character than either bulk water or water in a solid confinement (MCM41) at 298 K. Cooling the 70 wt % sorbitol−water solution from 298 to 213 K results in a further strengthening tetrahedral structure of water, with the orientational order of water in sorbitol at 213 K becoming similar to that of confined water in MCM41 at 210 K. This structural change takes place slightly above the calorimetric Tg of 202 K, whereas the tetrahedral structure remains the same upon further cooling from 213 to 100 K. Neither water nor sorbitol crystallizes, and the sample remains amorphous during the experiment at temperatures between 298 and 100 K. Considering both the WANS results from this study and the literature reports, the following description of processes during cooling 70 wt % sorbitol−water mixture is proposed. At room temperature, water clusters, which occupy voids in sorbitol matrix, have their mobility coupled with that of the matrix. Upon cooling from 298 to 213 K, the tetrahedral structure of water is enhanced and becomes similar to water confined in MCM41 pores with solid walls. At the same time, the macroscopic viscosity increases dramatically, and mobility of water molecules becomes uncoupled from the viscosity; i.e., water diffusion is no longer described by the Stokes−Einstein relationship. At this point, the walls of voids in the sorbitol matrix, as “seen” by water molecules, solidify and resemble those in solid pores in MCM41 or other solid porous material. Considering that water in pores do not freeze when the pore dimension reaches a critical value of approximately 20 Å,21 it is reasonable to propose that the prevention of ice formation in 70 wt % sorbitol−water solution is due to the confinement of water in the solidified sorbitol matrix. Note that it was suggested, based on studies of the freezing behavior of water and aqueous salt solutions under pressure, that the repression of water crystallization could be due to the breakup of tetrahedral structures from which ice Ih nuclei could form.56,57 While this is a logical suggestion, the results of this study demonstrate that the tetrahedrality is not broken and indeed enhanced in the presence of sorbitol, and therefore the repression of water crystallization is not due to the breaking 7294

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The Journal of Physical Chemistry B

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of water tetrahedrality, at least not in the sorbitol−water system. A follow-up neutron diffraction study of the 70 wt % sorbitol−water solution would be essential in order to determine if the orientational ordering of water molecules occurs at a well-defined temperature or if this transition takes place in a broad temperature interval. Such a study would also explore possible relationships between transition detected by the THz spectroscopy at 230 K, the break in the water diffusion from SE relationship (estimated to occur at approximately 250 K), and the enhanced tetrahedrality of water as detected via the O−O−O distribution triangles and the q parameter (detected between 298 and 213 K).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b06157. Estimation of viscosity for 70 wt % sorbitol at 213 K (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; phone 1-714-246-3372. *E-mail [email protected]; phone 44-1235 445543. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council. We appreciate Grace Chou’s assistance in the WANS experiment, and T. Narayanan and Michael Sztucki for the help in collecting WAXS patterns.

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REFERENCES

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