Nanophase Segregation in Supercooled Aqueous Solutions and Their

Dec 17, 2010 - We use large-scale molecular dynamics simulations to investigate the phase transformation of aqueous solutions of electrolytes cooled a...
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Nanophase Segregation in Supercooled Aqueous Solutions and Their Glasses Driven by the Polyamorphism of Water Ly Le and Valeria Molinero* Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, United States

bS Supporting Information ABSTRACT: We use large-scale molecular dynamics simulations to investigate the phase transformation of aqueous solutions of electrolytes cooled at the critical rate to avoid the crystallization of ice. Homogeneous liquid solutions with up to 20% moles of ions demix on cooling producing nanophase segregated glasses with characteristic dimensions of phase segregation of about 5 nm. The immiscibility is driven by the transformation of water to form a four-coordinated low-density liquid (LDL) as it crosses the liquid-liquid transformation temperature TLL of the solution. The ions cannot be incorporated into the tetrahedral LDL network and are expelled to form a solute-rich water nanophase. The simulations quantitatively reproduce the relative amounts of low and high-density liquid water as a function of solute content in LiCl glasses [Suzuki and Mishima, Phys. Rev. Lett. 2000, 85, 1322-1325] and provide direct evidence of segregation in aqueous glasses and their dimensions of phase segregation.

1. INTRODUCTION Water is a poor glass former and ice formation in pure water can only be avoided by hyperquenching micrometer-sized droplets at rates of about 106 K/s.1 Slower cooling rates result in crystallization of ice at or above ∼232 K, the temperature of homogeneous nucleation of ice (TH). While glasses have the structure of the liquids from which they form, the structure of the water glass, low-density amorphous ice (LDA), is significantly different from liquid water at ambient temperature: the number of molecules in the first neighbor shell decreases from about 5.1 to 4.0, the density drops by 7%, and the environment of the molecules becomes more tetrahedral.2-6 Structurally, LDA resembles more the crystal, ice I, than the high temperature liquid. Like ice, water molecules in LDA are four-coordinated.3,4 Different from ice, the orientation of neighboring tetrahedra is not correlated in LDA, resulting in an absence of long-range order.7 Compression of LDA leads to the formation of a distinct highdensity amorphous phase (HDA).5,6 The polyamorphic transition is sharp and reversible and appears to be first-order.6,8 Based on experiments and simulations, it has been hypothesized that these two glass phases correspond to two distinct liquids of water, low-density liquid water (LDL) and high-density liquid water (HDL), and that these two liquids interconvert through a first order liquid-liquid phase transition located deep in the supercooled region of water's phase diagram, below the temperature of homogeneous nucleation of ice.9-11 Thermodynamic analyses of the properties of liquid water above TH and in the glass indicate that the structural transformation from high temperature liquid to LDA is continuous at room pressure,12,13 consistent with the existence of a liquid-liquid critical point (LLCP) at positive pressures.11,14-17 r 2010 American Chemical Society

As the structure of liquid water changes dramatically on cooling, a key question is how this affects the properties of liquid water as a solvent. It has already been demonstrated that the solubility of small hydrophobic molecules or atoms increases significantly on cooling, as the density of the liquid decreases.18-24 Increased ion pairing has been reported for dilute aqueous solutions of salts as their temperature drops from 300 to 235 K.25 The extremely low solubility of salts in ice and the structural similarity between ice and LDA suggests that salts are not soluble in LDA26-30 and prompts the question of what is the structure of glasses of aqueous solutions of salts. It has long been known that the addition of salts to water favors vitrification.31 Little is known, however, of the microscopic structure of these glasses. Already more than forty years ago, the identification of two distinct glass transitions in calorimetric experiments of LiCl-water glasses prompted Angell and Sare to hypothesize the existence of liquid-liquid immiscibility in supercooled aqueous solutions analogous to that already known to occur in silicates.32 These authors assigned the lowest Tg to a saltrich phase and the highest Tg to a water-rich phase. The concentration and pressure dependence of the two glass transitions was subsequently investigated by Kanno.33 It should be noted that, in all cases, the highest Tg is not readily observable (its heat capacity signature is too weak) but it is deduced to appear just before the exothermic peak associated with the Special Issue: Victoria Buch Memorial Received: October 25, 2010 Revised: November 26, 2010 Published: December 17, 2010 5900

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The Journal of Physical Chemistry A crystallization of ice. It was not until the discovery of water polyamorphism by Mishima and co-workers,8 that a conceptual framework became available to explain the apparent phase segregation in aqueous glasses of electrolytes and the identity of the phases.34,35 A Raman spectroscopy study of aqueous glasses of LiCl by Suzuki and Mishima demonstrated that the OH stretching of the glass can be decomposed in two signals: one corresponding to a concentrated aqueous solution of the salt and the other to pure low-density amorphous ice (LDA).26 The decomposition of the spectra in these two components yielded a quantification of the fraction of water in each of the two phases. These results show that the fraction of water in the LDA phase decreases almost linearly with the molar percent of salt. Interestingly, the fraction of water on each phase was not overtly sensitive to a change in the cation.26 The phase diagrams of binary mixtures with a solvent that presents polyamorphism were also investigated by Debenedetti and co-workers using statistical mechanics theory for two model solvents: one that presents a first-order liquid-liquid transition that ends in a critical point, and another that presents a continuous transition between the two liquid phases.36,37 The two scenarios are consistent with the thermodynamic anomalies of liquid water.9-11,38 The calculations predict that if a solute couples strongly with the solvent (as would be the case for electrolytes or other strongly hydrophilic solutes in water), the binary system displays liquid-liquid phase separation regardless of whether the liquid-liquid transition in the pure solvent is firstorder or continuous.36 Thus, experiments and thermodynamic theory support the existence of liquid-liquid immiscibility in supercooled aqueous solutions. The glasses of LiCl-water solutions, however, do not present macroscopic phase segregation and they are transparent. This indicates that if the glasses consist of two phases, the dimensions of phase segregation are smaller than the wavelength of visible light. Dupuy et al. measured small angle neutron scattering (SANS) spectra of 10.5% hyperquenched LiCl/water glass and found no sign of phase segregation within the 20 nm to hundreds of nm length scales of their study.39 This suggests that if there is phase segregation in these glasses, the characteristic dimensions of phase segregation should be smaller than 20 nm. It should be noted, however, that the composition used in the SANS study is the one for which the temperature of homogeneous nucleation of ice in the mixture intersects the glass transition temperature of the solution40 and for which the fraction of four-coordinated water molecules (as indicated by the Raman spectra) is just about 10%.26 The short-range structure of LiCl-water glasses have been recently studied through neutron diffraction.41 The radial distribution functions obtained from empirical potential structure refinement (EPSR) analysis of the experimental structure factors evidence an increase in solventseparated ion pairs for ions of the same sign, and an oxygenoxygen rdf that is intermediate between that of LDA and of a water-LiCl solution. The neutron diffraction results are consistent with phase segregation in the glass but cannot confirm it (because of the need to use a small cell size, less than 3 nm side, for the expensive atomistic simulations needed for the EPSR analysis) and do not provide a picture of their long-range structure. Phase segregation in the order of a few to tens of nanometers could be proved with small-angle X-ray scattering (SAXS), but to the best of our knowledge this technique has not yet been used to study the structure of glasses of aqueous solutions of salts.

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Molecular simulations have an optimal spatial and time resolution to shed light on the structure of aqueous solutions and their glasses and the microscopic processes that lead to segregation of the mixtures. There are, however, significant challenges for the study of these processes through simulations. First, the large size of systems required for characterizing the heterogeneous structure of the segregated glasses. Second, the long simulation times needed to equilibrate the liquid in the supercooled region and to reach the critical rate for vitrification of water (the slowest rate that prevents the formation of ice). Finally, the simulation model has to properly represent the structure of water from the liquid to the glass state. In this work, we address these challenges through the use of an efficient and accurate coarse-grained model of water, the monatomic model of water mW.42 The mW model represents each water molecule as a single particle that interacts through very short-range anisotropic interactions that mimic hydrogen bonds. Although the model does not have explicit hydrogen atoms, it is able to reproduce the structures and phase relations between liquid water, low-density amorphous ice, and ice.3,7,42-44 Simulations with the mW model are 180 times more efficient than atomistic models with Ewald sums, while the intrinsic relaxation times of the monatomic water are faster than for atomistic models, making it possible to perform simulations slow enough to reach the critical rate for vitrification.42,43 In this work, we use large-scale molecular dynamics simulations of systems with more than 110000 molecules modeled with the mW model to investigate the structural evolution of aqueous solutions of ions as they are cooled in their pathway toward the glass state. We characterize the structure of the glasses, determine the characteristic dimensions of phase segregation, and identify the factors that prevent macroscopic phase-separation in glasses of aqueous solutions of salts.

2. MODELS AND METHODS Models. Water was modeled with the monatomic water model mW that represents each molecule as a single particle that interacts through anisotropic short-ranged potentials that favor “hydrogen-bonded” water structures.42 The mW model is based on the short-ranged Stillinger-Weber (SW) potential,45 and relies on the interplay between two-body attraction terms (φ2) that favor high coordination and three-body repulsion terms (φ3) that encourage tetrahedral configurations, to produce the “hydrogen-bonded” structures that are characteristic of water. The functional form of the SW potential is shown in eq 1,

E ¼

PP i

j>i

2

φ2 ðrij Þ þ

σij φ2 ðrij Þ ¼ Aεij 4B rij

!4

PPP i

j6¼i k>j

3

φ3 ðrij , rik , θijk Þ

σij - 15exp rij - aσ ij

!

φ3 ðrij , rik , θijk Þ

!   γσij γσ ik ¼ λijk εijk ½cos θijk - cos θ0  exp exp rij - aσij rik - aσ ik 2

ð1Þ where rij is the distance between particles i and j and θijk is the angle subtended by the vectors between the positions of the i-j 5901

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The Journal of Physical Chemistry A and i-k pairs of particles. The constants that define the potential are: A = 7.049556277, B = 0.6022245584, γ = 1.2, the cutoff parameter a = 1.8, the preferred angle θo = 109.48°, the characteristic size σWW = 2.3925 Å, the depth of the potential εWW = 6.189 kcal/mol, and the tetrahedral interaction parameter λWWW = 23.15. The parameters were adjusted in ref 42 to reproduce the experimental temperature of melting, density, and enthalpy of vaporization of liquid water. DeMille and Molinero recently developed a coarse-grained model of water and NaCl that accurately reproduces the structure of the aqueous solutions at room temperature with the use of very short-ranged interaction potentials.46 For this work, we used a simplified version that models a single strongly hydrophilic solute (that we call S) in water. The solute-solute and solutewater interactions were modeled using eq 1. We considered that the solute does not have a preference for tetrahedral coordination, so we set λSXY = 0 for triplets that involve a central solute molecule (where X and Y can be either water or solute), and that the water prefers to form tetrahedral configurations both when coordinated to other water molecules and to the solutes, λWXY = 23.15. The solute-solute potential is set to be mildly attractive, with εSS = 0.618 kcal/mol, and the water-solute potential to have εSW = εWW = 6.189 kcal/mol. All the other potential parameters involving the solute, and its mass, are identical to those for water. The result of this choice of parameters is a system in which the solute has a strong preference for water that is consistent with the high enthalpy of hydration of ions. The mild attraction between solutes ensures that their interactions are dominated by water and do not form strong solute-solute attractions that could lead to their separation into a pure solute phase. Although S was not parametrized to reproduce the property of a particular salt, we show in the next section that the water-S mixtures correctly predict several properties of water-LiCl solutions and that the dimensions of phase segregation and the fraction of water in each phase are not very sensitive to the details of the water-solute interaction. As the effect of the S on the structure of water and the phase segregation is equivalent to that of a monovalent salt in water, through the text we use also the term ions to refer to the coarse-grained solute S of this study. Simulations. Molecular dynamics (MD) simulations were carried out using LAMMPS.47 The equations of motion were integrated using the velocity Verlet algorithm with a time step of 10 fs. Simulations were performed in the NpT ensemble using the Nose-Hoover thermostat and barostat with damping constants of 1 and 5 ps, respectively. The pressure was 1 atm in all simulations. Periodic boundary conditions were used in all directions. Aqueous solutions with 5, 10, 15, 20, 30, 40, and 50% molar fraction of solute were prepared starting from equilibrated liquid water configurations at 300 K and randomly replacing water molecules with solutes. All compositions are indicated as percentage moles of solute, 100  nS/(nS þ nW). Systems with 4092 molecules were first equilibrated for 5 ns at 300 K and then cooled down to 100 K at the critical rate for vitrification of mW water,43 10 K/ns, to determine whether they form a two-phase glass, a homogeneous glass, or a crystal. Identical quenching protocol was repeated for larger simulation cells containing 32768 and 110592 molecules (about 10 and 15 nm cell side, respectively) for the 5, 10, 15, and 20% solute systems.

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Figure 1. Water-water radial distribution function in the liquid at 300 K in the presence of different molar percentage of solute S (labeled within the figure). The rdfs are vertically displaced to facilitate the visualization. The second peak in the rdf is due to tetrahedral configurations of water molecules and is strongly depressed by the solute. The effect of the solute of this study on the structure of water is identical to that observed for NaCl in experiments.48

3. RESULTS AND DISCUSSION A. Ions Decrease the Tetrahedrality of Liquid Water. Ions perturb the structure of water, leading to a significant decrease in tetrahedral ordering in the liquid. The structure of aqueous solutions of NaCl or KCl as a function of salt content was investigated by Mancinelli et al. using neutron diffraction.48 Their study shows that salts strongly depress the second peak of the oxygen-oxygen radial distribution function (rdf), which is associated with the formation of tetrahedral structures in the liquid. The disappearance of the second peak centered at 4.5 Å is accompanied by the formation of a spread peak at around 3.7 Å, close to the position of the first minimum in the rdf of pure water, 3.5 Å.48 The solute S of the present study produces the same effect as monovalent salts on the structure of liquid water. This is illustrated in Figure 1 that presents the water-water rdf of pure water and aqueous solutions of S with up to 20% moles of ions. The perturbation of the tetrahedral ordering in water arises from the strength of water-solute interactions that outcompete waterwater interactions. The addition of the hydrophilic solute produces a densification of the liquid at room temperature, as seen in Figure 2. The molecular mass of S is the same as for a water molecule, thus the densification reflects a genuine decrease in molar volume as more water is involved in the hydration shell of the ions. B. Solutions with Less than 20% Ions Produce a PhaseSegregated Glass. To investigate the effect of solute concentration on the structure of the hyperquenched mixtures, solutions containing 5-50% ions were cooled at the critical rate to avoid crystallization of ice in pure mW water, 10 K/ns.3,42 Hyperquenching of the solutions yield three classes of structures, depending on the solute concentration: (i) Crystalline Hydrate. Cooling of the 50% solution leads to the formation of an equimolar water-S crystal, shown in the lower right panel of Figure 3. The crystallization of the 5902

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Figure 2. Density of pure water and aqueous solutions of solute S as a function of temperature on cooling of the systems at the critical rate for vitrification of pure water in the model, 10 K/ns. The labels indicate the % moles of solute in the mixtures. The 50% mixture produces a 1:1 hydrate (note the densification on crystallization at around 260 K). The 40, 30, and 20% mixtures produce homogeneous glasses (the sharp change in slope signals the formation of a glass with lower thermal expansivity). The solutions with 5, 10, and 15% present a density maximum that moves toward lower temperatures with increasing ion content. The density maximum precedes a liquid-liquid transition at lower temperatures (evident in the sharp decrease in density) that is the locus where the phase segregation begins. The vitrification of the nanosegregated system does not produce a noticeable signature in the density.

solution at around 260 K is accompanied by a sudden increase in density, as seen in Figure 2. (ii) Homogeneous Glass. Cooling of the 20, 30, and 40% solutions results in the formation of a single-phase glass. A snapshot of the 30% S glass is shown in the lower left panel of Figure 3. The glass transition for the cooling rate 10 K/ns is signaled in Figure 2 by a change in thermal expansivity. We further estimated the glass transition temperatures that correspond to relaxation times of 100 ns and 100 s by computing the diffusion coefficients of the high-temperature solutions and extrapolating the relaxation times with the Vogel-Fulcher-Tammann equation (see Supporting Information). The resulting glass transition lines of the high-temperature liquids as a function of the composition are shown in Figure 4. (iii) Nanosegregated Glass. The glasses formed by quenching the 5, 10, and 15% solute mixtures contain two phases: a concentrated mixture of ions and water and pure LDA (see rdf in Figure 5). The 20% solute mixture is at the boundary of the phase-segregation regime. Incipient phase segregation can be detected for the 20% ion solution on cooling at rates of 1 K/ns. The two-phase 10% S glass formed by cooling a simulation cell with 110592 molecules is displayed in the upper panel of Figure 3. In the next section we show that phase segregation develops as water crosses the liquid-liquid transition temperature, forming a four-coordinated low-density liquid that, like ice, is solutophobic: the tetrahedral arrangement of the water molecules in LDL is topologically inconsistent with the strong solvation of the ions.

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Figure 3. Structures formed by hyperquenching the solutions. Upper panel: glass obtained with 10% solute (equivalent to 5% LiCl) contains two nanophases: a pure water LDA phase (blue) and a concentrated solution of ions (green) in water (red). The water in the LDA phase is four-coordinated; the water in the solution has five or more first neighbors. Lower panel: homogeneous glass obtained by quenching of the 30% ions solution (left) and crystal obtained by cooling of the 50% solute mixture (right). The simulation cell of the nanosegregated glass contains 110592 molecules; the cells of the homogeneous glass and the crystal contain 4092 molecules.

C. Liquid-Liquid Immiscibility is Driven by the Polyamorphism of Water. The mW water model predicts a density

maximum in liquid water and a continuous liquid-liquid transformation at room pressure.3,42 The liquid-liquid transition temperature (TLL) is defined as the inflection point of the density versus temperature. We have previously shown that TLL is also the locus of maximum change in tetrahedrality and maximum correlation length of structural fluctuations (also known as the Widom line49) in liquid water as a function of temperature.3 For pure water, the mW model predicts TMD = 250 K and TLL = 200 K, about 27 K lower than obtained in experiments and their extrapolations, respectively.3 We estimate the liquid-liquid transition temperature TLL of the solutions as the inflection point of density versus temperature in Figure 2. The inflection point of the fraction of four-coordinated water molecules as a function of temperature, shown for the 10% solute mixture in Figure 6, predicts the same value for TLL. The liquid-liquid transition appears continuous in the hyperquenching simulations. In the presence of solutes, however, the transition must be first order.36 This is evident in the coexistence of two distinct phases in the glass. The TLL we determine from the density versus temperature is probably between the actual liquid-liquid equilibrium temperature and the spinodal of the homogeneous high temperature phase. Lower 5903

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Figure 4. Phase diagram of the aqueous mixtures as a function of mole % of solute. The blue line signals the locus of the liquid-liquid transition of the solutions, computed as the locus of maximum change of the density with temperature for the simulation cell with 32768 molecules (identical TLL are obtained from the 4092 and 110592 molecules systems). Crossing of the line results in the formation of low-density liquid water, that, unable to solvate the ions, segregates as a pure phase from the mixture. Solutions with up to 20% ions produce a nanophase-segregated glass on cooling. Solutions with ion content between 20 and 50% vitrify to a single-phase homogeneous glass. Red lines locate the glass transition of the solutions as predicted from an extrapolation of the high-temperature diffusion coefficients of systems with 4092 molecules using the VFT equation (see Supporting Information). The line for relaxation times, 100 ns, is more relevant to the simulations than the one for a relaxation time of 100 s. The close lines for each time scale correspond to the predictions based on the mobility of water and the ions, which are practically identical in the homogeneous solutions. The arrow illustrates the pathway of cooling of the 15% ions mixture: below TLL, the liquid forms two phases, pure water (LDL) and a concentrated mixture. The tip of the arrow signals the temperature at which the concentrated solute phase (that should be read from the intersection of the tie with the LL line, as shown by the thin red line) reaches its glass transition temperature.

cooling rates that would produce a more accurate estimation of the equilibrium TLL result in formation of ice. The latter suggests that the temperature of the homogeneous nucleation of ice in the solutions closely follows the temperature of the liquid-liquid transition, as demonstrated elsewhere.50 Figure 4 summarizes the estimated TLL and Tg of the solutions as a function of S content. The phase behavior of the water-S solution is strikingly similar to the one for aqueous solutions of LiCl in experiments,40 with the caveat that TLL also signals the locus of homogeneous nucleation of ice in the solution, TH. Both the experiments and simulations indicate that TH decreases and Tg increases with solute concentration. The simulations predict that TH and Tg of the solution cross at 20% ions, in excellent agreement with the 10.5% LiCl (21% ions) in the experiments.40 The TMD decreases with ion content and it disappears for solutions with 20% solute and higher, in agreement with the experimental observation of a decreasing TMD with ion concentration in LiCl solutions and its disappearance for concentrations above 18% moles of ions.51,52 The quantitative agreement with the experiments indicates that the efficient

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Figure 5. Radial distribution function of water in LDA from the simulations of pure water in ref 3 (black) and the four-coordinated pure water domains segregated in the 10% aqueous glass (blue). The structure of the LDA in pure water and the mixture is identical. The g(r) of LDA in the nanosegregated glass reaches the asymptotic value of 1 at distances longer than the characteristic dimensions of phase segregation, 4.5 nm.

Figure 6. Advance of the segregation in the 10% solute concentration mixture followed by the fraction of four-coordinated water molecules (blue line) and the intensity of the diffraction peak at 0.14 Å-1 (red circles), extracted from the curves of Figure 8. The locus of maximum change in fraction of four-coordinated water molecules is the same as that of maximum change in density and defines the liquid-liquid transition temperature.

model of this work correctly captures the competition between crystallization and vitrification in LiCl solutions. Crossing of the liquid-liquid transition temperature produces separation of the solutions in pure LDL and a mixture of water and ions. The increase in the fraction of LDL is shown in Figure 6. If the phase-segregated system were in equilibrium the composition of the phases at each temperature could be read from the intersection of the tie line with the TLL line and the pure water axis in the phase diagram of Figure 4. Under equilibrium conditions, the chemical potential of water in the mixture should be the same as 5904

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Table 1. Dimensions of Phase Segregation in the Glasses (2π/qpeak), Fraction of Water in the LDA Phase (R), and Solute Fraction in the Solute-Rich Nanophase (XSrich) as a Function of the Solute Content in the System solute content (% of moles)

2π/qpeak (nm)

R

Table 2. Dimensions of Phase Segregation (2π/qpeak) and Fraction of Water in the LDA Phase (R) for the 10% Solute Glass as a Function of the Water-Solute Interaction Parameter εsw εSW (kcal/mol)

XSrich

2π/qpeak (nm)

R

5

4.7 ( 0.4

0.67

0.14

4.642 (75%)

5.0 ( 0.4

0.48

10

4.5 ( 0.4

0.46

0.17

6.189 (100%)

4.5 ( 0.4

0.46

15

4.0 ( 0.4

0.22

0.18

7.736 (125%)

4.5 ( 0.4

0.44

0.08

0.21

20a a

R and XS' for the 20% solute mixture were obtained after annealing of the system at 140 K.

Figure 7. Ratio R of water in the LDA phase in glasses of LiCl (circles), NaCl (triangles), and KCl (crosses) obtained from the analysis of the Raman spectra of the glasses. The four red circles show the results of this work; the green line is a guide through them. Adapted from Figure 3 of ref 26 [Y. Suzuki and O. Mishima, Phys. Rev. Lett. 2000, 85, 1322-1325], copyright 2000 by the American Physical Society, with permission from the authors.

in LDA, and thus the composition of the solute-rich phase XSrich at a given temperature should not depend on the initial solute content. Table 1 shows that the molar fraction of S in the solute-rich nanophase of the glasses, XSrich, increases slightly with solute content because, in practice, it is not possible to equilibrate the two-phase liquid system as the low-density liquid would crystallize within the time scale needed for its equilibration.43 Thus, the XSrich in the glass reflects the temperature at which the mixture falls out of equilibrium on the hyperquenching simulations. Figure 7 overlays the fraction of four-coordinated water molecules R in the glasses of this work (data in Table 1) and in glasses of LiCl, NaCl, and KCl as revealed by the Raman study of ref 26. The agreement between experiment and simulation is excellent, although the solute S was not parametrized to model the specific properties of these salts. The agreement is probably

due to the low sensitivity of the fraction of water in each phase to the details of the water-solute attraction, provided that this attraction is strong compared to the hydrogen bonding between water molecules. This is verified in the similarity in fraction of water in the LDA phase, R, for LiCl, NaCl, and KCl in the experiment and in very slight changes in R for the 10% solute mixture modeled with water-solute parameter εWS that are 125 or 75% of the 6.189 kcal/mol value of the model (see Table 2). The robustness of the results against significant changes in the water-solute potential supports the use of the present model to unravel the structure of aqueous solutions of monovalent salts and their glasses. D. Dimensions of Phase Segregation in the Glasses Are about 5 Nanometers. The dimensions of phase segregation in the supercooled solutions and glasses were computed from the position of the low-q peak in the diffraction patterns for the systems containing 110592 molecules. To compute the diffraction pattern as it would be measured through small-angle X-ray scattering (SAXS) we determined the structure factor S(q) of the solution using an artificial contrast factor between the phase that contains the solute and the pure water phase. The diffraction intensity S(q) was obtained from the Fourier transform of the solute-solute radial distribution function in the glass, as in ref 3. The position of the low-q peak determined the characteristic dimensions of phase segregation 2π/qpeak. Figure 8 shows the evolution of the low-q region of the diffraction pattern as the 10% solute mixture is cooled from the high temperature homogeneous liquid to the nanosegregated glass. It is worth noting that while the intensity of the diffraction peaks (circles in Figure 6) accompanies the extent of phase segregation, the position of the peaks moves only slightly toward smaller q as the liquid phase segregates and the four-coordinated water molecules consolidate into larger LDL domains. The dimensions of phase segregation indicate the characteristic spacing between the LDL domains and should not be confused with their size. The LDA and binary phases percolate through the structure in the 10% ion glass. Table 1 lists 2π/qpeak for the glasses with 5, 10, and 15% ions. The dimensions of phase segregation of the glasses are 4-5 nm and, like the fraction of LDA, not very sensitive to the solute content or to moderate changes in the solute-water interaction (Table 2). The agreement between the experiments and simulations in the fraction of LDA in the glasses and the weak dependence of the characteristic dimensions on 25% changes in the water-solute attraction suggest that the same characteristic dimensions of nanophase segregation would be measured in the glasses of aqueous solutions of monovalent electrolytes such as LiCl, NaCl, and KCl using SANS or SAXS sensitive to q within the range of Figure 8. What sets the maximum dimensions of phase segregation in the glass? The free energy of the segregated system increases with the area between the two phases, thus, it should be expected that the mixture minimizes its free energy by coarsening. While the dimensions of the domains would increase with time, the 5905

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arrangements around ions and the tetrahedral ordering in LDA and its liquid form, LDL. Liquid-liquid immiscibility develops in the solutions after crossing the liquid-liquid (LL) transformation temperature. The ions stabilize the high temperature liquid phase, leading to a decrease of the LL transition temperature with increasing ion content, as previously reported for NaCl solutions.53,54 The LL transformation in the solutions is first-order at p = 1 atm, as evidenced by the coexistence of two-distinct phases in the liquid, while the LL transformation in pure water is continuous at ambient pressure.3 This confirms the theoretical predictions of ref 36 and implies the first-order nature of the transition in solutions cannot be used to infer the order of the LL transition in pure water.

’ ASSOCIATED CONTENT

bS

Supporting Information. Estimation of the glass transition temperatures of the solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author Figure 8. Low-q diffraction pattern of the 10% solute mixture as it is cooled from the one phase liquid to the nanosegregated glass. The intensity was computed from the Fourier transform of the ion-ion radial distribution function in a 110592 molecule system cooled at 10 K/ns. The intensities at qpeak = 0.14 Å-1 (dashed line) are summarized in Figure 6.

effective limit to the phase separation is imposed by the rate of cooling needed to avoid the crystallization of ice from the solution. The results presented in this work correspond to the slowest cooling rate that leads to vitrification of the mixtures. Slower cooling rates result in nucleation and growth of ice within the pure water phase in a time scale that is shorter than that required to coarsen the structure and produce significantly larger domains. The crystallization of ice from supercooled aqueous solutions and its relation to the formation of domains of lowdensity liquid are discussed in a separate communication.50

4. CONCLUSIONS We investigated the structural transformation of aqueous solutions of salts as they are quenched from room temperature to the glass state using large-scale molecular dynamics simulations with an efficient model. The simulations reproduce quantitatively the experimental phase behavior of the aqueous solutions of LiCl, including the loss of the density anomaly between 15 and 20% ion content, the beginning of the one-phase glass forming regime just above 20% ions, and the fraction of LDA in the glasses as a function of salt concentration. Aqueous solutions with up to 20% moles of ions yield phasesegregated glasses. The characteristic dimensions of phase segregation in the glasses are about 4-5 nm and rather insensitive to modulations of the already strong water-ion attraction. This prediction should be compared with future SAXS or SANS results in the relevant q-range. Correlation lengths of about 5 nm are the maximum that can be attained within the liquid state: slower simulations that would produce coarsening of the two-phase structure (thermodynamically favorable, as it would decrease the interfacial free energy), result in the formation of ice within the LDL domains. The immiscibility is driven by the poor solvent quality of the fourcoordinated liquid for ions, a consequence of the incompatible water

*To whom correspondence should be addressed. E-mail: [email protected].

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