Mutual Effects of Glycerol and Inorganic Salts on Their Hydration

Article pubs.acs.org/JPCB

Mutual Effects of Glycerol and Inorganic Salts on Their Hydration Abilities Lishan Zhao,†,‡ Liqing Pan,† Zexian Cao,‡ and Qiang Wang*,‡ †

Department of Physics, University of Science and Technology Beijing, Beijing 100083, China Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

‡

ABSTRACT: It is a tough challenge to understand the mutual interactions among various components in aqueous solutions of inorganic mixed with organic solutes. The hydration number, nh, and critical hydration number, ncr, determined by the measurements of glass transition of the solutions, in conjunction with tracing the change in local water structure, can provide some insights into the complicated interplays in such a mixture. Here, the nh and ncr for aqueous solutions of glycerol, various chlorides, and mixtures of glycerol with a chloride are determined. The ratio of ncr/nh measures 4 for glycerol and 1.7 for all the chlorides, and for mixtures of glycerol with all of the chlorides except ZnCl2, it falls within these two extremes. Glycerol content dependence of nh and ncr reveals a rich and interesting scenario of mutual effects therein, in particular, the glycerol’s replacement and sharing of hydration water with salt. In the case of ZnCl2, at most, one hydration water molecule is replaced by glycerol, and the excess glycerol molecules continuously reduce the number of glycerol molecules that share hydration water with ZnCl2. Our results can help establish a pathway for the investigation of interactions among the organic and inorganic components in aqueous solutions, which is desirable for many applications.

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INTRODUCTION Aqueous solutions of mixed solutes are crucial for the understanding of a plethora processes and/or functions in biology, geology, and industrial applications. They are more complicated than the aqueous solutions of one single solute, though the latter is in no sense a simpler existence and is usually far from being well understood. Diverse interactions exist among solvent and mixed solutes, including ions, small organic molecules, and macromolecules, which results in many interesting mutual effects between them.1−16 For example, cations or anions were classified by Hofmeister according to their different salting-in or salting-out effects on proteins.2,3 This behavior has been attributed to either an indirect interaction between solutes through their structure-making or structure-breaking effects on water3−9 or a direct interaction between them mainly through electrostatic, hydrogen bonding, or van der Waals forces.10−16 The types of interactions among mixed solutes can be qualitatively estimated by some physicochemical properties of solutes, such as solvation free energy.6,13−15,17,18 For example, a “law of matching water aff inities” was proposed by Collins to predict the formability of ion pairs in dilute electrolyte solutions.17 This semiempirical model presumes that the oppositely charged ions with similar solvation free energy tend to form ion pairs in dilute solutions. On the contrary, for oppositely charged ions with very different solvation free energy, dehydration of the more strongly hydrated ions needs greater energy than that released by forming an ion pair with © XXXX American Chemical Society

the more weakly hydrated ions, and therefore, these ions tend to stay apart. Recently, the change of solvation free energy of proteins when moving them from water to solutions as well as the dominate interaction contributing to this change has also been analyzed to reveal the salting-in and salting-out effects of ions and small organic molecule on proteins.14,15 Within moderately or highly concentrated solutions, ions or organic molecules cannot be fully hydrated;19 therefore, the interactions between different components of the solution are much more complicated than what can be conveyed in the “law of matching water af f inities”. This work just aims to reveal the interactions among strongly hydrated ions, weakly hydrated glycerol molecules, and water in moderately concentrated aqueous solutions, based on a comparison of hydration number nh and critical hydration number ncr for individual solutes and their mixtures. Here, nh refers to the number of water molecules per solute molecule that can vitrify totally as a whole in a dilute solution after the precipitation of primary ice when cooling the solution; ncr refers to the maximum number of water molecules per solute molecule which can totally vitrify as a whole, yet part of the water in proportion to ncr − nh can crystallize in a slow cooling process or recrystallize during heating the devitrified solution.20−22 Received: August 31, 2016 Revised: November 27, 2016 Published: December 2, 2016 A

DOI: 10.1021/acs.jpcb.6b08778 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. (a) State diagram of aqueous solutions showing how the hydration number, nh, and the critical hydration number, ncr, are determined and the status of a dilute solution in zone III at different temperatures; (b) ncr vs nh for various inorganic salts (blue solid square), and a mixture of ZnCl2 and MgCl2 with a molar ratio of 1:1 (magenta solid diamond); those data can be fitted by ncr = 1.7nh (blue solid line); glycerol (blue solid circle), mix fitted by ncr = 4nh (blue dashed-dotted line); and nmix cr vs nh for the glycerol−salt mixtures with a molar ratio of glycerol to salt at 3:1 (black open mix = 2.5n (black dashed line). square), which was roughly fitted by nmix cr h

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RESULTS AND DISCUSSION In the following discussion, for brevity, the aqueous solutions of inorganic salts used here will be specified with the formula M· nH2O, where M stands for the solute. When glycerol is added, the solutions are specified with the formula M·Xgly·nH2O, where X denotes the molecular ratio of glycerol to inorganic salts. The hydration ability is discussed in the context of nh and ncr, where ncr > nh, which corresponds to a solution specified by the formula M·nhH2O and M·ncrH2O, respectively. In the case of the solute mixture, the superscript “mix” will be occasionally added to these parameters to prevent any confusion. mix The parameters nh and ncr, or nmix h and ncr , when particular emphasis on the cases of mixed solutes is necessitated, are extracted from the state diagram of the glass transition behavior of aqueous solutions, which is quite universal at least for aqueous solutions of inorganic salts and for those even with the addition of glycerol,20−22 as presented in Figure 1a. As can be seen in Figure 1a, the aqueous solution of a given solute can be divided into three distinct concentration zones, labeled as I, II, and III, according to the concentration dependence of the solution vitrification and ice crystallization behaviors.22 Within zone I, solutions can easily vitrify totally when cooled at moderate or slow rates, and no recrystallization (also called cold crystallization) of ice is observed in the reheating process. Within zone III, part of the water in the solutions may spontaneously crystallize into primary ice upon cooling, and the residual liquid portions, often referred to as the freezeconcentrated solution, vitrify at an almost constant temperature, Tg′ (Figure 1a), which is independent of the initial concentrations of solutions. The concentration of the freezeconcentrated solutions resulting from icing of the dilute solutions in zone III corresponds to a formula of M·nhH2O (for a detailed argument, see ref 22). Solutions in zone II deserve particular attention. They can vitrify as a whole when cooled at moderate cooling rates, but spontaneous icing of part of the water may occur at very slow cooling rate, and icing as well as recrystallization into ice in the reheating process can also be provoked by elaborately designed thermal treatment procedures.21,22 These three zones are separated by just two

Differential scanning calorimetric (DSC) measurements were performed to determine nh and ncr for glycerol, various chlorides of mono-, di-, and trivalent metallic ions, and glycerol mixed with chloride salts at different molecular ratios. Raman spectroscopies of some particular samples are also obtained to reveal the dependence of the local structure of water and glycerol molecule on the molar ratio of glycerol to salts. The glycerol content dependence of nh and ncr was determined and scrutinized, which reveals a rich and interesting scenario of mutual effect in aqueous solutions of mixed solutes, in particular, glycerol’s replacement of hydration water and sharing of hydration water with the salt. Moreover, it is found surprisingly that ZnCl2 behaves very exotically in comparison with other chlorides described here. These results help establish a pathway for the investigation of interaction between salts and organic molecules in aqueous solution, which is desirable, for instance, for the design of more effective cryoprotectant agents.23

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EXPERIMENTAL SECTION Aqueous solutions were prepared with Millipore water and high-purity solutes, including LiCl (anhydrous, 99.9%), NaCl (anhydrous, 99.9%), KCl (anhydrous, 99.9%), RbCl (anhydrous, 99.8%), CsCl (anhydrous, 99.99%), CaCl2 (anhydrous, 99.99%), MgCl2·6H2O (99%), MnCl2·4H2O (99.99%), ZnCl2 (anhydrous, 99.99%), AlCl3·6H2O (99%), and glycerol (99.5%), all purchased from Sigma-Aldrich. Differential scanning calorimetric (PerkinElmer DSC8000) measurements of solution droplets (∼5.0 μL) were performed at a cooling/ heating rate of 20 K/min. When cooled to 123 K, the samples were held at that temperature for 1 min before subsequent reheating procedure was performed. Raman spectra were measured on a confocal Raman system (Jobin-Yvon HR800) with the 532 nm diode laser excitation at room temperature. The laser power of 1 mW was focused onto the sample surface through a fused SiO2 film with a thickness of 0.3 mm. The integration time was set at 40 ms per point with a spectral resolution of 1.4 cm−1. B

DOI: 10.1021/acs.jpcb.6b08778 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Figure 2. Hydration ability of glycerol−ZnCl2 and glycerol−CrCl3 at different molar ratios of glycerol to salt, X. (a) Hydration number nmix h (solid symbol), Δnh (open symbol), and Δnh + Xngly h (open symbol in inset) for the mixtures of glycerol−ZnCl2 (square) and glycerol−CrCl3 (circle), here salt gly gly mix Δncr = nmix h − (nh + X*nh ). The dashed line refers to −Xnh . (b) Critical hydration number, ncr (solid symbol), and the change in Δncr (open salt gly mix mix symbol) for glycerol−ZnCl2 (square) and glycerol−CrCl3 (circle); here, Δnh = nmix cr − (ncr + X*ncr ), and (c) ncr /nh vs X for the mixture of glycerol−ZnCl2 (square), −CrCl3 (circle), and −NaCl (triangle). The horizontal dashed line indicates the value for pure glycerol.

parameters nh and ncr, which together characterize the hydration ability of a given solute. When an aqueous solution of two solutes is involved, these two parameters can effectively reveal the mutual effects of the two solutes in water. Figure 1b displays the data of ncr versus nh for a few chlorides (blue solid squares) and glycerol (blue solid circle), and nmix cr versus nmix h for the mixtures a sample of MgCl2−ZnCl2 with a molar ratio of 1:1 (magenta solid diamond) and glycerol− inorganic salt all in a molar ratio of 3:1 (open squares), that is, M·3gly·nH2O. Here, X = 3 is chosen for comparison because mix ZnCl2·3gly·nH2O shows a nmix cr /nh particularly larger than that for pure glycerol (see Figure 2). In fact, the cases with X other than 3 have also been investigated. Remarkably, for all individual inorganic salts discussed herein, ncr/nh is almost independent of the nature of the salts and ncr varies in accordance with nh. Because the hydration sphere around ions, which consists of nh water molecules, is in some sense a rigid and compact structure, the vitrification of more water, at maximum, corresponds to ncr − nh water molecules for each solute, mainly resulting from the spatial confinement constituted by the hydration spheres. A salt-type independent ncr/nh can be attributed to almost the same spherical symmetry of different hydrated ions and their uniform distribution in solutions. The constant ratio of ncr/nh for inorganic salts does not apply to glycerol molecules. For this simple organic molecule, nh = 1.5, which is obviously smaller than those for salts; however, ncr/nh = 4, which is much larger than the value of 1.7 for salts. The small value of nh for glycerol can be proposed to result from a weak interaction between hydroxyl groups of glycerol and water, and the significantly large value of ncr/nh can be attributed to a nonspherical symmetry of a hydrated glycerol mix molecule. For glycerol−salt mixtures, nmix usually falls cr /nh within a region between the corresponding values of pure mix glycerol and salts (Figure 1b). For example, at X = 3, nmix cr /nh scatters around 2.5 for glycerol mixed with KCl, MnCl2, MgCl2, CrCl3, or AlCl3 (dashed black line in Figure 1b). The mixture of glycerol with ZnCl2 seems quite exotic that, for ZnCl2·3gly· mix mix mix nH2O, nmix h = 6.8 and ncr = 29, accordingly it has ncr /nh ∼ 4.3, which, surprisingly, outruns that of pure glycerol (dashed blue line in Figure 1b).

As can be seen from Figure 1b, glycerol−ZnCl2 shows a peculiarity in its hydration ability, which deserves particular attention. To understand this exotic hydration behavior of mix glycerol−ZnCl2, we measured the nmix h and ncr of ZnCl2·Xgly at different X values and plotted them in Figure 2a,b, respectively. Obviously, nmix decreases at first and then increases with h increasing X. A decrease in nmix h suggests a dehydration for one or both of the solutes when they are mixed, which indicates the presence of a direct interaction between salt and glycerol. Therefore, aqueous solutions of glycerol−ZnCl2 deviate from the ideal one within which there is no interference between different solutes; consequently, the hydration performance for each solute is maintained. To qualitatively describe the possible deviation of M·Xgly·nH2O from the ideal solution, the gly difference Δnh = nmix − (nsalt h h + X*nh ) is introduced and is plotted in Figure 2a as a function of X (open square), where superscripts “salt” and “gly” refer to the salt and glycerol, respectively. For an ideal solution, Δnh = 0. Next, let us consider an extreme case where all glycerol molecules share hydration water with salt. In that case, Δnh = −Xngly h , as represented by a dashed blue line in Figure 2a. For clarity and mix salt to facilitate comparison, Δnh + Xngly h , that is just nh − nh , is also shown in the inset of Figure 2a. We see that Δnh + Xngly h < 0 for X < ∼3.5, and it reaches a minimum of −1 when X ∼ 2. This observation suggests that, in the mixture solution of glycerol−ZnCl2 specified by ZnCl2·Xgly·nmix h H2O, at most, one hydration water of ZnCl2 is replaced by the glycerol molecule and the other hydration water molecules are shared by glycerol and ZnCl2 for X < ∼3.5. By further increasing X, interestingly, Δnh begins to increase and Δnh + Xngly h becomes greater than zero. This behavior is possible only when, for X from 3.5 onward, the hydration behavior of the further added glycerol molecules is hardly affected by the presence of salt, and at the same time, the number of glycerol molecules that share hydration water with the salt becomes gradually fewer when X is increased greater than 3.5. Dependence of nmix h or Δnh on the content of glycerol, that is, the value of X, is also investigated on the aqueous solution of glycerol−CrCl3, bearing in mind that CrCl3 has a much stronger interaction with water than ZnCl2 (see circle symbols in Figure 2a). It can be seen that Δnh + Xngly h also minimizes to C

DOI: 10.1021/acs.jpcb.6b08778 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Figure 3. (a) Raman spectra for the aqueous solutions of glycerol−ZnCl2 with an X ranging from 0.25 to 6 measured on samples corresponding to −1 refer to the symmetric and asymmetric CH2 stretching band of glycerol, and those ZnCl2·Xgly·nmix cr H2O. The peaks around 2890 and 2950 cm around 3270 and 3470 cm−1 refer to the OH stretching band of water with tetrahedral and non-tetrahedral local structures. (b) Area ratio of the peaks at 2950 and 2890 cm−1 for the solutions with glycerol−ZnCl2 (square) and glycerol−CrCl3 (circle) recorded on samples corresponding to M· mix Xgly·nmix h H2O (solid symbol) and M·Xgly·ncr H2O (open symbol). The red line segment refers to that of glycerol aqueous solution at a concentration corresponding to its ncr. (c) Area ratio of the peaks at 3270 and 3470 cm−1 for the solutions with glycerol−ZnCl2 (square) and mix glycerol−CrCl3 (circle) recorded on samples corresponding to M·Xgly·nmix h H2O (solid symbol) and M·Xgly·ncr H2O (open symbol).

−1 at X ∼ 1, implying that at most one hydration water molecule for CrCl3 is replaced by glycerol and the other hydration water molecules are shared by glycerol and CrCl3. This case is very similar to that observed in ZnCl2·Xgly· nmix h H2O. Remarkably, the decrease of Δnh of glycerol−CrCl3 becomes saturated at X = ∼5. As Δnh can remain unchanged only when additionally added glycerol molecules are fully hydrated, we might conclude that five glycerol molecules share hydration water with a molecule of salt in the solution of CrCl3· mix Xgly·nmix h H2O (it is ∼3.5 for ZnCl2·Xgly·nh H2O). When more glycerol is added, the excess glycerol molecules are hydrated as in an aqueous solution of pure glycerol, and the number of glycerol molecules that share hydration water with the salt remains unchanged. Figure 2b illustrates the same story for the parameter nmix cr and and Δn shown in Figure 2a. The measured Δncr as that for nmix h h is found to increase almost linearly with X for both nmix cr mixtures of glycerol−ZnCl2 and glycerol−CrCl3. The differsalt gly ence, Δncr, accordingly defined as Δncr = nmix cr − (ncr + X*ncr ), is also plotted as a function of X; see the open symbols in Figure 2b. Obviously, very different trends concerning the dependence of Δncr on X are observed in the two systems. For glycerol−CrCl3, the monotonically decreasing Δncr becomes saturated at a value of −7 from X = 5 onward, similar to the situation for Δnh (circle in Figure 2a). For glycerol−ZnCl2, however, Δncr immediately becomes saturated at X = 0.5 and is maintained at a value of only −2, very different from the change of Δnh with X (square in Figure 2a). This implies that in ZnCl2· Xgly·nmix cr H2O, when X > 0.5, two hydration water molecules are shared by ZnCl2 and glycerol, and the other glycerol molecules display hydration behavior similar to that in glycerol·ngly cr H2O. This is clearly different from the situation in ZnCl2·Xgly· nmix h H2O, where the hydration of glycerol molecules is always associated with the presence of ZnCl2 for all the X values used here. In other words, the interaction between ZnCl2 and glycerol obviously becomes weakened when the water content mix increases from nmix h H2O to ncr H2O. In this respect, the ZnCl2−

glycerol is quite particular. For the CrCl3−glycerol system, Δncr and Δnh vary in accordance to each other that both of them are saturated at a value of ∼−7 from X ∼ 5 onward. In other words, the mutual effects for the CrCl3−glycerol combination exhibit a weak dependence on water content, probably due to the stronger interaction of CrCl3 with water. The observation on CrCl3 also holds for other inorganic salts used here, though a small difference is present. In order to explain the exotic hydration behavior of glycerol− mix ZnCl2, the data of nmix cr /nh are plotted as a function of X for glycerol−ZnCl2, glycerol−CrCl3, and glycerol−NaCl (Figure 2c). NaCl is also taken into consideration mainly due to its comparatively weaker interaction with water. For both mix glycerol−CrCl3 and glycerol−NaCl, nmix cr /nh increases monotincreases more rapidly with onically with increasing X, or nmix cr (cf. Figure 2a,b). However, for glycerol− increasing X than nmix h mix ZnCl2, its nmix cr /nh is larger than those for glycerol−CrCl3 and glycerol−NaCl, and it reaches a maximum at X ∼ 4. mix Remarkably, within ∼2.5 < X < ∼5.1, nmix cr /nh for glycerol− ZnCl2 is even larger than that for pure glycerol, which arises dominantly from the nearly V-shaped X dependence of Δnh (Figure 2a). The exotic hydration behavior of glycerol−ZnCl2 seemingly cannot be well explained simply with the concepts of ion−water interaction (it is stronger for CrCl3 but weaker for NaCl than ZnCl2) or ion size (Mg2+ and Zn2+ ions have roughly the equal size). To further reveal the interaction in aqueous solution between glycerol and salt, Raman spectra were measured on samples of mix M·Xgly·nmix h H2O and M·Xgly·ncr H2O for various X values, where the salt is either ZnCl2 or CrCl3. Figure 3a displays the area-normalized Raman spectra of ZnCl2·Xgly·nmix cr H2O. Four characteristic peaks are of particular interest, of which the two at 3270 and 3470 cm−1 are attributable to the OH stretching vibrations arising from water with tetrahedral and nontetrahedral local structures, respectively. The other two peaks around 2890 and 2950 cm−1 refer to the symmetric and asymmetric CH2 stretching vibrations in glycerol molecules, D

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mutual effect of inorganic salts and organic molecules in aqueous solution.

respectively. The area ratio between the two Raman peaks at 2950 and 2890 cm−1, A2950/A2890, reflects qualitatively the structural change of glycerol molecules induced by interaction with surroundings.24−30 At present, it is still difficult to provide a one-to-one relation between A2950/A2890 and the structure of glycerol molecule because glycerol molecules have six possible conformations,31 and some conformations may coexist at the same time in a given solution. In spite of this, a decrease (increase) in A2950/A2890 here can still be qualitatively related to the weakening (strengthening) of interaction between glycerol and salt in the solution.29−31 Figure 3b represents the A2950/A2890 measured in solutions of mix M·Xgly·nmix h H2O and M·Xgly·ncr H2O as a function of X for a mixture of both glycerol−CrCl3 and glycerol−ZnCl2. The data of A2950/A2890 for an aqueous solution of pure glycerol (i.e., at gly·ngly cr H2O) are also included for comparison. In all cases, A2950/A2890 decreases rapidly at first and then continues to decrease slowly. Two points need be particularly addressed. First, the solution of glycerol−CrCl3 has a A2950/A2890 higher than that of glycerol−ZnCl2 at concentrations referring to both mix nmix h and ncr of each. This fact directly supports the observation in Figure 2 that hydrated CrCl3 exerts stronger influence on glycerol than does hydrated ZnCl2. Second, in the solution specified by ZnCl2·Xgly·nmix cr H2O, A2950/A2890 decreases rapidly first and then immediately becomes saturated at X > 1, approaching finally a constant which is, we believe, the value of A2950/A2890 ∼ 1.53 for gly·nmix cr H2O. This near indifference of A2950/A2890 to the increasing content of glycerol for X > 1 and that of Δncr for X > 0.5 both support the conclusion that ZnCl2 has a negligible effect on the hydration behavior of most glycerol molecules in ZnCl2·Xgly·nmix cr H2O for sufficiently large X. Furthermore, the interactions among glycerol, hydration water, and the salt can also be provided by X-dependent Raman spectra of water, for example, by tracking the change of the ratio A3270/A3470.32−34 Let us first consider the case of glycerol− CrCl3. As shown in Figure 3c, A3270/A3470 for the solutions of glycerol−CrCl3 remains nearly constant for X > 1 at concentrations referring to both M·Xgly·nmix h H2O and M·Xgly· nmix cr H2O, implying that the local structure of hydration water does not change with the presence of more glycerol. This observation indicates that the interaction between hydration water and CrCl3 is strong enough to prevent the local structure of hydration water from being disturbed by glycerol molecules, though they share some hydration water with CrCl3. In sharp contrast, however, the ratio of A3270/A3470 for the solutions of glycerol−ZnCl2 increases steadily with increasing X, while the gap between the values for M·Xgly·nmix h H2O and M·Xgly· nmix cr H2O become narrowed (Figure 3c). This behavior probably results from two factors. First, with increasing content of glycerol, the fraction of hydration water shared by ZnCl2 and glycerol may become less in M·Xgly·nmix h H2O. Second, in M· Xgly·nmix H O, an increase in the number of glycerol molecules cr 2 weakens the interaction between hydration water and ZnCl2, consequently reducing the fraction of the non-tetrahedral structure of water. Those results support the conclusion deduced from the DSC measurement that the interaction between glycerol and ZnCl2 depends on the contents of both water and glycerol. These results also indicate that a combination of DSC measurement (in determining the hydration number and critical hydration number) and Raman spectra can provide an applicable pathway for revealing the

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CONCLUSIONS In summary, the hydration number nh and the critical hydration number ncr for glycerol, various inorganic salts, and the glycerol−salt mixtures at different compositions were determined from the quite universal concentration dependence of glass transition behavior. Among the inorganic salts, ZnCl2 was found exceptional in that the mixture of glycerol−ZnCl2 has values of ncr/nh higher than that for pure glycerol. Detailed investigation of the mixture of glycerol−ZnCl2 was carried out in reference to that of glycerol−CrCl3. In ZnCl2·Xgly·nmix h H2O with X less than 3.5, one hydration water of ZnCl2 is replaced by glycerol molecule, while the others are shared by glycerol and ZnCl2. When even more glycerol is available, the hydration behavior of excess glycerol molecules is hardly affected by the presence of salt, and the number of glycerol molecules which share hydration water with ZnCl2 observed at low X levels also becomes fewer at high X levels. In ZnCl2·Xgly·nmix cr H2O, the hydrated ZnCl2 can affect the hydration behavior of one glycerol molecule at most within the whole measured range of X. In comparison with ZnCl2, CrCl3 has a stronger interaction with hydration water, which can effectively prevent both the structure of hydration water and the interaction between hydrated CrCl3 and glycerol from being altered by excess glycerol molecules. Hydrated CrCl3 can affect the hydration behavior of five glycerol molecules at most in both CrCl3·Xgly· nhmixH2O and CrCl3·Xgly·ncrmixH2O. In spite of the strong interaction between CrCl3 and the hydration water, still one hydration water molecule is replaced by glycerol. The peculiarity in hydration behavior of ZnCl2 with the addition of glycerol, in comparison to other chlorides at least, needs further explanation, and now we are trying to verify it in other series of salts including ZnBr2 and Zn(ClO4)2. Our results have proven that determination of hydration numbers provides an effective pathway for the understanding of mutual effects in aqueous solutions of multiple solutes, and they are very helpful for the understanding the interaction between salts and organic molecules and for the revealing specific ionic effects on the organic molecules in aqueous solution, which are desirable for the design of more effective cryoprotectant agents and many other applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 (010) 82649090. ORCID

Qiang Wang: 0000-0002-8529-9325 Author Contributions

Q.W. initiated the research; L.Z. performed the measurement; Q.W., Z.C., and L.P. interpreted the result; L.Z., Q.W., and Z.C. compiled the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Knowledge Innovation Project of Chinese Academy of Sciences on Water Science Research KJZD-EW-M03, the National Natural Science Foundation of China (Grant Nos. 11474335, 11474325. and 11290161). E

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DOI: 10.1021/acs.jpcb.6b08778 J. Phys. Chem. B XXXX, XXX, XXX−XXX