J. Phys. Chem. B 2006, 110, 12191-12197
12191
Peculiar Properties of Water as Solute Luigi Dei* and Scilla Grassi Department of Chemistry & CSGI Consortium, UniVersity of Florence, Via della Lastruccia, 3 I-50019 Sesto Fiorentino (FI), Italy ReceiVed: January 30, 2006; In Final Form: March 27, 2006
Water has been investigated for a long time as the most important solvent; the peculiar behavior of water as solute has been studied in binary mixtures with organic solvents, mainly exploring the whole phase diagram. In this Article, we studied the behavior of water in binary mixtures with propylene carbonate in the phase diagram region where water acts as a solute as a function of the water molar fraction Xwater. Surface tension measurements, differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) have been used to investigate the state of water molecules and hydrogen bonds when water is to be considered a solute instead of a solvent, and peculiar and interesting properties were discovered. The interaction of water molecules among themselves and between water and propylene carbonate has been shown to be dependent on the water concentration in the mixtures. All of the measured properties showed a break at Xwater ≈ 0.150.20 similar to the break due to the critical micellar concentration in surfactant solutions. In particular, from the FTIR spectra, it was possible to deduce that at this concentration water has a transition from pure solute (“multimers” solvated by PC) to cosolvent (“intermediate” and “network” water).
Introduction Despite its relatively simple molecular structure, liquid water exhibits unusual thermodynamic behavior and some anomalous properties that differentiate it from other liquids; this is the reason why the nature of liquid water and how H2O molecules are organized and interact have attracted the interest of chemists for many years.1 Many studies have been carried out to develop models2,3 that attempt to explain the structure and behavior of water when it behaves as a bulk liquid,4 but much more interesting appears the study of the water structure when it is encapsulated in confined geometries.5,6 It is well known that water, like all of the other solvents, changes its physical properties upon confining:7 it has been demonstrated that the intermolecular structure of water is modified in confined systems. A lot of papers8-10 demonstrated the existence of two well-separated dynamical processes: the first associated with the bulklike molecules that seems to be unaffected from external interactions, and the second one connected to an interfacial layer of molecules strongly influenced in their reorientation dynamics. Typical systems where this phenomenon has been studied are reverse micelles,11,12 mesoporous solids,13 polymeric membranes,14-16 silica gels,17 zeolites,18 and activated charcoal.19 Literature regarding the structure of water and when it is to be considered a solute rather than a solvent is abundant,20-26 mainly dealing with thermodynamic, rheological, spectroscopic, and dielectric aspects. Particularly interesting is the phase diagram region where 0 < Xwater e 0.3, because in these systems the structure of a relatively small number of water molecules in an organic solvent environment can show a peculiar behavior. Often the literature on such systems investigates the water-poor phase diagram portion in not as much detail as for the water* To whom correspondence should be addressed. Phone: +39 0554573045. Fax: +39 0554573036. E-mail:
[email protected].
rich portion.27,28 On the other hand, many studies have been carried out about binary systems in the presence of electrolyte.29 Indeed, the aim of this paper was to add some insight into the role of water as a solute and the effect of dissolving a small amount of water in an organic solvent, carrying out measurements on many samples all falling in the water-poor region. In particular, in the present work, we report the results of an investigation of the propylene carbonate-water system in the range 0.041 < Xwater e 0.33, where the water can be considered a solute. The choice of propylene carbonate (PC) was determined because of its peculiar characteristics: it is a polar solvent with high dielectric constant30 and dipole moment, but, in contrast with a lot of other polar solvents which are H-donors, propylene carbonate has an H-acceptor ability,31 which may be one of the reasons for its peculiar behavior. Moreover, PC is an excellent solvent for many organic and inorganic materials in such applications as surface cleaners, degreasers, dyes,32 plastics, batteries,33 and natural gas. It is also widely used for lithium battery electrolytes34 because of its high relative permittivity, high boiling point, and good solubilizing power for lithium salts.35-37 A comprehension of its local liquid structure and its interaction with water in the PC-rich region of the phase diagram would be interesting to deeper understand its peculiar and anomalous properties and its potentiality as a solvent and reaction medium. Moreover, PC has been studied but mainly with the aim of characterizing some electrochemical properties in the presence of electrolytes.29 Therefore, we investigated the changes in the water structure in water-PC mixtures where the “solute” (water) content is ranging from 0.041 water molar fraction up to saturation (0.33 water molar fraction), studying a region of the phase diagram that is almost unknown. The techniques we used to achieve information on the water structure in the presence of PC molecules as solvent were differential scanning calorimetry (DSC), surface tension measurements, and Fourier transform infrared (FTIR) spectroscopy.
10.1021/jp060633l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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Figure 1. Phase diagram composition versus temperature for the propylene carbonate-water system.
Dei and Grassi °C up to room temperature at 5 °C min-1 both in the cooling and in the heating modes. The calibration of the DSC apparatus was made through the melting of indium. The accuracy in the determination of ∆H was (0.5 J g-1. 2.4. Infrared Spectroscopy. FTIR spectra have been collected in the mid-Ir range (4000-400 cm-1) with a Nexus 870FTIR (Thermo-Nicolet), in transmittance mode (beam splitter, KBr; detector, DTGS), with a resolution of 4 cm-1, with 32 scans per spectrum, and no mathematical correction (e.g., smoothing) was performed. The spectrum of pure PC was used as a reference in the absorbance calculations, to extract the bands of the solvent and to achieve only the spectrum relative to water. The thickness of the liquid film, and therefore the irradiated amount of sample, was fixed by a spacer (∼0.025 mm) placed between the CaF2 windows.
Experimental Section
Results and Discussion
2.1. Materials. Propylene carbonate (4-methyl-1,3-dioxolan2-one) was purchased from Merck, Darmstadt, Germany (g99% grade) and used as received without further purification. Water was purified by a Millipore Organex system (R g 18 MΩ cm). Aqueous solutions of PC were prepared with water/PC weight ratios depending on the different techniques employed. All samples were prepared at concentrations allowing a single stable and transparent isotropic phase at the selected temperature. Figure 1 reports the binary phase diagram of the system water/ PC. The diagram was obtained using the cloud-point method. Mixtures were prepared by weighing the appropriate amount of water and PC in 4 mL vials. The sealed tubes were immersed in a water bath at a temperature below the demixing temperature and heated at ca. 2 °C min-1. The cloud point temperature was measured with a mercury-in-glass thermometer. The temperatures recorded were the mean of several observations on each mixture. Our study focused on water weight concentrations between 0.9% and 7% (i.e., 0.041 e Xwater e 0.33), a range in which the H2O/PC mixtures are monophasic in the temperature range 20-35 °C. It is worthwhile to notice that the shape of the phase diagram shown in Figure 1 resembles, even if reverted, those found for short-chain nonionic surfactants,38-41 even if the systems are profoundly different with PC not being an amphiphilic molecule with surfactant behavior. 2.2. Equilibrium Surface Tension. Equilibrium surface tension was determined by the De Nouy ring method (ring of platinum) using a KSV Sigma 70 digital tensiometer (accuracy 0.1 mN/m) (Helsinki, Finland). The measurements were performed at 25 ( 0.5 °C diluting an aqueous propylene carbonate solution with propylene carbonate in a thermostated vessel. The solution was stirred for 3-5 min and allowed to rest 3-5 min before each measurement. The final curve was obtained as the result of seven measurements, and the data reported are the average values with corresponding standard deviation. 2.3. Differential Scanning Calorimetry (DSC). DSC measurements have been performed in sealed (to avoid water evaporation) aluminum pans (TA Instruments). The weight of the PC/water sample was checked at the beginning and at the end of the measurements, and only measurements where no evaporation had occurred were considered. The accuracy on the weight was (0.01 mg. The DSC curves were carried out by means of a Q1000 differential scanning calorimeter (TA Instruments) equipped with an Universal Analysis 2000 Version 3.7A software. Measurements were performed in a dry nitrogen flow of 40 cm3 min-1 with the following temperature cycle: from room temperature down to -80 °C and then from -80
Before discussing the results obtained, it is important to remember that water as a solute has a very peculiar characteristic due to its very low molecular weight: a very small amount of water in an organic solvent can lead to quite high molar concentration. Therefore, the aim of this section is to illustrate how the addition of water to propylene carbonate affects the interaction between the two components and possible restructuring of water, by measuring both macroscopic properties (surface tension and calorimetric measurements) and molecular ones (FTIR spectroscopy). Accordingly, we first present experimental data obtained through macroscopic measurements, and subsequently data on a possible molecular interpretation of the interactions. 3.1. Equilibrium Surface Tension. In the past, a lot of studies about the determination of surface tension of binary mixtures were reported: Benson and Lam42 studied binary systems of alcohols such as methanol-n-decanol, ethanol-ndecanol, n-propanol-n-decanol, n-butanol-n-decanol, and n-hexanol-n-decanol; Kudrya and Telbiz43 studied the surface tension of mixtures of dioxane and nitromethane with n-butanol, and dioxane has also been investigated in mixture with n-butylamine;44 and Papaionnou and co-workers45 measured the surface tension of the acetone plus isooctane system, as well as of the mixtures of 1-propanol and n-propylammine.46 Instead, surface tension of binary mixtures between water and organic solvents was rarely measured, especially in the region where water acts as a solute; our aim was to deeply investigate the surface tension behavior in such a portion of the phase diagram. Figure 2 reports the surface tension versus the water molar fractions for the PC/water system. The plot can be divided into two parts: on the right side, we report the surface tension of the system where the main phase is water and the solute is propylene carbonate added until Xwater ) 0.958 (19.7 wt % of PC), and on the left side we report the surface tension values when water behaves as a solute. A particular behavior can be observed in the PC-rich part of the curve where the water content is ranging from zero to saturation (in particular to 7.96 % wt, i.e., Xwater ) 0.33, or log[H2O] ) 0.719). For a small amount of water, the surface tension presents a value close to that of pure PC that remains almost constant (plateau) up to ca. log[H2O] ) 0.2-0.3; in correspondence with this region (Xwater ≈ 0.14), a further increase in the water content resulted in an increase of the surface tension. This trend is pointed out in Figure 3 where surface tension is reported as a function of log[H2O] or Xwater; the shape of the plot resembles the cmc diagrams reverting the position of the plateau (the plateau is here in the low concentra-
Peculiar Properties of Water as Solute
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Figure 2. Surface tension of the propylene carbonate-water system as a function of water molar fraction (O). Full line shows the solubility gap with the corresponding temperature values on the y-axis on the right. The diagram on the right is the magnification of the water-rich region.
Figure 4. Surface tension versus water molar fraction of (b) DMSO, (0) MP, and (×) PC. Figure 3. Surface tension values as a function of log[H2O] or water molar fraction in the region where water acts as a solute.
tion region, whereas the contrary occurs in cmc plots). From the surface tension data, it was possible to deduce that the first water molecules added to PC did not affect the air/water interface structure, keeping the surface tension identical to that of pure PC. In this region (plateau), water molecules are completely solvated by PC molecules in the bulk phase without any interactions at the air/water interface. When the molecular ratio water/PC becomes approximately 1:6 (log[H2O] ) 0.242, i.e., Xwater ≈ 0.14), water starts to restructure the interface and the slope of the surface tension versus log[H2O] begins to have a value different from zero. At the highest concentration before the miscibility gap (Xwater ) 0.33, or log[H2O] ) 0.719), the molecular ratio water/PC is ca. 1:2, and we can state that the region of the plot in Figure 3 where surface tension varies as a function of water concentration represents the transition between water as a pure solute to water as a cosolvent. When water behaves as a pure solute, being completely solvated in bulk, it does not change the surface properties of the solvent (PC). When water starts to behave as a cosolvent, the water-air interface begins to be more structured due to the presence at the surface of both water-PC and water-water interactions, and the surface tension increases. Concerning the right side of the diagram, the trend of the curve was of clearer interpretation. The surface tension of water is 72 mN/m at 25 °C; PC, having a lower surface tension (almost one-half that of water), produces a strong decrease of the surface tension versus concentration. The addition of 19.7 wt % of PC to pure water (Xwater ) 0.958) causes a drop in the surface tension of the mixture up to ∼44 mN/m; the decrease of surface tension is the consequence of
the enrichment of the component with lower surface tension in the Gibbs surface region. Despite its simple interpretation, even this trend shows some particularities: comparing the slope of this part of the curve (see Figure 4) to those obtained for other systems (where the nonaqueous component is dimethylsulfoxide (DMSO) or 1methyl-pyrrolidinone (MP)), both having similar dipolar moments and surface tensions with respect to PC and also quite high dielectric constants, it is easy to note that they are characterized by different slopes. In particular, PC seems to destructure the water-air interface, producing a sharp decrease of surface tension much stronger than the other two solvents. This finding suggests a peculiar interaction between PC and water molecules that reflects the behavior already found in the PC-rich part of the phase diagram. It is interesting to compare these surface tension and phase diagram results with some old studies by Corti et al.47,48 and Kahlweit et al.49 on mixtures with nonionic surfactants (CiEj, where 4 e i e 12 and 4 e j e 8). A self-evident and clear difference is that the water-rich region did not show any features typical of PC micellar formation as in the nonionic surfactants, well explainable with the too poor hydrophobicity of PC. In the water-poor portion of the diagram, neutron scattering experiments showed50 that, for example, in C12E8 aqueous solutions at high volume fraction of the amphiphile, water originated in clusters confined in the bulk amphiphile. In our case, we observe a break in the surface tension versus concentration curves that could be associated with the formation of similar clusters of water in bulk PC. 3.2. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry has been frequently used to study confined
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Figure 5. FWI deduced from DSC measurements as a function of water concentration for the various mixtures examined.
water.51,52 For our purposes, we carried out DSC measurements to determine possible re- or destructuring of water following the amount of the so-called freezable water, that is, water able to melt and solidify reversibly. PC-H2O mixtures in various molar ratios of water were investigated by DSC, to understand the behavior of water as a function of temperature. From the calorimetric behavior of the solid / liquid transitions, it is possible to classify water into three different species: freefreezable and bound-freezable,53 and water that does not freeze, which is called nonfreezable water.53 Free-freezable water is not significantly different from bulk water and may be called “free water” (we will adopt this one). The second type involves water that still succeeds in freezing and melting but at different temperatures with respect to bulk water, being more or less bound and/or more or less confined. The third one exhibits too large differences from bulk water and may be called “strongly bound water” not able to freeze and melt. From the melting curves, we calculated the enthalpy of fusion, and by comparing this value with that in the literature (∆Htheor ) 336.3 J/g), it was possible to determine the free water index (FWI ) [∆Hexp (J/g)/∆Htheor] × 100),54 that is, the amount of freezable water for each of the examined mixtures. Figure 5 shows that the FWI values group in two clusters, the first involving 0.2 e FWI e 0.35, and the second with FWI around 0.5. The interesting finding was that the abrupt variation of FWI as a function of water content is approximately at the same concentration as found in surface tension behavior (see Figure 3). The change of the water properties from pure solute to cosolvent results in a net increase of the amount of freezable water, confirming that when Xwater is around 0.14 water molecules start to interact among themselves, originating into network structures (see next paragraph). Despite that the method of studying frozen solvents in mixed systems has been already developed also in the presence of PC,55 with our approach we succeeded in detecting the melting of water in supercooled samples without any problems associated with the crystallization/melting of PC or PC glass formation.55 The supercooling of liquid water is a well-known phenomenon in the literature56 and may be attributed to the decrease of the free water amount and/or to confinement. Taking into account this datum, we plotted the temperature of solidification (tsolid) (Figure 6) as a function of the water amount in the mixtures, and the result was extraordinarily in agreement with the findings reported in Figures 3 and 5. Again, we have two clusters of values, the first in the interval -70 °C e tsolid e -50 °C and the second one -30 °C e tsolid e -20 °C, and the abrupt change from one cluster to the other was at Xwater ≈ 0.2.
Dei and Grassi
Figure 6. Temperature of solidification in °C as a function of water amount for the various mixtures examined.
It was interesting to notice that these values confirmed that below Xwater ≈ 0.2, water is mainly highly bound (not freezable), and the freezable water is highly confined in multimeric species such as monomers or dimers (see below). Above Xwater ≈ 0.2, the amount of freezable water reaches about 50% and the poor supercooling effect (pure water has a supercooling of 15-20 °C) infers that water molecules that freeze are highly interconnected in network structures (see below), leading to a situation similar to that of bulk water. A comparison of our DSC results with some investigations in similar systems evidenced an interesting feature concerning the above-discussed break. In fact, Piekarski et al.28 find some singular points (minima) in the behavior of the enthalpy of solution of NaCl, NaI, and urea in water-1,2-dimethoxyethane mixtures as a function of the water molar fraction. Interestingly, these minima are at Xwater around 0.1-0.2, that is, at almost the same values found in the present study. The attribution of such a trend to the change of the mixed solvent structure57,58 seems perfectly adherent also to our water/PC system without any dissolved salts. Indeed, to have more information about such changes in the mixed solvent structure and to establish whether the PC behaves as a conventional hydrotrope, a good research strategy could be to carry out measurements of the solubilization of a dye in the mixtures as recently set up by Bauduin et al.,59 who succeeded in developing a unified concept dealing with hydrotropes and cosolvents. 3.3. Infrared Spectroscopy. The infrared spectrum of liquid water in the range 200-4000 cm-1 consists of four main bands,60 but the dominating feature of the IR water spectrum is the band relative to the OH stretching that falls in the 30003800 cm-1 region with the presence of various subbands, shifted with respect to the fundamental OH mode, originating a broad spectral distribution, unambiguously indicating the presence of different structural environments. The evolution of the water/PC FTIR spectrum upon changing water content in the O-H stretching region is depicted in Figure 7. It can be seen from Figure 7 that the intensity of the absorbance increases as water content increases, as is predicted from Beer’s law. To quantify the changes in the O-H stretching region arising from increasing the water amount in the mixtures, we fit each spectrum as a sum of two or three Gaussian curves according to similar previous studies.11,12,60,61 This deconvolution was carried out according to a computer program for multiple
Peculiar Properties of Water as Solute
J. Phys. Chem. B, Vol. 110, No. 24, 2006 12195 different kinds of water in the various systems. As an example, the absence of the peak at lower energies for the 0.08 water molar fraction sample showed that the network state (NW) water was not present for a very small amount of the solute (water) in the PC (solvent). As water is represented by three different states in the systems (NW, IW, MW), it is reasonable to assume that the total peak area corresponding to the water band is the sum of the peak areas of the different states of water.62 Therefore, labeling the area of the peak at ∼3300 cm-1 as A1, that at ∼3450 cm-1 as A2, and that at ∼3570 cm-1 as A3, then the total peak area is given by the expression:
Atotal ) A1 + A2 + A3
Figure 7. Infrared spectra (3000-4000 cm-1 region) of water in water/ PC mixtures at Xwater ) 0.29, 0.22, 0.13, 0.08, and 0.05.
Gaussian curve analysis. Figure 8 illustrates the two- or threecomponent fits with the 0.08 and 0.22 molar fraction sample. The different Gaussian contributions are assumed to account for different water “populations” associated with a particular type of hydrogen bond. The low-energy Gaussian (centered at ca. 3300 cm-1) represents the so-called “network water” (NW) molecules that are most likely connected tetrahedrally, and it is assumed to be originated from linear, fully developed hydrogen bonds that break and form continuously.11,12,60,61 The medium energy Gaussian (centered at ca. 3450 cm-1) stands for the “intermediate water” (IW): these molecules are somewhat connected to other water molecules, although unable to develop fully connected patches, and that, having distorted H-bonds, may be arranged in short-lived aggregates. In other words, this component of water has an average degree of connection larger than that of dimers or trimers, but lower than those participating to the percolating networks. The highest energy Gaussian (centered at ca. 3570 cm-1) is ascribed to water molecules called “multimer water” (MW), being poorly connected to their environment and standing as free monomers, or as dimers, or trimers. This latter assignment is supported by the fact that, frequency-wise, these MW molecules are close to those found in the vapor phase, just as the NW Gaussian is positioned at a frequency close to that of the OH absorption in ice. Looking at Figure 8, it was possible to understand that the deconvolution of the broad band resulted in the presence of the
At the same time, we can calculate the fractions of network water, intermediate water, and multimer water weighed on the total amount of water, and we can report the variation of the ratio between the area of each Gaussian component (Ai) to the total peak area (Atotal) as a function of Xwater (water molar fraction). The observed behavior could be interpreted in terms of distinct types of water molecules, whose relative concentrations change with Xwater. It was very interesting to notice that the graph reported in Figure 9 is very similar to that found for water confined in
Figure 9. Ratio of the area of the ith Gaussian component (Ai) to the total peak area (Atotal) versus the water molar ratio (XH2O). The different symbols represent: (O) component centered (MW) at ∼3570 cm-1, (0) component centered (IW) at ∼3450 cm-1, and (]) component centered (NW) at ∼3300 cm-1.
reverse micelles.12,56 In particular, when the amount of water is very small and it behaves as a true solute solvated by PC,
Figure 8. FTIR spectra of water/PC mixtures: (a) Xwater ) 0.08, (b) Xwater ) 0.22. Dotted line, experimental spectrum; dashed line, Gaussian components to evaluate the theoretical spectrum; full line, theoretical spectrum.
12196 J. Phys. Chem. B, Vol. 110, No. 24, 2006 there is a high degree of MW water (almost 100% with very high degree of confinement). Increasing the water amount resulted in a strong decrease of the MW component with the corresponding increase of the other two components, the network and intermediate water. The behavior reported in Figure 9 confirms the previous findings by surface and DSC measurements, indicating that at Xwater ≈ 0.20 there is a break in the properties of the system, reminding us of the concept of critical micellar concentration. Interestingly, the water/PC system at Xwater ≈ 0.3 (i.e., close to the miscibility gap limit) has the same water population composition in terms of Gaussian contributions as AOT reverse micellar aggregates at W g 7:12 maybe are there water micelles in PC solvent close to the miscibility gap? This is a question worth answering in future studies. In conclusion, all three experimental techniques evidenced a break in the solvation mechanism between water and PC: what can be deduced at present is that when Xwater ≈ 0.15-0.20, there is a drastic passage from a preferential solvation of water by PC molecules to a preferential interaction among water molecules leading to the formation of water clusters. This effect is particularly interesting in relationship to some studies on analogous mixed systems63 (acetonitrile/PC63 and dioxane/ water64) where the observed behavior about reciprocal solvation was monotone without any sharp break. Conclusions The present study evidenced that water acting as a solute presents very interesting and peculiar properties. The investigation on the binary mixtures water/PC showed that in the dependence of the water amount the behavior of both macroscopic (surface and calorimetric) and molecular structures (FTIR) was very peculiar: in particular, there is a discontinuity in the various plots at around Xwater ≈ 0.14, suggesting that at this concentration there is an abrupt change of the interaction among water molecules and between water and PC molecules. A small amount of water added to PC leads to the formation of monomers, dimers fully solvated by PC, indicating that water acts as a true solute. FWI, temperatures of solidification, surface tension, and FTIR spectra converge to a picture where water molecules interact among themselves at maximum with formation of trimers, with a high degree of solvation by PC molecules. The change from water as a solute to water as a cosolvent is not gradual as expected, but is characterized by a discontinuity in the observed properties in a way resembling the behavior of surfactants in aqueous solution, that is, critical micellar concentration. In fact, the same properties above-described are subjected to a drastic variation in correspondence of Xwater ≈ 0.2, that is, when the molecular ratio between water and PC is ca. 1:6. FTIR spectra enabled us to classify the water structure and to distinguish among “network water”, “intermediate water”, and “multimer water”, and the role of these different contributions in the various mixtures matched the observed macroscopic (surface and calorimetric) properties. Our work can be concluded with a question that could be the start for future research: is Xwater ≈ 0.15-0.20 a critical concentration for the transition between almost single molecules to supramolecular aggregates? In other words, may we envisage water micelles formation above a certain water concentration? Further studies should clear this important question. Acknowledgment. Thanks are due to the Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI) for financial support.
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