Local Hydration Pressures in Methanol Aqueous Solution: A Raman

Article pubs.acs.org/JPCB

Local Hydration Pressures in Methanol Aqueous Solution: A Raman Spectroscopy Analysis Nubia Judith Mendoza,† Laura Jiménez Bonales,†,‡ Valentín García Baonza,† and Mercedes Cáceres*,† †

MALTA-Consolider Team and QUIMAPRES, Departamento de Química Física I, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain ‡ CIEMAT, Departamento de Energía, Unidad de Residuos de Alta Actividad, Av. Complutense, 40, 28040 Madrid, Spain ABSTRACT: Raman spectra of methanol−water mixtures were measured over the whole composition range at room conditions. The spectra are used to quantify the strength of intermolecular interactions in terms of local internal pressures. The conclusions derived from the spectroscopic analysis are discussed within the framework of the solvation pressure model using values of the cohesion energy density expected in the mixture. This work demonstrates that an appropriate analysis of Raman spectroscopy experiments can be used to quantify the local internal pressures due to intermolecular interactions in liquid mixtures, provided that high pressure results of the pure liquids are available.

1. INTRODUCTION Research on liquid mixtures and solutions experienced a tremendous increase between the 1970 and 1990 decades1 with the development of equations of state models based on Statistical Thermodynamics2 and with the advancement of computer simulation methods.3 This allowed the development of theoretical models4 to analyze the large amount of experimental data for thermodynamic properties of pure liquids and mixtures that was available as a function temperature, pressure, and composition. Of fundamental importance were the rapid advances occurring in the understanding of aqueous solutions, given their relevance in biological processes5 and their countless technological applications. Aqueous solutions of short chain alcohols have historically been considered as models to study and to clarify hydrogenbonding phenomena, covering areas of interest in physics, chemistry, biology, and in many technological applications.6 Among the most important discoveries in the study of aqueous solutions was that water and methanol do not mix at the molecular level,7 as revealed by the analysis of the oxygen− oxygen distribution function deduced from neutron diffraction experiments in the 30% (v/v) methanol−water mixture. This study showed that, although the two liquids are miscible in all proportions, their mixtures are not complete at the molecular level due to molecular clustering, thus explaining the abnormal measure for their entropy of mixing. Such discovery triggered an increasing number of studies8−11 devoted to analyzing the structural characteristics of alcoholic aqueous solutions at the molecular level. Both liquid water and methanol are themselves far from being simple liquids at the molecular level, and this complexity is increased with the application of external pressures. For instance, despite the efforts devoted to understand the structure and dynamics of liquid water, novel phenomena, like the lowdensity-water to high-density-water structural transition,12 are still under exploration. The pressure behavior of methanol, © 2014 American Chemical Society

being the simplest alcohol, has demonstrated to be also quite complex, showing different equilibria between the hydrogenbonded clusters under different pressure regimes.13,14 It is therefore not surprising that the structure of their aqueous mixtures is even more complex and quantifying the strength of such intermolecular interactions is still a difficult issue.11 Soper and Finney15 studied water-rich mixtures and concluded that the structure of water remains quite similar to that of the pure liquid (i.e. a hydration shell composed by tetrahedral-like water clusters surrounding the methanol molecules), whereas the addition of a small amount of water to neat methanol causes a decrease of the C−H bond and an elongation of the O−H bond in the methanol molecules/ clusters. Such bonding changes should reflect an effective local pressure (or stress) that can be quantified with an appropriate model, and this is the aim of the present study. In the past, molecular interactions have been quantified in terms of internal pressure using the cohesion energy density (CED) concept, which was introduced more than 60 years ago by Hildebrand et al.,16 and it is defined as CED = δ 2 =

ΔUvap V

L

≈

ΔH vap − RT VL

(1)

where δ is the Hildebrand solubility parameter, ΔUvap and VL are the vaporization energy and the molar volume of the liquid, respectively, and T is the absolute temperature. Since the CED has pressure units, it can be compared to the internal pressure (Pint) in the pure liquid, ⎛ ∂U ⎞ PInt = −⎜ L ⎟ ≈ CED ⎝ ∂V ⎠T

(2)

Received: May 5, 2014 Revised: July 12, 2014 Published: July 31, 2014 9919

dx.doi.org/10.1021/jp5043975 | J. Phys. Chem. B 2014, 118, 9919−9925

The Journal of Physical Chemistry B

Article

Figure 1. Raman spectra at room conditions for selected methanol aqueous solutions at different water compositions expressed in molar fraction (xH2O). F.R. stands for Fermi resonance bands (see text).

pressures if high-pressure spectroscopy experiments are available for the pure liquids. Within the framework of aqueous solutions, vibrational frequencies are used to evaluate both hydrophobic and hydrophilic effects in terms of either positive or negative pressures, respectively. In this work we analyze the effect of water addition on methanol over the whole composition range by using Raman spectroscopy. We show that a detailed analysis of the Raman spectrum leads to a precise description of the changes in the local structure of the methanol molecules. Using existing highpressure experiments in pure methanol,29 we are able to analyze and quantify the strength of the intermolecular interactions (both hydrophobic and hydrophilic) in this system related to the solvation effects in terms of local internal pressures. These results are used for discussing the validity and the limits of applicability of the SPM through calculation of the cohesion energy density of the mixture, CEDmix.

leading to the so-called solvation pressure model (SPM), which was introduced by Wood et al.17−19 to analyze the interactions of single-wall carbon nanotubes (SWCNT) dispersed/immersed in different liquids. These authors first correlated the Raman shift of the D* band of SWCNT with the CED of the liquids, and then they measured the Raman shift of the same band under hydrostatic pressures using a diamond anvil cell for comparison. The good correlation obtained between the Raman shifts obtained in both immersion and high-pressure experiments lead these authors to conclude that the CED acts like a real hydrostatic solvation pressure. Later, Dixit et al.20 highlighted the possibility of solvation pressure effects in the ethanol/water system, and van Uden et al.21 studied ethanol in both water and chloroform mixtures, introducing the mixing cohesion energy density, CEDmix, which was defined as CEDmix =

v1LCED1 + v2LCED2 v1L + v2L

(3)

where vL indicates the volume, CED is the cohesion energy density, and 1 and 2 refer to the components in a binary mixture, so the solvation pressure in the mixture is defined as Psol,2 =

v1L v1L

+ v2L

(CED1 − CED2 )

2. MATERIALS AND METHODS Liquid methanol (pro analysi, >99%) purchased from SigmaAldrich was used without further purification. Milli-Q water used in this work has a TOC lower than 5−10 ppb and resistivity higher than 18 mΩ·cm−1. The solutions were prepared by weight in an analytical balance and housed in a glass tray. Raman spectra were measured in a confocal Raman spectrograph (Voyage, BWTek) using the 532 nm line of a solid state laser for excitation. The laser beam was focused through a microscope (Olympus BX5), with a ×10 longworking distance objective and the scattered light was analyzed on an air-cooled CCD (Hamamatsu S10141-1107S, 2048(h) × 122(v) pixels). The spectral resolution is 4−5 cm−1, and exposure times needed to maximize the signal/noise ratio in all the spectral features ranged between 20 and 100 s.

(4)

According to eq 4, the relative values of CED1 and CED2 determine that the solvation pressure can be either positive or negative in a given component of the mixture. These studies confirm the long-known advantages of using vibrational spectroscopy to probe intermolecular interactions and structural information in water and methanol solutions.22−28 The changes observed in the frequency, ν, of a given stretching vibration are directly related to the strengthening/weakening of those bonds involved in the intermolecular interactions in water solution. This strengthening or weakening of the bond can be quantified in terms of local 9920



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3. RESULTS AND DISCUSSION The evolution of the Raman spectrum at room conditions of the water−methanol mixture at selected compositions is shown in Figure 1. For the sake of comparison, the spectra of the pure liquids have been also included at the top and the bottom of the figure. It is evident that as the composition varies some Raman bands of methanol change their characteristic shapes and frequencies. In the following discussion we shall use the band assignment for pure methanol, given in refs 29 and 30 for the CH stretching region, and 13 for the OH region. Let us start with a qualitative analysis of the main Raman bands of methanol. The bands involved in the Fermi-resonance (FR) of methanol, the asymmetric methyl-stretch CH3 (as) and the out-of-plane CH3 antisymmetric stretching vibration: (FR−, 2835 cm−1; FR+ + out-CH3−As, 2945 cm−1; and CH3(as), 2988 cm−1 in the pure liquid) show substantial changes. All increase their frequency as xH2O increases and the relative intensities of the Fermi doublet changes dramatically, which indicates the large effect that hydrophobic interactions have in the molecular force field of methanol. The CO-stretch band, which appears at 1036 cm−1 in pure liquid methanol and gradually shifts to lower frequencies as xH2O increases, now reflects the strong effect of hydrogen-bonding on the COH group. The bands corresponding to OCH-deformations (ca. 1107 and 1154 cm−1) and both symmetric and asymmetric CH-bending (ca. 1448 and 1462 cm−1, respectively) show little frequency changes, although slight changes are observed in their relative intensities. The most interesting conclusion from this preliminary analysis is that the overall changes observed in the Raman spectrum of methanol in aqueous solutions are similar to those observed under moderate compression (below 0.7 GPa) in pure liquid methanol.13,29 Such changes clearly reflect the effect on the hydrogen bonding interactions in the mixture, and deserve further analysis in order to express the frequency changes observed in terms of local pressures/stress exerted by the surrounding water molecules on the bonds of individual methanol molecules. A reliable quantitative analysis of the frequencies requires computing the second derivative of the measured Raman spectra, instead of using the standard band-shape analysis of using several bands to fit the spectrum. Thus, the frequency of a given band corresponds to a minimum in the second derivative function regardless of the band-shape supposed for each spectral feature. We have analyzed the Raman spectrum of 20 methanol−water mixtures covering the whole composition range as a function of xH2O. In Figure 2 we represent the results of this analysis for the main bands involved in the CH stretch region (CH3,(as), and the Fermi doublet); for the sake of comparison, we include in Figure 2 the frequencies reported by Dixit et al.20 for the FR− band, which show excellent agreement with our data. The shifts of the three Raman bands considered in Figure 2 show similar variations as a function of xH2O. A global analysis of both Raman shifts and band-shapes suggests the existence of three different regions (broken lines in Figure 2) as a function of composition, in close agreement with previous results published by Dixit et al.20 (solid symbols in Figure 2) and by Ebukuro et al.31 In the methanol-rich (xH2O < 0.2) and in the water-rich (xH2O > 0.8) regions the changes are much less pronounced than in the intermediate region (0.2 < xH2O < 0.8),

Figure 2. Raman shifts (left axis) of the Fermi doublet (FR− and FR+) and the asymmetric methyl-stretch (CH3(as)) of methanol (open symbols) and calculated local pressures Pi (right axis, see text for explanation) as a function of xH2O. Solid symbols are results reproduced from Dixit et al.20 Broken lines are guides to the eyes.

where the Fermi doublet and the CH3(as) bands upshift in about 10 and 13 cm−1, respectively. Ebukuro et al.31 found a nearly constant Raman shift for the CH-stretch for xH2O < 0.21, pointing out the possibility that the local structure of methanol remains very similar to that of pure methanol. In the intermediate region the blue-shifts can be interpreted as an increase in the CH bond strength (and a decrease in the CH bond length), thus revealing the noticeable hydrophobic effect of water over the methyl group. The notion that such a compressive force could be assimilated to an internal pressure has been previously discussed by Kamogaka and Kitagawa.32 These authors conducted a study to discern whether the Raman shift variation was due to the secondary effect of the interaction of the hydroxyl groups or to the direct intermolecular interactions, concluding that the latter were responsible for the decrease of the CH bond length. However, to our knowledge, no attempts to quantify such hydrophobic interaction have been put forward to date. To quantify this effect, we recall existing experiments in pure liquid methanol under hydrostatic conditions using a sapphire anvil cell at room temperature.29 In these experiments we analyzed the Raman shifts of the methyl-stretch related bands as a function of hydrostatic pressure (PH). From these results, we have calculated the local internal pressures (Pi) indicated in the right axis of Figure 2, obtaining a pressure increase ranging between 1.0 and 1.2 GPa across the whole composition range. Figure 3 shows the averaged values for Pi(CH3) where the three composition regions discussed above can be again approx9921



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Figure 5. Raman shift of the CO-stretch band of methanol as a function of the molar fraction of water, xH2O. Solid symbols are results reproduced from Dixit et al.20 Broken lines are guides to the eyes.

Figure 3. Mean local internal pressures over the methyl group calculated from the data shown in Figure 2 as a function of xH2O. Error bars correspond to deviations between the three values calculated from the Raman shifts of the Fermi doublet (FR− and FR+) and the asymmetric methyl-stretch (CH3(as)) of methanol.

again distinguish three different behaviors on this band in concentration ranges almost coincidental with those found in Figure 2. Again, for the water-rich and methanol-rich regions the shift is less pronounced than in the intermediate region, but now a strong red-shift in the frequency is found, which must be interpreted as a weakening in the CO bond (i.e., bond-length increase) with increasing xH2O, in clear contrast to the behavior found for the methyl group. A good agreement is found with the previous results of ref 20, except in the methanol-rich region, very likely due to the different numerical treatment of the signal in this complex feature of the Raman spectrum of methanol.13 The changes observed in the CO-stretch region reveal a strong hydrogen-bonding interaction between methanol and water molecules. This behavior of methanol in aqueous solution is in clear contrast with that observed in pure liquid methanol under hydrostatic pressure.13,33,34 Although the maximum of the CO-stretch band shows a shallow downshift at moderate pressures (ca. 0.5 GPa) it has been ascribed to rearrangement of the methanol clusters,13 instead of hydrogenbonding effects among methanol molecules in the cluster; the CO-stretch, in fact, does not shift with pressure for the different methanol clusters in the liquid. Only under further compression is when the CO-stretch band in liquid methanol shows the expected blue-shift at hydrostatic pressures from 0.7 GPa up to several GPa. With this in mind, the constancy in the Raman shift of the CO-stretch observed in the methanol−water mixtures at low values of xH2O suggests that the effect of solvation pressures mainly leads to a reorganization of the methanol clusters. However, at higher water contents, the decrease in the COstretch frequency is a consequence of the attractive effect caused by hydration water on the COH terminal group of methanol, very likely inducing a gradual disruption and compression of the existing methanol molecules/aggregates. To quantify this attractive interaction, we have calculated the local (tensile) pressure from the CO Raman shifts of Figure 5 using the linear slope of 3.34 cm−1·GPa−1 found above 0.7 GPa in pure liquid methanol under hydrostatic pressure.13 The results are plotted in Figure 6, where it can be noticed that the addition of water leads to tensile pressures on the CO bond up to 4.5 GPa in the limit of xH2O = 1. It should be noted that this is an effective pressure, and it should be only considered like a relative measure of the attractive (hydrophilic) interaction on

imately distinguished. The maximum value of Pi(CH3), roughly 1.2 GPa, corresponds to the compressive (hydrophobic) pressure felt by the methyl group of an isolated single methanol surrounded by water molecules, somewhat short of a solvation pressure value at infinite dilution. The precedent analysis allows us to discuss the validity of the SPM (eq 4) in a quantitative fashion. The value of the solvation pressure in the mixture, psol, might be assimilated to a local internal pressure applied on the molecules of the solute due to solvation/hydration effects, plus an eventual external hydrostatic pressure. In the present study, since the experiments have been performed at room pressure, we can approximate psol ≈ Pi. This is confirmed in Figure 4, where a nice correlation between

Figure 4. Psol vs internal pressure obtained at each studied concentration of the methanol−water mixture. The line corresponds to the Psol = Pi(CH3) condition.

both quantities is found for all the concentrations studied, except a slight disagreement in the water-rich region. This result demonstrates that the hydrophobic effect acts like a real external hydrostatic pressure applied to the methyl group of methanol. However, as it was expected, hydration effects in the COH group of methanol are opposite to those found in the methyl group, as confirmed in Figure 5, where the Raman shifts of the CO-stretch show a marked decrease as xH2O increases, confirming previous observations.13,20,23,32 Interestingly, we 9922



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Figure 6. Internal pressure, Pi, as a function of water molar fraction, xH2O. Figure 7. Second derivative of the Raman spectrum of methanol, methanol−water mixture (at xH2O = 0.5), and water.

the COH group as a whole, in comparison with the repulsive (hydrophobic) interaction on the methyl group. In any case, such comparison highlights the huge attractive effect that hydration causes on the CO bond of methanol. Thus, the changes in the CO bond strength, as measured by an straightforward Raman experiment, provides an easy way to measure the relative strength of the hydrogen-bond interactions in alcohol−water and related mixtures. Although this idea was early pointed out by Kecki,22 who concluded that the downshift in the CO-stretch frequency originates from hydrogen bonding between the oxygen of the COH group (acceptor) and the hydrogen of water, here we show that comparison with high pressure experiments allows quantifying this attractive interaction that, according to our present analysis, is four times stronger than the repulsion on the CH3 methyl group of methanol. In any case, the precedent results and discussion indicate that the SPM lacks the desired generality; although we find an excellent correlation for the local pressures applying on the methyl group, the model is unable to predict the weakening of the CO bond. The straightforward explanation is that the psol calculated from eq 4 averages the local pressures applied by the surrounding molecules of the solvent on a given molecule in the solution, and such average local pressure does not necessarily reflect the local intermolecular solvent−solute interactions. However, we believe that the equivalence found between the local pressures calculated from the SPM and those derived from the combined Raman/high pressure experiments is not fortuitous in the present case. The neutron diffraction results of ref 7 reveal that that this mixture could be understood as a fluid of close-packed methyl groups with water clusters bridging the methanol OH groups through hydrogen bonding. It is therefore difficult, if not impossible, to distinguish between the OH groups within the complex hydrogen-bonding structure in the mixture. In other words, methanol−water solutions can be classified into a complex mixture of OH and CH3 groups, so the only relevant differences between pure liquid water and the mixture is just the hydrophobic interaction with the CH3 group. Unfortunately, the analysis of the OH stretch region at 3200−3700 cm−1 is somewhat complex.35 As an example, Figure 7 shows the second derivate analysis spectra of pure methanol, pure water, and methanol/water mixture (xH2O = 0.5) to highlight the complex combination and overlapping of

the different Raman features which correspond to different kinds/strengths of OHs. In Figure 7 we use the assignment given in ref 13 for the different Raman features of pure liquid methanol, that is, the band at ∼3650 cm−1 is assigned to the OH of free monomer methanol (free), while the OH stretch in clusters appear at lower frequencies (dimers at ∼3532 cm−1 (n = 1), trimers at ∼3445 cm−1 (n = 2), tetramers at ∼3338 cm−1 (n = 3), and higher aggregates at ∼3338 cm−1 (n ≥ 4)). For water, the names H2O-3 and H2O-2 refer to multiple and single hydrogen-bonded molecules, while H2O-1 stands for nonbonded water molecules.36,37 It is possible to differentiate four peaks in the analysis corresponding to the mixture. The peak centered around 3250 cm−1 corresponds to the overlap of n = 4 of the methanol molecules and the H2O-3 band of water. The feature around 3450 cm−1 corresponds to a combination band of n = 3, n = 2, and H2O-2, and that centered around 3530 cm−1 corresponds to the n = 1 (for xH2O above 0.3 it is not possible to calculate its position with accuracy). As expected, the feature centered around 3660 cm−1 is a combination of the free OH in both methanol and water. To have a general view of the behavior of the Raman shift in the OH stretch region at 3200−3700 cm−1, we show in Figure 8, as an example, the variation of the Raman shift corresponding to the vibrational feature centered around 3250 cm−1. A clear redshift from 3270 to 3220 cm−1 is observed as xH2O increases, which can be interpreted as a weakening of the OH bond in methanol tetramers and the OH bond involved in multiple hydrogen bonds in water. In Figure 9 we compare the Raman shift taken from Figure 8 vs psol calculated with the SPM model (eq 4) and the Raman shift of the methanol tetramers (n = 4) of pure methanol vs hydrostatic pressure reproduced from 29. It can be appreciated that both data follow the same trend and shift to lower frequencies. This behavior can be interpreted as an elongation of the OH bond of the large aggregates of methanol and water due to an increase in the intermolecular interactions through hydrogen bonding between these molecules, which probably causes the rupture of these aggregates into smaller ones. Summarizing, the behavior exhibited by the different Raman features analyzed in previous sections can be interpreted as 9923



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Figure 10. Scheme of the effect provoked by water addition in the different bonds of the methanol molecule: (A) methanol-rich mixtures, xH2O 0.8.

diffraction data of mixtures of xH2O = 0.36 and xH2O = 0.915 and the molecular dynamics simulation.11 Figure 8. Variation of the vibrational feature centered around 3250 cm−1 as a function of water molar fraction, xH2O.

4. CONCLUSIONS We have shown in this work how the shift of the Raman frequency can be used to analyze the structure of the methanol molecules in methanol−water mixtures and how it can be used as a local probe to measure intermolecular interactions. We highlight that solvation effects due to the hydration of methanol molecules act as a hydrostatic pressure (repulsion effect) in the methyl group, which decreases the C−H bond in a similar way to the changes occurring in pure methanol due to a compression at high pressure. The analysis of the CO stretch shows an increase in the CO bond due to the addition of water (attraction effect), which has been related to hydrogen bonding through the oxygen of the hydroxyl group. The analysis of the intermolecular interactions has been obtained by comparison with high pressure experiments of pure methanol published elsewhere,29,13 and with the analysis of the cohesion energy density of the mixture, psol. This comparison shows that water hydration causes a stronger attractive effect on the CO bond length (ca. 4 GPa) than the repulsive effect (ca. 1.2 GPa) on the terminal methyl group.

Figure 9. Pressure dependence of OH Raman feature centered around 3270 cm−1. The solid data are reproduced from Arencibia et al.13

■

(1) For xH2O < 0.22 water does not cause significant changes in the bond distances of CH or CO in the methanol molecules, which indicates that methanol molecules in mixtures at low water content show aggregation properties and conformations similar to those of pure liquid methanol. (2) For 0.22 < xH2O < 0.80 the methyl group shows a continuous decrease of the CH bond length; on the contrary, the CO bond length shows a continuous increase, as it occurs with the O−H bond of methanol, due to the great hydrophilic effect of this group caused by the strengthening in the hydrogen bond between the different molecules. (3) For xH2O > 0.8 the CO bond distances seem to be unaffected (Raman shift is almost constant in this range). Figure 10 sketches the ideas expressed above in a simple molecular model, in which the hydrophilic and hydrophobic effects on a methanol molecule for the different mixtures are shown: (A) methanol-rich mixtures, xH2O < 0.2−0.3, (B) intermediate mixtures 0.2 < xH2O < 0.8, and (C) water-rich mixtures, xH2O > 0.8. It should be noted that this interpretation is in agreement not only with the previous Raman data, but also with the neutron

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 913944206. Fax: +34 913944135. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Comunidad de Madrid and the EU through QUIMAPRES program (S2009/PPQ-1551) and by MICINN/MINECO through MALTA-Consolider (CSD2007-00045), CTQ2009-14596-C02-01 and CTQ201238599-C02-02 projects.

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