Temperature-Dependent Hydrophobic Crossover Length Scale and

Feb 8, 2018 - Experimental Raman multivariate curve resolution and molecular dynamics simulations are performed to demonstrate that the vibrational ...
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Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 1012−1017

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Temperature-Dependent Hydrophobic Crossover Length Scale and Water Tetrahedral Order Xiangen Wu,† Wanjun Lu,† Louis M. Streacker,‡ Henry S. Ashbaugh,¶ and Dor Ben-Amotz*,‡ †

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ¶ Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, United States ‡

S Supporting Information *

ABSTRACT: Experimental Raman multivariate curve resolution and molecular dynamics simulations are performed to demonstrate that the vibrational frequency and tetrahedrality of water molecules in the hydration-shells of short-chain alcohols differ from those of pure water and undergo a crossover above 100 °C (at 30 MPa) to a structure that is less tetrahedral than pure water. Our results demonstrate that the associated crossover length scale decreases with increasing temperature, suggesting that there is a fundamental connection between the spectroscopically observed crossover and that predicted to take place around idealized purely repulsive solutes dissolved in water, although the water structure changes in the hydration-shells of alcohols are far smaller than those associated with an idealized “dewetting” transition.

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case for the predicted hydration entropy crossover of idealized hydrophobic solutes.2,8,10−12 This agreement suggests a fundamental connection between the two crossovers, although the observed water structure changes are far smaller than those associated with an idealized dewetting transition. Additional implications of the results are discussed, including crossoverinduced hydration-shell density changes and the influence of the alcohol OH headgroup. Figure 1a shows the Raman spectra of pure water (dashed blue curve) and a 5 wt % (∼1 M) solution of ethanol in water (solid blue curve), both measured at 20 °C and 30 MPa. The inset diagram in Figure 1a shows a schematic of the highpressure spectroscopic cell used to obtain these spectra, and the inset graph in Figure 1a shows the temperature and density range over which the Raman measurements were performed. The experimental densities23 are in excellent agreement with our MD simulations, as shown in Table S1 of the Supporting Information, which also contains further details regarding our experimental and simulation methods. Figure 1b shows the Raman-MCR spectra of pure water (dashed curves) and the solute-correlated (SC) spectra obtained from a 5 wt % solution of ethanol in water (solid curves) over a temperature range of 0−374 °C at 30 MPa. Note a SC spectrum is essentially equivalent to the smallest positive difference spectrum obtained when subtracting the spectrum of pure water from the spectrum of an aqueous solution.17,25 Thus, SC spectra reveal vibrational features arising from the solute itself as well as water molecules whose vibrational spectrum is perturbed by the solute. Previous studies have demonstrated that the SC spectra of alcohols result primarily

ydrophobic crossover phenomena arise from the influence of the size of an oily solute on water structure and hydration thermodynamics1−4 and are thought to play an important role in biological binding, folding, and selfassembly.5−7 Previous molecular dynamics (MD) simulations2,4,6−13 and experiments14−19 are consistent with a crossover length scale of ∼1 nm near ambient temperatures. However, it is not yet clear how the experimentally measured and theoretically predicted crossover phenomena are related to each other. More specifically, while many theoretical crossover predictions pertain to idealized purely repulsive solutes,2,6,8−11,20 for which the crossover is associated with a dramatic “dewetting” (or “drying”) transition with a large change in hydration-shell density and structure, the experimental crossover phenomena pertain to more subtle changes in hydration-shell rotational dynamics14,21,22 and vibrational spectra,17−19 as well as polymer chain unfolding free energies.15,16 Although some of the differences between the experimental observations and idealized dewetting predictions may be linked to the predicted suppression of dewetting by solute−water attractive interactions,4,6,13 it remains unclear if the experimental and theoretical crossovers are in fact fundamentally related. Here we address this question by performing Raman multivariate curve resolution (RamanMCR) measurements to reveal that the hydration-shells of short-chain alcohols undergo a crossover transition above 100 °C (at 30 MPa), from a structure with a lower to one with a higher OH frequency than pure water. Additionally, the associated changes in hydration-shell tetrahedrality (relative to pure water) are quantified by correlating the measured hydration-shell OH frequency distributions with tetrahedrality distributions obtained from molecular dynamics (MD) simulations. Most importantly, our results reveal a decrease in crossover length-scale with increasing temperature, as is the © 2018 American Chemical Society

Received: December 28, 2017 Accepted: February 8, 2018 Published: February 8, 2018 1012

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Figure 1. Aqueous ethanol and pure water Raman and Raman-MCR spectra. (a) The dashed and solid blue curves represent measured Raman spectra of pure water and a 5 wt % aqueous ethanol solution, respectively, at 20 °C and 30 MPa. The inset diagram in panel a is a schematic of the capillary optical cell used to obtain these Raman spectra. The inset graph in panel a shows the temperature−density range over which Raman spectra were collected (red curve), as well as the liquid−vapor coexistence curve of pure water (dotted-blue curve). The low-frequency region in these spectra include some features arising from the polymer material of the microcapillary high pressure cell.24 (b) Raman-MCR bulk water (dashed curves) and solute-correlated (SC, solid curves) spectra obtained from a 5 wt % aqueous ethanol solution, at 30 MPa and T = 0−374 °C. (c) RamanMCR spectra obtained from a 5 wt % aqueous ethanol solution, at 0.1 MPa and T = 1−100 °C.17 The ethanol SC spectra in panels b and c are normalized to the same CH area (and essentially identical, but noisier, Raman-MCR were obtained from solutions of lower concentration, down to ∼1 wt %, see the Supporting Information).

surrounding bulk water. Above 100 °C the results in Figure 1b show the emergence of a SC hydration-shell OH band of quite different shape, peaked near ∼3600 cm−1 with a higher average frequency and narrower width than the OH band of the surrounding bulk water at the same temperature. The higher frequency of this SC OH band implies that the hydration-shell has transformed to a structure with weaker hydrogen bonds than bulk water at the same temperature. To obtain more precise and quantitative measures of the crossover temperature and hydration-shell tetrahedrality, we have performed the following additional combined RamanMCR and MD analysis. More specifically, MD predictions of the number of water molecules in the first hydration-shell around the −CH2CH3 tail of ethanol were combined with our Raman-MCR results to reconstruct the experimental spectrum of the OH band arising from the entire hydrophobic hydrationshell of ethanol. In other words, we produced a linear combination of the SC and pure water spectra whose OH area is equivalent to that arising from all the OH groups in the hydrophobic hydration-shell of ethanol, as shown in the upper two panels of Figure 2 (see the Supporting Information for further details). These spectra reveal the very small differences between the first hydration-shell OH band (solid curves) and pure water OH band (dashed curves) at −10 °C (Figure 2a) and 374 °C (Figure 2b). Note that at −10 °C the ethanol hydration-shell OH band is slightly red-shifted (lower in frequency) than pure water, while at 374 °C it is slightly blueshifted (higher in frequency). The solid points in Figure 2c

from the hydration-shell of their hydrophobic tails, with little or no influence of the OH headgroup, as evidenced by the fact that the SC spectra of benzene and phenol are virtually identical,26 as are the SC spectra of n-hexanol and 1,2hexanediol.17 Our Raman-MCR results further reveal that there is little aggregation of ethanol at a concentration up to 5 wt %, because essentially identical Raman-MCR results are obtained from aqueous ethanol solutions of lower concentration (see Figures S1 and S2). Moreover, the results shown in Figure 1b,c demonstrate that pressures up to 30 MPa do not significantly perturb water structure, as nearly identical SC spectra are obtained at 30 and 0.1 MPa at temperatures between 0 and 100 °C.17 The results in Figure 1b,c further reveal that at low temperatures (≤20 °C) the shape of the hydration-shell of ethanol differs from bulk water in having a more prominent shoulder near 3200 cm−1, as well as a depleted intensity above 3500 cm−1. The more prominent 3200 cm−1 shoulder implies that the hydration-shell is more tetrahedral than pure liquid water, as evidenced by the fact that the freezing of liquid water to ice, as well as the formation of solid clathrate-hydrates, are both accompanied by an increase in the intensity of the lowfrequency OH sub-band near 3200 cm−1.27,28 The depleted intensity of the ethanol SC spectrum above ∼3500 cm−1 further indicates that the hydration-shell has fewer weak hydrogen bonds than bulk water.17 As the temperature is increased to 100 °C, the SC hydration-shell OH stretch band nearly vanishes, indicating that at this temperature the hydration-shell of ethanol has a structure that closely resembles that of the 1013

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show the differences between the average OH frequencies in the hydration-shell and pure water obtained at these and other intermediate temperatures. More specifically, the pure water and hydration-shell average OH frequencies were obtained by evaluating ⟨ν⟩ = ∫ νI(ν) dν, where ν is the OH vibrational frequency; I(ν) is the OH stretch band shape (normalized to unit area); and the integral is performed over the OH band area (excluding the CH band region). These results clearly reveal a crossover temperature of 127 ± 10 °C below which the hydration-shell OH band is red-shifted and above which it is blue-shifted, relative to pure water. This crossover temperature is quite robust, as it does not depend on the precise value of the assumed number of perturbed water molecules in the hydration-shell of ethanol, as indicated by the open and starred points in Figure 2c; the open points are obtained by assuming that only half of the first hydration-shell OH groups are significantly perturbed, and the starred points are obtained from the average OH frequency of the entire aqueous ethanol solution and pure water. In other words, the latter results indicate that the experimental crossover temperature may be obtained without any MD simulation input. However, the magnitude of the average OH frequency shift depends on the number of OH groups whose vibrational frequencies are assumed to be significantly perturbed. Prior comparisons of Raman-MCR and femtosecond infrared rotational dynamics measurements suggested that only ∼8 water molecules were significantly perturbed in the hydrophobic hydration-shell of tert-butyl alcohol.29 This suggests that the “1/2 Shell” points in Figure 2 likely represent a lower bound to the magnitude of the actual average OH frequency differences arising from the smaller number of water molecules that are significantly perturbed by ethanol. Figure 3 shows our additional Raman-MCR and crossover temperature results obtained from 5 wt % aqueous solutions of methanol and n-propanol at 30 MPa. Both the SC spectra (a

Figure 2. Comparison of the average OH frequency of pure water and the first hydration-shell of ethanol. Panels a and b show the pure water (dashed curves) and first hydration-shell (solid curves) spectra obtained at −10 and 374 °C (at a pressure of 30 MPa), revealing that the OH band has a lower frequency than pure water at low temperature but a higher frequency than pure water at high temperature. The resulting average OH frequency differences obtained at various temperatures are plotted in panel c, revealing the crossover temperature of ∼127 ± 10 °C, obtained either from the first hydration-shell spectra (solid points) or from the entire solution (star points), or when it is assumed that only half the molecules in the first hydration-shell produce a spectrum that differs significantly from pure water (open points).

Figure 3. Raman-MCR (a and b) and crossover temperature (c and d) results obtained from aqueous solutions of methanol (a and c) and npropanol (b and d) at a concentration of 5 wt % and pressure of 30 MPa (see Figures 1 and 2 for further details regarding the description of the how the Raman-MCR spectra and crossover temperatures were obtained). 1014

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Figure 4. Correlation between the OH frequency and tetrahedral order of pure water. (a) Temperature dependence of the experimental pure water OH stretch Raman band. (b) MD simulations results for the temperature dependence of the pure water tetrahedral order distributions. (c) Correlation between the average tetrahedral order ⟨q⟩ and the average OH frequency ⟨ν⟩ of pure water. (d) MD results for the difference between the tetrahedral order in the alcohol hydration-shells and pure water, plotted as a function of temperature at 30 MPa. The points at which the curves cross the horizontal line produce the predicted q-crossover temperatures T* (see the text for further details).

double (and the estimated magnitudes of Δq would further increase if fewer water molecules are significantly perturbed). The above experimental crossover results are qualitatively (but not quantitatively) consistent with predictions obtained from the MD simulations alone, as the MD simulations yield somewhat lower crossover temperatures and greater tetrahedrality differences than the Raman-MCR hydration-shell spectra. This is illustrated in Figure 4d, which shows the MD predictions of the difference between the average tetrahedralities of the alcohol hydration-shells and pure water. These tetrahedrality predictions pertain to water molecules whose centers are localized between the first maximum and first minimum in the g(r) of the water-oxygen and alcohol-carbon atoms. The MD results confirm that at low temperature the alcohol hydration-shells have tetrahedral order greater than that of bulk water (Δq > 0) while at high temperature the converse is true (Δq < 0). The MD results also confirm that the predicted tetrahedrality crossover temperatures T* (at which Δq = 0) decrease with increasing alcohol chain length. However, the predicted crossover temperatures are somewhat lower (and less strongly chain length dependent) than the experimental crossover temperatures. Moreover, the temperature derivatives of Δq obtained from the MD simulations are 2−3 times larger than those inferred from the Raman-MCR measurements. However, the experimental and predicted crossover temperatures would become nearly identical if the horizontal (Δq = 0) line in Figure 4d were moved down by about 0.01. Thus, experimental and MD crossover temperatures are in qualitative agreement with each other. Most importantly, both the MD and Raman-MCR results confirm that the crossover length scale decreases with increasing temperature

and b) and the hydration-shell OH frequency differences (c and d) indicate that the crossover temperature decreases with increasing alcohol chain length. More specifically, the results in Figure 3c,d indicate that the hydration-shells of methanol and n-propanol have crossover temperatures of ∼164 ± 10 and ∼90 ± 10 °C, respectively, on either side of the ethanol crossover temperature of ∼127 ± 10 °C. In order to quantitatively link the above crossover results to a crossover in the tetrahedrality of water, we used MD simulations to evaluate the Errington−Debenedetti30 tetrahedral order parameter (q) of pure water and aqueous alcohol solutions, as further described in the Supporting Information. Note that q quantifies the extent to which a central water molecule and its four nearest neighbors adopt a tetrahedral angular arrangement, with q = 1 corresponding to a perfect tetrahedron and 0 to a random (ideal gas) distribution. The upper two panels in Figure 4 compare the OH Raman bands (a) and tetrahedral order distributions (b) of pure water over a temperature range of −10 to 374 °C at 30 MPa. Figure 4c shows the nearly linear correlation between average tetrahedrality ⟨q⟩ and average OH frequency ⟨ν⟩ of pure water. We have used this correlation to estimate the tetrahedrality difference between pure water and hydrophobic hydration-shells. More specifically, if we assume that all the first hydration-shell molecules are perturbed, then the −5 cm−1 OH shift at −10 °C (see Figure 2a,c), combined with the correlation shown in Figure 4c, would imply a first hydrationshell tetrahedral order increase of ∼1% (Δq ∼ +0.008), while the +4.4 cm−1 OH shift at 374 °C would imply a tetrahedral order decrease of ∼1% (Δq ∼ −0.005). If only half of the water molecules in the first shell are significantly perturbed, then the estimated magnitudes of Δq for those water molecules would 1015

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OH headgroup on the hydrophobic crossover length scale and temperature remains an open question. Nevertheless, comparisons of the present results with previous Raman-MCR results pertaining to longer aqueous n-alcohol solutions,17 as well as aqueous solutions of neutral n-carboxylic acid and charged carboxylate, and tetraalkylammonium ions19 clearly imply that the enhanced tetrahedral order in the hydration-shells of oily molecules at low temperatures transforms to a structure that is more disordered than the surrounding bulk water molecules above a crossover temperature that decreases with increasing solute size. Although the associated changes in water structure are quite small, the temperature dependence of the crossover length-scale suggests that the observed crossover behavior is related to the more dramatic “dewetting” transition predicted to occur around idealized purely repulsive solutes.

and is associated with a crossover in hydrophobic hydrationshell tetrahedrality. Comparisons of experimental and MD results further indicate that the hydration-shell crossover is accompanied by a decreased hydration-shell density, relative to pure water. More specifically, the density of pure water decreases by ∼9% between 100 and 200 °C at 30 MPa (as indicated in the inset graph in Figure 1a), in good agreement with our MD simulation results (see Table S1). Over the same temperature range, our MD results indicate that the number of water molecules in the first hydration-shell of ethanol decreases by ∼13%. The larger hydration-shell density change is also qualitatively consistent with the ∼18% increase in the partial molar volume of ethanol, as compared to the ∼9% increases in the partial molar volume of pure water, over this same temperature range, at 30 MPa (as obtained from global fits to the equations of state of pure water and aqueous ethanol solutions).23,31 Thus, the hydration-shell OH frequency and tetrahedrality crossover is evidently accompanied by a greater thermal expansion of the hydration-shell, relative to pure water. The temperature and length scale dependent crossover phenomena that we have identified are expected to influence other hydration thermodynamic functions. For example, the temperature dependence of the hydration free energies2,8,10−12,31−34 and activity coefficients33 of idealized hydrophobic purely repulsive solutes,8,10−12,32 and more realistic oily solutes (including alcohols,31,33 rare-gases, and alkanes31,34) have been found to be nonmonotonic, with an extremum at a temperature that decreases with increasing solute diameter8,10−12,32 or chain length.33 This implies that the hydration entropy is negative below the free-energy maximum and then changes sign to become positive at higher temperatures, thus providing a thermodynamic measure of the crossover temperature, above which water molecules in the solute’s hydrationshell evidently have a higher entropy than bulk water molecules. Although the crossover length scales and temperatures associated with various spectroscopic, structural, and thermodynamic observables are not identical, they all imply a decrease in crossover length-scale with increasing temperature. One might expect the alcohol OH headgroup to influence the corresponding hydrophobic crossover temperature. A recent infrared difference spectroscopic study of methane in water (at 19 ± 7 °C and 3.5 ± 2 MPa) presented evidence suggesting that the hydration-shell of methane has a substantially clathrate-like structure,35 thus implying an increase in tetrahedral order in the hydration-shell of methane that far exceeds the very small (∼1%) increase in tetrahedrality that we have found in the hydration-shells of methanol, ethanol, and npropanol at 0 °C. If so, that would suggest that the OH headgroup of n-alcohols has a substantially disruptive influence on the surrounding tetrahedral water structure; thus, one might expect alcohols to have a lower crossover temperature than alkanes (of the same size). On the other hand, a global fit to the experimental equation of state of methane dissolved in water predicts that methane’s partial molar volume increases by ∼32% when the temperature of the aqueous solution is increased from 100 to 200 °C (at 30 MPa).31 The fact that this percent increase is much larger than the 16% increase in the partial molar volume of methanol31 suggests that the hydrationshell of methane may undergo a crossover transition at a lower temperature than methanol, thus implying that the crossover length scale for alkanes is smaller than that of alcohols. In view of these conflicting expectations, the influence of the alcohol

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b03431. Experimental and MD simulation methods and additional results, including ethanol hydration-shell spectra obtained at concentrations down to 1 wt %; comparisons of experimental and MD water densities; and MD hydrophobic hydration-shell coordination numbers for methanol, ethanol, and n-propanol from −10 to 374 °C (at 30 MPa) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (765) 494-5256. ORCID

Henry S. Ashbaugh: 0000-0001-9869-1900 Dor Ben-Amotz: 0000-0003-4683-5401 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1464904 for DBA and CBET-1403167 for HSA) and the National Sciences Foundation of China (Nos. 41102154 and 41176047, for X.W. and W.L.).

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