Properties of Polyvinylpyrrolidone in a Deep Eutectic Solvent - The

Mar 10, 2016 - Deep eutectic solvents (DES) are mixtures of two or more components with ... Glycerol Hydrogen-Bonding Network Dominates Structure and ...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCA

Properties of Polyvinylpyrrolidone in a Deep Eutectic Solvent Liel Sapir,† Christopher B. Stanley,*,‡ and Daniel Harries*,† †

Institute of Chemistry and The Fritz Haber Research Center, The Hebrew University, Jerusalem 91904, Israel Biology and Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: Deep eutectic solvents (DES) are mixtures of two or more components with high melting temperatures, which form a liquid at room temperature. These DES hold great promise as green solvents for chemical processes, as they are inexpensive and environmentally friendly. Specifically, they present a unique solvating environment to polymers that is different from water. Here, we use small angle neutron scattering to study the polymer properties of the common, water-soluble, polyvinylpyrrolidone (PVP) in the prominent DES formed by a 1:2 molar mixture of choline chloride and urea. We find that the polymer adopts a slightly different structure in DES than in water, so that at higher concentrations the polymer favors a more expanded conformation compared to the same concentration in water. Yet, the osmotic pressure of PVP solutions in DES is very similar to that in water, indicating that both solvents are of comparable quality and that the DES components interact favorably with PVP. The osmotic pressure measurements within this novel class of promising solvents should be of value toward future technological applications as well as for osmotic stress experiments in nonaqueous environments.

1. INTRODUCTION The past decade has seen the emergence of a promising class of solvents termed deep eutectic solvents (DES).1,2 Sometimes considered a unique family of ionic liquids, DES are composed of at least two components, whose melting temperature when pure is relatively high (usually over 100 °C) but when mixed have a much reduced melting temperature. Thus, DES are typically liquid at room temperature. Because they are different from water and other widely used solvents, DES offer unique solution environments for biological and other macromolecules and many technologically important chemical processes. These new solvents afford several practical advantages: they are generally cheap, nontoxic, renewable, and biodegradable. Given these virtues, DES hold potential as green solvents for chemical reactions.3 Many known DES parallel natural solvating bioenvironments;4 in most cases a DES is obtained by mixing a quaternary ammonium salt with metal salts or a hydrogen bond donor (HBD) that is able to complex with the halide anion of the quaternary ammonium salt. This propensity is manifested in the degree to which the melting temperature of the DES mixture is lowered with respect to the pure components. Generally, the stronger the complexing propensity of the ion with the HBD, the lower the melting temperature of the DES. A prominent example is the mixture of urea and choline chloride, Figure 1A,B. Whereas the pure compounds melt at high temperatures (choline chloride at 302 °C and urea at 133 °C), a 1:2 molar mixture of these melts at 12 °C.2 Both components serve as precursors for biochemical reagents in the chemical industry and are biocompatible and readily available; in fact, both are used as animal feedstock, and furthermore urea is used for fertilization. © XXXX American Chemical Society

Figure 1. Schematic of (A) choline chloride, (B) urea, and (C) polyvinylpyrrolidone (PVP). Rendering is in the CPK representation.

Since their discovery, DES have been scrutinized for the way chemical processes are carried out within them. Applications have been explored in electrochemical deposition,5,6 metal extraction,7 and catalyst aldol reactions8 and as solvent and template in the synthesis of zeolite analogues.9 Resolving molecular interactions in DES opens new possibilities for future applications for catalysis in nonaqueous solvents for the purpose of recycling and synthesis of new materials. Despite increasing interest in DES as a solvent, the mechanisms of Special Issue: Ronnie Kosloff Festschrift Received: December 6, 2015 Revised: February 21, 2016

A

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

quartz windows, both at 1 mm path length, and SANS measurements were performed at 25 °C. Data reduction followed standard procedures using MantidPlot. 22 The measured scattering intensity was corrected for the detector sensitivity and scattering contribution from the solvent and empty cells and then placed on an absolute scale using a calibrated standard.23 The analysis of the SANS measurements was conducted using the SasView software.24 2.3. Vapor Pressure Osmometry. The osmotic pressure of PVP aqueous solutions was measured using a VAPRO 5520 vapor pressure osmometer (Wescor, Logan, UT). Each data point represents the average of at least two repeats. The osmometer was initially calibrated using standard NaCl solutions of 290 and 1000 Osmolal (Osm), and recalibrated after ca. every 5−10 measurements. The ambient temperature was set to 295 ± 1 K. Because the osmometer used is designed for measuring osmotic pressure in H2O solutions, an additional calibration step was necessary for measurements in D2O. First, the osmolality of NaCl in D2O was measured over the range of 0−1.42 m. Then, a calibration curve was constructed by comparing our measured results with the known values for NaCl in D2O as reported in ref 25. This calibration curve is shown in Figure S1 of the Supporting Information. The measured values for PVP in D2O were then read from the calibration curve accordingly.

macromolecular solvation and solubility, as well as the underlying specific interactions in this unique alternative to typical ionic liquids, is only starting to be explored.10,11 Specifically, although polymer properties in regular ionic liquids are beginning to be probed,12,13 the structure of polymers in DES has hardly been studied. Polyvinylpyrrolidone (PVP, also known as povidone or polyvidone) is a linear homopolymer, Figure 1C, available at various molecular weights, and highly soluble in both water and various organic solvents.14 These properties, along with PVP’s ability to complex with polar molecules, and its biodegradability, have led to its multitude of industrial applications, including in pharmaceuticals15 and processed foods.16 All these make PVP of particular interest for future potential polymerDES applications. Beyond this, PVP has been extensively used in biophysical studies as an osmotic stressor for measuring forces between macromolecules, a technique that allows the determination of distance vs force curves, i.e., equations of state.17−20 Taken together, this wide variety of potential applications underscores the importance of the study of PVP and its properties in DES. Here, we investigate the properties of PVP of molecular weight 10 000 g mol−1 in both water and, the now prototypical, DES formed by a 1:2 molar mixture of choline chloride and urea (sometimes referred to as Reline). We use small-angle neutron scattering (SANS) to probe the structural properties across a range of concentrations. We complement this structural analysis with measurements of solution osmotic pressure, obtained from the scattering measurements and from vapor pressure osmometry. The structural and thermodynamic analyses indicate that DES acts as a solvent similar to water for PVP.

3. RESULTS AND DISCUSSION 3.1. PVP Characteristic Length Scales. The SANS spectra for various concentrations of PVP in D2O and DDDES are presented in Figure 2. Using the deuterated solvents allowed for an improved contrast of the PVP, with a difference in neutron scattering length density, ΔSLD = −4.944 × 1010 and −4.556 × 1010 cm−2 for D2O and DD-DES, respectively. In both solvents, the overall scattering intensity of PVP increased with increasing polymer concentration, but with incrementally smaller increases at the upper concentration range. By 248 mg mL−1 PVP in D2O, the low-Q scattering intensity slightly decreased, as polymer−polymer interactions suppress the lower Q-range. The SANS curves in Figure 2 were fitted to a polymer model that incorporates excluded volume contributions,26,27

2. METHODS 2.1. Materials. PVP (MW = 10 000 g mol−1), D2O, and NaCl (all from Sigma), deuterated choline chloride (trimethyld9, from Cambridge Isotope Laboratories, CIL), and deuterated urea (d4, from CIL) were all used without further purification. The DES was made by mixing choline chloride and urea at a 1:2 molar ratio, heating (ca. 80 °C), cooling back to room temperature, and later dehydrating the samples in a vacuum oven (at 50 °C). This protocol produced DES with water content of less than 1% by weight, as gauged with a refractometer, and verified by Karl Fischer analysis. Solutions of PVP in DES or D2O were prepared by mixing thoroughly and placing in a dry bath at 50 °C overnight. Deuterated choline chloride and deuterated urea (that form the deuterated DES, DD-DES), and D2O, were used to maximize contrast between polymer and solvent in the SANS experiments. Each PVP solution concentration was determined gravimetrically. 2.2. Small Angle Neutron Scattering. SANS experiments were performed on the extended Q-range small-angle neutron scattering (EQ-SANS, BL-6) beamline at the Spallation Neutron Source (SNS) located at Oak Ridge National Laboratory (ORNL). Measurements were made with two configurations: (1) in 30 Hz operation mode, a 4 m sample-todetector distance with 2.5−6.1 and 9.4−13.4 Å wavelength bands was used, and (2) in 60 Hz mode, a 1.3 m sample-todetector distance with 4−8.1 Å wavelength band21 was used. The scattering curves were merged to obtain the full wavevector transfer, Q = 4π sin(θ)/λ, where 2θ is the scattering angle. Samples were loaded into either circular-shaped quartz cuvettes (Hellma USA, Plainville, NY) or assembled cells with

I (Q ) =

mI(0) mI(0) ⎛ m ⎞ γ ⎜ ,U ⎟ − γ(m ,U ) m /2 ⎝ ⎠ 2 Um U

(1)

where γ is the incomplete gamma function and the parameter U satisfies U=

Q 2R g 2(2/m + 1)(2/m + 2) 6

(2)

The least-squares fitting procedure accounts for constant incoherent background and results in the polymer radius of gyration, Rg, the Porod exponent, m (corresponding to the scaling law at high wavevector), and the scattering intensity at Q = 0, I(0), as optimized fit parameters. This fitting procedure has the added benefit of seamlessly accounting for both high and low scattering wavevector regimes. The resultant Rg values as a function of PVP density, c, are presented in Figure 3A. These should be regarded as apparent values that do not necessarily reflect the single chain property. The values of Rg in the most dilute cases measured (5 or 6 mg mL−1), Rg,0, indicate that PVP chains appear larger, and therefore more swollen, in D2O than in DD-DES. At low B

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

The trends in polymer size reflected in ξ and Rg are also evident in the pair−distance distribution function, P(r), calculated from the SANS data (Figure 3C,D), using SasView.24,30 The function P(r), which reflects the real-space intramolecular correlations, also indicates that PVP in the dilute regime is more expanded in water than in DES. This is observed in the P(r) plots by the larger maximum linear dimension, Dmax, in D2O compared to DD-DES (Figure S2A in the Supporting Information). With increasing PVP concentration, Dmax becomes, and remains, higher in DD-DES compared to D2O. Also, the Rg and I(0) values determined from P(r) fits are consistent (in the trends and the absolute values, within error) with our model-dependent analyses (Figure S2B,C in the Supporting Information). It is important to note that the P(r) calculations at the highest polymer concentrations can be influenced by intermolecular correlations that become increasingly dominant. This is apparent at 248 mg mL−1 PVP in D2O, where a portion of P(r) is negative. 3.2. PVP Structure. To shed more light on the morphology of the polymer within each solvent, we follow the Porod exponent, m, Figure 4A. The value of m approaches 1.6 ± 0.2 at low concentrations, as expected for a swollen chain in a good solvent (m = 5/3). In D2O, the Porod exponent then decreases rapidly with concentration and approaches unity matching the value for stiff, rodlike structures, but it does not appreciably change in DD-DES. The same trend can be derived from the normalized Kratky plots of (QRg)2I(Q)/I(0) vs QRg for PVP in both solvents, Figure 4B,C. The normalized Kratky plot allows shape information to be derived from the scattering data. These plots indicate that although the polymer does not appreciably change its solution conformation in DD-DES with increasing concentration, it does undergo a clear shift away from a random coil toward a rodlike structure in water. We note that at the higher PVP concentrations in D2O, the normalized Kratky curves begin to extend beyond the rodlike model. This may be within experimental error, or possibly the effect of polymer− polymer interactions that are strongly influencing at these higher concentrations; note that we are comparing the data to single chain models, as described previously.31 Notwithstanding, the Kratky plot analysis is consistent with our Porod exponent analysis, which lends confidence in the salient features. 3.3. Osmotic Pressure of PVP Solutions. To further investigate the nature of polymer−solvent interactions, we used the Zimm equation32

Figure 2. SANS intensity curves of PVP in (A) D2O and (B) DDDES. Colors represent different PVP concentrations (given as mg mL−1 in the legend). Black lines represent fits based on a polymer chain model with excluded volume contributions (see text). For clarity, the constant incoherent background determined from each fit has been subtracted from both the data and fit lines.

concentrations, Rg in DD-DES is almost constant with polymer content, and gradually decreases at higher concentrations, but is larger compared to its value in D2O. This conclusion is also supported by the correlation length, ξ (Figure 3B) extracted from a fit of the scattering data to the Ornstein−Zernike equation28 I (Q ) =

I(0) 1 + Q 2ξ 2

(3)

For PVP in both D2O and DD-DES, ξ approaches a limiting value at low concentrations and decreases more sharply at the overlap concentration, c*, which marks the lower limit of the semidilute regime. In the semidilute regime we find for D2O that ξ ∝ c−0.93±0.08, where the exponent is close to the value expected for a solvent between the Θ and good solvent regimes (i.e., between −1 and −3/4).29,28 The turnover region from one scaling behavior to the other (Figure 3A,B) suggests that c* occurs for D2O at ∼60 mg mL−1, which is in good agreement with an estimate of Rg,0, as c* ≈ 3MW/(NAV4πRg,03) ≈ 70 mg mL−1, where NAV is Avogadro’s number. Interestingly, on the basis of the concentration dependence of ξ, the value of c* in DD-DES appears slightly higher than in water and the polymer does not fully reach the semidilute regime before arriving at the solubility limit, thus precipitating from solution. As a result, we can only resolve the onset of the semidilute regime for PVP in DD-DES and still before the characteristic scaling for a good solvent becomes fully apparent.

KMW c = 1 + 2MW B2 c + 3MW B3c 2 + 6(c 3) I(0)

(4)

to derive a virial expansion for the osmotic pressure from the scattering at zero wavevector. In eq 4, B2 and B3 are the second and third virial coefficients, respectively, and K is the contrast factor, K = (ΔSLD/ρPVP)2NAV−1, where ρPVP = 1.2 g mL−1 is the pure PVP density. A least-squares fit to eq 4, Figure 5A, gives the second (and higher) virial coefficients. These can be integrated to give the Osmolarity, π/RT as33 ⎤ ⎡ 1 π = c⎢ + B2 c + B3c 2 + 6(c 3)⎥ RT ⎦ ⎣ MW

(5)

This expression, in turn, can be converted to Osmolality, Π/RT, by accounting for solution density. We calculated the second virial, B2, to be (1.03 ± 0.03) × 10−3 and (1.43 ± 0.14) C

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 3. Structural properties of PVP in water and in DES. (A) log−log plot for the radius of gyration, Rg, versus PVP density. Black and red colors represent the D2O and DD-DES solutions, respectively. (B) log−log plot of the correlation length, ξ, versus PVP density (fit to the Ornstein− Zernike model). (C) and (D) Pair−distance distribution function, P(r), for PVP in D2O and in DD-DES, respectively, for various concentrations.

× 10−3 mL mol g−2 for D2O and DD-DES, respectively. The value for PVP in D2O is close to those previously reported for the same molecular mass polymer in H2O: 1.06 × 10−3 (ref 34) and (1.54 ± 0.11) × 10−3 mL mol g−2 (ref 35). The resulting osmolality is plotted against solution molality in Figure 5B. For comparison, we plot our VPO measurements of PVP in D2O, and data from ref 36 for PVP in H2O on the same figure. The osmotic pressure of PVP solutions in the three solvents appears almost indistinguishable and all appear to fit one osmotic pressure master curve (gray line). Nonetheless, the values of B2 suggest that the effective interactions between PVP chains in DD-DES are more repulsive than in water, implying a stronger interaction between PVP and DES. 3.4. Mixture Interactions in Terms of the Flory− Huggins Solution Theory. The more favorable interaction (compared to those in water) between PVP and DES is also manifested in the Flory−Huggins interaction parameter,37−41 which satisfies χ = 1/2 − B2vs̅ olventvP̅ VP2MW2.42 To evaluate χ, we calculated the partial molar volume, ν,̅ for each solvent and PVP from their pure densities (for DD-DES we determined ρDD−DES = 1.271 ± 0.004 g mL−1). The derived interaction parameters are χ = 0.47 for D2O and χ = 0.05 ± 0.04 for DD-DES, suggesting that although both act as good solvents for PVP, water is closer to a Θ solvent (χ = 0.5) solvent whereas DES approaches the ideal solution limit (i.e., where there are no net effective interactions, χ = 0). The fact that χ is lower for DES, whereas the solubility limit of PVP in DES is also lower, might suggest a concentration dependence of the interaction parameter,43−46 which we do not consider here. Taken together, the emerging picture is that PVP in water enters the semidilute regime at lower concentrations. In that

regime, where the polymer forms an extended network, PVP seems stiffer in water than in DES (Figure 4) and has a smaller mesh size (ξ, Figure 3B), which furthermore decreases more rapidly in water with increasing concentration. These differences could be related to changes in the packing of PVP within the solvent, which directly reflect polymer−solvent interactions. Possibly, these differences also correspond to larger-amplitude fluctuations for PVP in DES that give rise to a larger apparent correlation length. A similar conclusion can be drawn by considering the second virial coefficient, B2, and the interaction parameter, χ, of PVP in the two solvents. Water interacts favorably with PVP, and the second virial is positive (i.e., “positive deviations” from ideality). DES interacts even more favorably with the polymer, echoing a stronger interaction of the polymer with the components of the choline chloride−urea mixture, as reflected in a second virial coefficient that is even more positive. It is interesting to compare the second virial coefficient of PVP (MW = 11 800 g mol−1) in another solvent, methanol, where B2 = 0.33 × 10−3 mL mol g−2 (yielding the interaction parameter χ = 0.48).47 The increasing value of the second virial coefficient (and the decreasing value of χ) for the series methanol, water, and DES implies that the (overall) interaction of a solvent unit with PVP monomers, relative to the reference solvent−solvent and polymer−polymer interactions, is strongest for DES (compared to water and methanol). This implies that the DES components form intermolecular interactions with the polymer which are of similar strength to solvent− solvent and polymer−polymer interactions. Methanol has a large hydrophobic component and may interact less strongly with the polar polymer monomers. To contrast, PVP is known D

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 5. Osmotic pressure of PVP solutions. (A) The Zimm formula at scattering vector Q = 0 as a function of PVP concentration (given in g mL−1) in both solvents. Solid lines are fits to eq 4. (B) The solution osmolality, Π/RT, vs molality, m, for D2O (SANS and VPO), H2O (Sadeghi et al.36), and DD-DES (SANS). The gray line is the thirdorder polynomial least-square fit for the combined VPO and scattering data for PVP in D2O, and is given by Π/RT = 1.12m + 84.5m2 + 3285m2. The inset shows the osmotic coefficient, φ = Π/mRT, as a function of molality.

hydrogen bonds with the pyrrolidone moiety. Urea seems like a prime candidate for interacting with PVP monomers, because it is present in twice the molar concentration of choline and chloride and shares similar chemical groups with the PVP monomer. We note that our derivation of the interaction parameter χ is based on the Flory−Huggins solution theory for binary mixtures. Because PVP in DES is a quaternary solution, the exact derivation should include additional interaction parameters that are considered here through a single interaction parameter. Therefore, the χ we derived above should be regarded as an effective interaction parameter, which subsumes additional mixing free energy components; perhaps most prominently, it should include the mixing entropy associated with the solvation of the polymer within the three-component deep eutectic solvent. The effective χ is therefore necessarily temperature dependent.28,48,49 This could in part explain the considerably lower value we calculated for DD-DES compared to that for D2O. Additional studies could resolve this dependence through measurements of osmotic pressure at different temperatures.

Figure 4. (A) Porod exponent versus PVP density (based on the same fit as for Figure 3A). Solid, dashed, and dotted lines represent the values for Θ solvent (m = 2), good solvent (m = 5/3), and an extended chain (m = 1) model, respectively. (B) and (C) Normalized Kratky plots for PVP in D2O and DD-DES, respectively. The values of Rg and I(0) used in these plots are taken from the same fit as for Figure 3A. Orange and black dotted curves represent the characteristic curves for a rigid rod model and a random coil chain (Debye model), respectively.

to interact with water through hydrogen bonds that compete with water−water hydrogen bonds and may also facilitate its ability to form aggregates.35 Thus, it is tempting to speculate that the Flory−Huggins interaction parameters that we find in water and DES are related to PVP’s ability to form hydrogen bonds as well as charge−dipole interactions. However, our results suggest that these hydrogen bonds are probably weaker than water−water hydrogen bonds. Overall, this may single out DES as an optimized solvent for PVP, because its intermolecular interactions are weaker than those of water and may be more similar to PVP−PVP interactions. Though our current measurements cannot dissect the preferential interactions of each DES component with PVP, it is noteworthy that both urea and choline molecules can form

4. CONCLUDING REMARKS We have used SANS measurements to follow the solvation of PVP in the novel and promising deep eutectic solvent of choline chloride and urea (1:2 molar ratio). Comparing the polymer properties of polyvinylpyrrolidone in water and DES, we have shown that the conformation of the polymer in DES is E

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

through the R. Rahamimoff travel grant program of the USIsrael Binational Science Foundation (BSF). A portion of this research at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. L.S. is supported by the Adams Fellowship Program of the Israel Academy of Sciences and Humanities. The Fritz Haber Research Center is supported by the Minerva Foundation, Munich, Germany.

more swollen, particularly in the semidilute regime. As the polymer concentration increases in DES, the polymer maintains a flexible coil structure, whereas in water it shifts toward a rodlike conformation concomitant with a decrease in the correlation length. These findings suggest that the intermolecular interaction of PVP with DES components is somewhat different from those in water. Indeed, the difference is also reflected in the osmotic pressure of PVP in both solvents. Although the osmotic pressure itself is not considerably different between solvents, the second virial coefficient implies that the underlying intermolecular interactions are distinct. We find DES to be an optimized solvent for PVP as the interactions between the polymer and solvent components are of comparable strength to those between solvent (or polymer) components and themselves, as reflected in the Flory−Huggins interaction parameter. In future investigations, it would be interesting to resolve the interactions of different components in DES to determine which acts more favorably with PVP. It should be noted that both DES and PVP are known to be highly hygroscopic10 and that water content is known to have a large impact on the solvent’s properties.50 Hence, in future studies it would be interesting to examine the role that water might play in the solvation of PVP and other polymers within DES. The similarity between the osmotic pressure of PVP in water and in DES might prove useful for future applications of DES as a solvent for polymers. For example, our results could facilitate the use of PVP for resolving force−distance equations of state of macromolecules in DES using the osmotic stress strategy. This strategy has been employed extensively using other watersoluble polymers, such as polyethylene glycol (PEG) and dextran, to probe macromolecules.51,52 It would be interesting to measure the osmotic pressure of these polymers in DES, too, for future potential studies and applications.

■

■

(1) Abbott, A. P.; Capper, G.; Davies, D. L.; Munro, H. L.; Rasheed, R. K.; Tambyrajah, V. Preparation of Novel, Moisture-Stable, LewisAcidic Ionic Liquids Containing Quaternary Ammonium Salts with Functional Side Chains. Chem. Commun. 2001, No. 19, 2010−2011. (2) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel Solvent Properties of Choline Chloride/urea Mixtures. Chem. Commun. 2003, 2003 (1), 70−71. (3) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids. J. Am. Chem. Soc. 2004, 126 (29), 9142−9147. (4) Choi, Y. H.; van Spronsen, J.; Dai, Y.; Verberne, M.; Hollmann, F.; Arends, I. W. C. E.; Witkamp, G.-J.; Verpoorte, R. Are Natural Deep Eutectic Solvents the Missing Link in Understanding Cellular Metabolism and Physiology? Plant Physiol. 2011, 156 (4), 1701−1705. (5) Abbott, A. P.; El Ttaib, K.; Frisch, G.; McKenzie, K. J.; Ryder, K. S. Electrodeposition of Copper Composites from Deep Eutectic Solvents Based on Choline Chloride. Phys. Chem. Chem. Phys. 2009, 11 (21), 4269. (6) Florea, a.; Anicai, L.; Costovici, S.; Golgovici, F.; Visan, T. Ni and Ni Alloy Coatings Electrodeposited from Choline Chloride-Based Ionic Liquids - Electrochemical Synthesis and Characterization. Surf. Interface Anal. 2010, 42 (6−7), 1271−1275. (7) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Shikotra, P. Selective Extraction of Metals from Mixed Oxide Matrixes Using Choline-Based Ionic Liquids. Inorg. Chem. 2005, 44 (19), 6497−6499. (8) Hu, S.; Jiang, T.; Zhang, Z.; Zhu, A.; Han, B.; Song, J.; Xie, Y.; Li, W. Functional Ionic Liquid from Biorenewable Materials: Synthesis and Application as a Catalyst in Direct Aldol Reactions. Tetrahedron Lett. 2007, 48 (32), 5613−5617. (9) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.; Morris, R. E. Ionic Liquids and Eutectic Mixtures as Solvent and Template in Synthesis of Zeolite Analogues. Nature 2004, 430 (7003), 1012−1016. (10) Zhang, Q.; De Oliveira Vigier, K.; Royer, S.; Jérôme, F. Deep Eutectic Solvents: Syntheses, Properties and Applications. Chem. Soc. Rev. 2012, 41 (21), 7108−7146. (11) Smith, E. L.; Abbott, A. P.; Ryder, K. S. Deep Eutectic Solvents (DESs) and Their Applications. Chem. Rev. 2014, 114 (21), 11060− 11082. (12) Triolo, A.; Russina, O.; Keiderling, U.; Kohlbrecher, J. Morphology of Poly(ethylene Oxide) Dissolved in a Room Temperature Ionic Liquid: A Small Angle Neutron Scattering Study. J. Phys. Chem. B 2006, 110 (4), 1513−1515. (13) Werzer, O.; Warr, G. G.; Atkin, R. Conformation of Poly(ethylene Oxide) Dissolved in Ethylammonium Nitrate. J. Phys. Chem. B 2011, 115 (4), 648−652. (14) Haaf, F.; Sanner, A.; Straub, F. Polymers of N-Vinylpyrrolidone: Synthesis, Characterization and Uses. Polym. J. 1985, 17 (1), 143−152. (15) Bühler, V. Polyvinylpyrrolidone Excipients for Pharmaceuticals; Springer: Berlin/Heidelberg, 2005. (16) Pande, H. Synthectic Hydrocolloids. The Complete Book on Gums and Stabilizers for Food Industry; Asia Pacific Business Press: Delhi, 2010. (17) Stanley, C.; Rau, D. C. Preferential Hydration of DNA: The Magnitude and Distance Dependence of Alcohol and Polyol Interactions. Biophys. J. 2006, 91 (3), 912−920.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11927. Calibration curve for osmotic pressure measurements in D2O using the Wescor VAPRO 5520 vapor pressure osmometer and derived quantities from the pair-distance distribution function (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*C. B. Stanley. E-mail: [email protected]. *D. Harries. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Werner Kunz for introducing us to DES. The authors thank Uri Raviv for allowing us use of the osmometer and for his continuing support. This project was supported by the International Network Grant of the Leverhulme Trust. L.S.’s travel to ORNL was funded through the Aharon and Ephraim Katzir Fellowship of the Batsheva de Rothschild Fund and F

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A (18) Parsegian, V. A. Protein-Water Interactions. Int. Rev. Cytol. 2002, 215, 1−31. (19) Parsegian, V. A.; Rand, R. P.; Fuller, N. L.; Rau, D. C. Osmotic Stress for the Direct Measurement of Intermolecular Forces. In Methods in enzymology; Academic Press: New York, 1986; pp 400− 416. (20) Rau, D. C.; Lee, B.; Parsegian, V. A. Measurement of the Repulsive Force between Polyelectrolyte Molecules in Ionic Solution: Hydration Forces between Parallel DNA Double Helices. Proc. Natl. Acad. Sci. U. S. A. 1984, 81 (May), 2621−2625. (21) Zhao, J. K.; Gao, C. Y.; Liu, D. The Extended Q -Range SmallAngle Neutron Scattering Diffractometer at the SNS. J. Appl. Crystallogr. 2010, 43 (5), 1068−1077. (22) Arnold, O.; Bilheux, J. C.; Borreguero, J. M.; Buts, A.; Campbell, S. I.; Chapon, L.; Doucet, M.; Draper, N.; Ferraz Leal, R.; Gigg, M. A.; et al. Mantid - Data Analysis and Visualization Package for Neutron Scattering and μ SR Experiments. Nucl. Instrum. Methods Phys. Res., Sect. A 2014, 764, 156−166. (23) Wignall, G. D.; Bates, F. S. Absolute Calibration of Small-Angle Neutron Scattering Data. J. Appl. Crystallogr. 1987, 20 (1), 28−40. (24) http://www.sasview.org/. (25) Craft, Q. D.; Van Hook, W. A. Isotope Effects in Aqueous Systems. VI. Partial Molal Free Energies in NaCl−H2O−D2O by Freezing-Point Measurements. The Heat of Fusion of D2O. J. Solution Chem. 1975, 4 (11), 923−947. (26) Hammouda, B. Small-Angle Scattering from Branched Polymers. Macromol. Theory Simul. 2012, 21 (6), 372−381. (27) Benoit, H. The Diffusion of Light by Polymers Dissolved in a Good Solvent. Compt. Rend. 1957, 245, 2244−2247. (28) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: Oxford; New York, 2003. (29) Daoud, M.; Cotton, J. P.; Farnoux, B.; Jannink, G.; Sarma, G.; Benoit, H.; Duplessix, C.; Picot, C.; Gennes, P. G. De. Solutions of Flexible Polymers. Neutron Experiments and Interpretation. Macromolecules 1975, 8 (6), 804−818. (30) Moore, P. B. Small-Angle Scattering. Information Content and Error Analysis. J. Appl. Crystallogr. 1980, 13 (2), 168−175. (31) Sitar, S.; Aseyev, V.; Kogej, K. Microgel-like Aggregates of Isotactic and Atactic Poly(methacrylic Acid) Chains in Aqueous Alkali Chloride Solutions as Evidenced by Light Scattering. Soft Matter 2014, 10 (39), 7712−7722. (32) Zimm, B. H. Molecular Theory of the Scattering of Light in Fluids. J. Chem. Phys. 1945, 13 (4), 141. (33) Noda, I.; Kato, N.; Kitano, T.; Nagasawa, M. Thermodynamic Properties of Moderately Concentrated Solutions of Linear Polymers. Macromolecules 1981, 14, 668−676. (34) Rathbone, S. J.; Haynes, C. A.; Blanch, H. W.; Prausnitz, J. M. Thermodynamic Properties of Dilute Aqueous Polymer Solutions from Low-Angle Laser-Light-Scattering Measurements. Macromolecules 1990, 23 (17), 3944−3947. (35) Sun, T.; King, H. E. Aggregation Behavior in the Semidilute poly(N-Vinyl-2-Pyrrolidone)/water System. Macromolecules 1996, 29 (9), 3175−3181. (36) Sadeghi, R.; Zafarani-Moattar, M. T. Thermodynamics of Aqueous Solutions of Polyvinylpyrrolidone. J. Chem. Thermodyn. 2004, 36 (8), 665−670. (37) Flory, P. J. Thermodynamics of High Polymer Solutions. J. Chem. Phys. 1942, 10, 51−61. (38) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, New York, 1953. (39) Huggins, M. L. Thermodynamic Proerties of Solutions of LongChain Compounds. Ann. N. Y. Acad. Sci. 1942, 43 (1), 1−32. (40) Huggins, M. L. Some Properties of Solutions of Long-Chain Compounds. J. Phys. Chem. 1942, 46 (1), 151−158. (41) Hill, T. L. An Introduction to Statistical Thermodynamics; Dover Publications, Inc.: New York, NY, 1986. (42) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997.

(43) Haynes, C. A.; Beynon, R. A.; Blanch, H. W.; Prausnitz, J. M. Thermodynamic Properties of Aqueous Polymer Solutions: Poly(ethy1ene glycol)/Dextran. J. Phys. Chem. 1989, 93, 5612−5617. (44) Eckelt, J.; Sugaya, R.; Wolf, B. a. Pullulan and Dextran: Uncommon Composition Dependent Flory-Huggins Interaction Parameters of Their Aqueous Solutions. Biomacromolecules 2008, 9 (6), 1691−1697. (45) Bercea, M.; Nichifor, M.; Eckelt, J.; Wolf, B. A. Dextran-Based Polycations: Thermodynamic Interaction with Water as Compared With Unsubstituted Dextran, 2 - Flory/Huggins Interaction Parameter. Macromol. Chem. Phys. 2011, 212, 1932−1940. (46) Eliassi, A.; Modarress, H.; Mansoori, G. A. Densities of Poly (Ethylene Glycol) + Water Mixtures in the 298. 15 - 328 15 K Temperature Range. J. Chem. Eng. Data 1998, 43 (5), 719−721. (47) Ahmad, N.; Ahmed, B.; Bhettani, A. K. Viscometric and Light Scattering Studies of Poly(vinyl Pyyrolidone). J. Chem. Soc. Pak. 1991, 13 (3), 153−156. (48) Pedersen, J. S.; Sommer, C. Temperature Dependence of the Virial Coefficients and the Chi Parameter in Semi-Dilute Solutions of PEG. Prog. Colloid Polym. Sci. 2005, 130, 70−78. (49) Sapir, L.; Harries, D. Macromolecular Stabilization by Excluded Cosolutes: Mean Field Theory of Crowded Solutions. J. Chem. Theory Comput. 2015, 11 (7), 3478−3490. (50) Leron, R. B.; Li, M.-H. High-Pressure Density Measurements for Choline Chloride: Urea Deep Eutectic Solvent and Its Aqueous Mixtures at T = (298.15 to 323.15)K and up to 50 MPa. J. Chem. Thermodyn. 2012, 54, 293−301. (51) Cohen, J. a; Podgornik, R.; Hansen, P. L.; Parsegian, V. a. A Phenomenological One-Parameter Equation of State for Osmotic Pressures of PEG and Other Neutral Flexible Polymers in Good Solvents. J. Phys. Chem. B 2009, 113 (12), 3709−3714. (52) Strey, H.; Parsegian, V.; Podgornik, R. Equation of State for DNA Liquid Crystals: Fluctuation Enhanced Electrostatic Double Layer Repulsion. Phys. Rev. Lett. 1997, 78 (5), 895−898.

G

DOI: 10.1021/acs.jpca.5b11927 J. Phys. Chem. A XXXX, XXX, XXX−XXX