Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 3922−3927
Amphiphilically Nanostructured Deep Eutectic Solvents Samila McDonald,† Thomas Murphy,† Silvia Imberti,‡ Gregory G. Warr,§ and Rob Atkin*,∥ †
Priority Research Centre for Advanced Fluids and Interfaces, Newcastle Institute for Energy and Resources (NIER), The University of Newcastle, Newcastle, New South Wales 2308, Australia ‡ STFC, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom § School of Chemistry and Sydney Nano Institute, University of Sydney, Sydney, New South Wales 2006, Australia ∥ School of Molecular Sciences, The University of Western Australia, Perth, Western Australia 6009, Australia Downloaded via WASHINGTON UNIV on July 4, 2018 at 10:30:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Deep eutectic solvents (DESs) are neoteric liquids produced by mixing a highmelting-point salt and a molecular hydrogen-bond donor. Amphiphilic (self-assembled) liquid nanostructure, which is key for many of the useful properties of the related ionic liquid class, has not previously been experimentally demonstrated in DESs. Here we show how amphiphilically nanostructured DESs can be prepared using primary ammonium cations. The bulk structure of alkylammonium bromide (alkyl = ethyl-, propyl-, and butyl) and glycerol DESs at a 1:2 mol ratio is examined using neutron diffraction and empirical potential structure refinement fitting. Analysis reveals cation alkyl chain association, which is the signature of amphiphilic liquid nanostructure, in all systems, which becomes better defined with increasing chain length. The ability to form amphiphilically nanostructured DESs will enable the translation of ionic liquid properties associated with liquid nanostructure to DESs.
A
of ionic liquids, from solvency to reaction outcomes, lubrication, and so on.21 Molecular dynamic simulations on DESs composed of alkylamides and LiClO4 have been performed, and X-ray and neutron structure factors have been extracted from the simulation boxes.22 These structure factors had a low-Q peak suggestive of a repeating length scale in the DESs, which is consistent with liquid nanostructure. However, experimental validation of these simulated structure factors is absent. Here the bulk structure of alkylammonium bromide (alkyl = ethyl-, propyl-, and butyl) and glycerol DESs at a 1:2 mol ratio is examined using neutron diffraction (ND), a well-established technique for probing inter- and intramolecular interactions in ionic liquids.21 The ND spectra are fit using empirical potential structure refinement (EPSR)23,24 to generate atomic and molecular-level structural information. Recently, the first ND/ EPSR study of a DES was performed for the most widely studied DES, ChCl:urea.25 Attractive interactions between the choline polar groups (−N(CH3)3+ and −OH) and Cl− meant these groups associated together, whereas urea preferentially H-bonded to chloride. Details regarding the synthesis of multiple neutron diffraction contrasts for the three DESs and the neutron diffraction experiments are provided in the Supporting Information. Figure 1 shows the chemical structures of the alkylammonium cations, bromide anion, and glycerol molecules, along with the labels used to identify each atom. Where
decade ago Abbott et al. introduced the term deep eutectic solvent (DES) to describe a liquid produced by mixing a high melting point salt and a molecular hydrogenbond donor at the eutectic composition, where the melting point (MP) depression is greatest.1−3 DESs are typically formed using inexpensive, biocompatible, low-toxicity components.2−5 The most widely studied DESs combine choline chloride with urea, ethylene glycol, or glycerol, but many other salts (e.g., phosphonium or sulfonium cations with various halides6−10) and molecular componentsacids (e.g., malonic acid, leuvilinic acid, etc.), amides, and polyols11−15have been examined. Whereas it is frequently held that the stoichiometry of the eutectic mixture must be an integer ratio related to Hbond geometry, recent molecular dynamic modeling suggests that the situation is more complex than this and that DES formation is the result of multiple interactions between various DES species.7,16−19 DESs were originally conceived as cheaper and more sustainable alternatives to ionic liquids, retaining the advantages of their low vapor pressures, wide liquid-state temperature range, and broad electrochemical window while avoiding the downsides of costly synthesis and toxic components.20 A critical structural feature that has emerged for ionic liquids is the existence of amphiphilic liquid nanostructure,21 which arises from strong cohesive interactions between the charged moieties of the ions, leading to the formation of polar domains that solvophobically exclude (usually cation) alkyl chains into apolar regions. These polar and apolar domains percolate throughout the liquid, forming a bicontinuous sponge-like structure.21 Amphiphilic nanostructure has been found to be key to many of the useful properties © XXXX American Chemical Society
Received: June 4, 2018 Accepted: July 1, 2018 Published: July 2, 2018 3922
DOI: 10.1021/acs.jpclett.8b01720 J. Phys. Chem. Lett. 2018, 9, 3922−3927
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The Journal of Physical Chemistry Letters
regions rich in different groups. Hydrogens are colored to match the atom to which they are bonded, and glycerol carbon atoms are yellow to differentiate them from the gray cation carbons; that is, NH3 atoms are blue, cation alkyl CHx atoms are gray, bromide atoms are maroon, glycerol OH atoms are red, and glycerol CHx atoms are yellow. Visual inspection of the simulation boxes reveals that bromide usually resides near polar atoms (red glycerol OH and blue cation −NH3+). In all three systems, distinct regions rich in glycerol can be discerned. However, from the image, the extent to which cation alkyl chains associate into continuous domains is unclear for EABr:glycerol. Segregation of cation alkyl chains into separate domains becomes obvious for PABr:glycerol (Figure 3D) and pronounced for BABr:glycerol (Figure 3G), where it is clear that cation alkyl chains form continuous regions that separate domains rich in the ammonium, bromide, and glycerol polar groups. The extent and nature of the amphiphilic nanostructure is more clearly seen in Figure 3B,E,H, which shows the same snapshots but with ions only (glycerol invisible), and Figure 3C,F,I, which shows the complementary snapshots for glycerol only (ions invisible). From these it is clear that glycerol forms a continuous, sample-spanning network with all three cations, which is not surprising given its concentration. The ions-only boxes show the association of alkyl chains to produce ion clusters filling the voids in the glycerol-only boxes. Both the extent of clustering of alkyl chains and the voids in glycerol become more pronounced as cation alkyl chain length increases. This evolution in nanostructure is consistent with the increase in the peak intensity noted for the deuterated contrasts in Figure 2. Pair correlation functions (gij(r)) reveal the positions of atom−atom pairs as a function of their radial separation, normalized to their bulk density. The first peaks in the gij(r) functions correspond to the first coordination shell of nearest neighbors. Figure 4 shows cation alkyl−alkyl gij(r) functions for EABr:glycerol, PABr:glycerol, and BABr:glycerol. In all DESs, the maximum in the CM···CM correlations between terminal methyls is at 3.8 Å, with a shoulder at 5 Å. This 3.8 Å peak position is similar to the CM···CM peak positions found in protic ionic liquids: 3.8 Å for ethylammonium nitrate (EAN),28 3.7 Å for propylammonium nitrate (PAN),29 and 4.0 Å for ethylammonium formate (EAF).26 Notably, the position of the CM···CM maximum for a 50:50 vol % mixture of EAF:glycerol, which is conceptually similar to the systems probed here, is 3.9 Å,30 which is essentially the same as that for the three DESs
Figure 1. Structure and atomic labels for the ethylammonium (EA+) cation (top left), propylammonium (PA+) cation (top center), butylammonium (BA+) cation (top right), bromide (Br−) anion (bottom left), and glycerol molecule (bottom right). Nitrogen atoms are blue, carbon atoms are gray, hydrogen atoms are white, oxygen atoms are red, and bromide atoms are maroon.
the methyl carbons on the three different cation alkyl chains are described collectively, the label CM is used. Figure 2 shows the measured neutron diffraction (ND) data (colored circles) along with the EPSR fitted F(Q) (solid black lines) for the 1:2 DES EABr:glycerol, PABr:glycerol, and BABr:glycerol for wavevectors (Q) from 0.2 to 20 Å−1. For each DES, all contrasts have a peak at ∼1.5 to 1.6 Å−1, which corresponds to a 4.2 Å repeat spacing in the bulk liquid. This peak, which is also present in ChCl:urea,25 originates from multiple short-range atom−atom correlations, such as ion−ion, ion−glycerol, or glycerol−glycerol interactions.25,26 Notably, the deuterated contrasts in Figure 2 have an additional peak at low Q, which is absent from ChCl:urea. As the cation alkyl chain length increases, the peak intensity increases, and its position shifts to lower Q for EABr:glycerol, PABr:glycerol, and BABr:glycerol; the peak positions of 0.5, 0.4, and 0.3 Å−1 correspond to real-space repeat distances of ∼13, 16, and 21 Å, respectively. This clearly indicates structure in the DESs larger than the nearest-neighbor distances between ions or glycerol and is consistent with self-assembled liquid nanostructure.21 The trends in peak position and intensity are consistent with those seen in ionic liquids, where increasing the cation alkyl chain length leads to larger domains and better defined nanostructure.26,27 Snapshots of an equilibrated EPSR simulation box after convergence to the data with all four deuteration schemes for EABr:glycerol, PABr:glycerol, and BABr:glycerol are shown in Figure 3. Figure 3A,D,G shows all atoms colored to highlight
Figure 2. Measured neutron diffraction F(Q) (colored circles) and fitted EPSR F(Q) (black solid lines) for (A) EABr:glycerol, (B) PABr:glycerol, and (C) BABr:glycerol as indicated. The data are offset for clarity. 3923
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Figure 3. Snapshots of fitted bulk structure for 1:2 mol/mole CxABr:glycerol DESs at 300 K. In the first column, all atoms are shown (300 CxA+, 300 Br−, and 600 glycerol) colored as follows: Cation CnH2n+1 (gray), NH3+ (blue), bromide (maroon), glycerol OH (red), and CH2 (yellow). The second and third columns show ions only (300 CxA+ and 300 Br−) and glycerol only (600 glycerol), respectively, in conventional colors, viz. N (blue), C (gray), H (white), O (red), and Br (maroon).
domains. This is consistent with a disordered bilayer-like arrangement of alkyl chains with terminal methyls closest to their core. Table 1 presents the corresponding average CM···CM coordination numbers, which increase with cation alkyl chain length: 1.7 for EABr:glycerol, 1.9 for PABr:glycerol, and 2.05 for BABr:glycerol. This is consistent with the observation from the simulation boxes (Figure 3) that cation alkyl group segregation increases with chain length. These values are lower than the average CM···CM coordination numbers determined for alkylammonium protic ionic liquids, which generally lie between 2.4 and 3.2. These coordination numbers represent the averages of a broad distribution (standard deviation 0.8 to
and clearly shows that amphiphilic nanostructure can be present even at high concentrations of the molecular H-bond donor (in this case, glycerol). For all DESs, the CM···CM−1 gij(r) functions are bimodal, with two moderately intense peaks at ∼3.8 and 4.7 Å. The maximum in the C1···C1 gij(r) function is significantly wider than that for CM···CM for all DESs and increases with alkyl chain length from 5.5 to 6.0 Å to 7.0 Å for EABr:glycerol, PABr:glycerol, and BABr:glycerol, respectively. The order of separations between cation alkyl chains is therefore CM···CM > CM···CM−1 > C1···C1, which indicates that terminal methyl carbons are on average closer together and more strongly associated than the methylene carbons within the nonpolar 3924
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Figure 4. Partial pair correlation functions gij(r) for cation carbons in (A) EABr:glycerol, (B) PABr:glycerol, and (C) BABr:glycerol.
Table 1. CM···CM Coordination Numbers (average ± standard deviation) and Cut-off Range (r, in Å) for EABr:Glycerol, PABr:Glycerol, and BABr:Glycerol Derived from gij(r) Functions DES
CM···CM
r (Å)
EABr:glycerol PABr:glycerol BABr:glycerol
1.7 ± 0.8 1.9 ± 0.8 2.05 ± 0.9
4.50 4.73 4.89
0.9), which is also consistent with disorder in the apolar domains. This is attributed to the high volume fraction of glycerol in the DES, which reduces the apolar volume fraction and changes the packing arrangements of the cation alkyl chains. Spatial density functions (SDFs), which represent the most probable 3D arrangement of terminal methyls on adjacent cations, are presented in Figure 5. The highest 20% probability surfaces show a strong preference for bilayer-like, tail-to-tail orientation that wraps around the methyl terminus due to various forms of disorder including intercalation. For both EABr:glycerol and PABr:glycerol, the CM···CM SDF wraps around the central atom, but for BABr:glycerol, four distinct lobes are observed, indicating a greater degree of ordering at the core of the nonpolar domains. Better defined methyl packing arrangements with increasing cation chain length have also been noted for nanostructured protic ionic liquids.31 Cluster analysis reveals the extent to which long-range structures arise from such local arrangements. Figure 6 shows cluster analyses for CM···CM or CM···CM−1 atom pairs in cation alkyl chains in all three DESs, lying within the second coordination shell of 6 Å (see Figure 4). This captures the
Figure 6. Cluster analysis of CM···CM or CM···CM‑1 neighbors in cation alkyl chains in the alkylammonium bromide:glycerol DESs. The power law line is the percolation threshold defined as p(n) = n‑2.2.
entirety of the ethylammonium alkyl chain and the same hydrocarbon moiety length for propylammonium and butylammonium, enabling direct comparison. The predicted power law for a 3D percolation threshold (N ∝ n−2.2, where N is the number of clusters of size n)32,33 is represented by the straight line. EABr:glycerol and PABr:glycerol follow the theoretical line up to large cluster sizes but then deviate clearly above the percolation line for clusters larger than ∼80 molecules. BABr:glycerol additionally shows some smaller clusters above the percolation threshold. Such a positive
Figure 5. SDF plots of CM···CM correlations for EABr:glycerol, PABr:glycerol and BABr:glycerol. A 20% probability surface is shown up to 5 Å distance. 3925
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micelle concentrations. The salt concentration in the DESs probed here is significantly higher again, so electrostatic repulsions between ions in charged regions will be even lower. Thus the low curvature sponge structure is consistent with the results for ionic surfactants in low dielectric media. The ability to produce amphiphilically nanostructured DESs opens new pathways for DES design, control of forces between ions, and the dissolution of more hydrophobic solutes. This provides a mechanism for enhancing the utility of DESs by incorporating properties related to nanostructures from their ionic liquid cousins while retaining the many advantages (low cost, ease of preparation, etc.) intrinsic to DESs.
deviation up to the simulation box size (300 cations) indicates that the nonpolar domains are bicontinuous on this scale. However, unlike comparable ionic liquid systems,34,35 the probability of finding such large clusters is rather low. This is primarily due to the high volume fraction of glycerol in these systems. Pair correlation functions, coordination numbers, and SDF plots for ion−ion, ion−glycerol, and glycerol−glycerol interactions for the three DESs are presented in Supporting Information Figures S1−S6 and Tables S2−S4. Across all of this data, there is very little variation between the three DESs, indicating that charge group packing and glycerol arrangements in the polar regions do not depend on the cation alkyl chain length. The stark exception is the coordination number data for Br···CM in Table S3, which is 1.6 for EABr:glycerol, 1.1 for PABr:glycerol, and 0.8 for BABr:glycerol. Reduced contact between bromide and the methyl group as alkyl chain increases is consistent with increased clustering of alkyl chains and greater alkyl chain volume. Broadly speaking, the polar domain structure depends on Coulomb and H-bonding interactions that determine local packing of ion charge groups, charge groups, and glycerol and between glycerol in these DESs. This is similar to what has been reported previously for 50:50 vol % EAF:glycerol.30 For both of the DESs studied here and the EAF:glycerol mixture, the glycerol C···C correlations are similar to those for pure glycerol36 and glycerol water mixtures;37 that is, relative to one another, the glycerol carbon atoms occupy spatially similar positions. In contrast, the O···O correlation peaks have lower intensities in the DESs and EAF:glycerol compared with pure glycerol due to the replacement of hydrogen bonds between glycerol and the salt replacing glycerol−glycerol hydrogen bonds. It is likely that amphiphilic nanostructure has not been experimentally demonstrated previously in DES due to the widespread use of cholinium cations. Cholinium is terminated by a hydroxyl group, which disrupts solvophobic interactions between alkyl chains, thereby preventing apolar domains from forming, as seen in the protic ionic liquid ethanolammonium nitrate.28 In this work, we have shown that using a methyl-terminated primary cation “switches on” solvophobic self-assembly of cation alkyl chains in DESs to produce nanostructured solvents. This grows conceptually out of our finding that even ethyl groups are sufficient to engender amphiphilic nanostructure in the bulk liquid state of protic ILs38 and their solutions.39,40 The analysis of the neutron diffraction results reveals that alkyl chains associate into disordered, bilayer-like structures even for the cation ethyl chain in EABr:glycerol and that association and ordering increases with alkyl chain length. These DESs differ from their closest corresponding ionic liquids, the alkylammonium nitrates, thiocyanates, and formates in the structure of their polar domains, viz. the volume fractions of polar phase (glycerol), the charge density, and H-bond capacity of the anion. This affects the extent to which polar/apolar segregation propagates into a bicontinuous network. Changing anion and molecular H-bond donor should be a route to tune their amphiphilic nanostructure. Previous studies of ionic surfactant aggregation in ethylene glycol41 and glycerol−water42 revealed that electrostatic interactions at aggregate surfaces decrease rather than increase compared with in water, even though the solvent dielectric constant is lower. This was attributed to the higher solution ionic strength resulting from the much higher surfactant critical
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01720. Experimental and data fitting details, pair correlation functions, coordination numbers, and SDF plots for ion−ion, ion−glycerol, and glycerol−glycerol interactions for the three DESs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Gregory G. Warr: 0000-0002-6893-1253 Rob Atkin: 0000-0001-8781-7854 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by an ARC Discovery Project (DP130102298), an ARC Future Fellowship (FT120100313) for R.A., and a grant of beamtime from ISIS. S.M. is grateful to the Australian Government and the University of Newcastle for a Research Training Program for a PGRA.
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