The structure of protic ionic liquids based on sulfuric acid, doped with excess of sulfuric acid or with water †

Neutron scattering with isotopic substitution was used to study the structure of concentrated sulfuric acid, and two protic ionic liquids (PILs): a Brønsted-acidic PIL, synthesised using pyridine and excess of sulfuric acid, [Hpy][HSO 4 ] (cid:2) H 2 SO 4 , and a hydrated PIL, in which an equimolar mixture of sulfuric acid and pyridine has been doped with water, [Hpy][HSO 4 ] (cid:2) 2H 2 O. Brønsted acidic PILs are excellent solvents/ catalysts for esterifications, driving reaction to completion by phase-separating water and ester products. Water-doped PILs are eﬃcient solvents/antisolvents in biomass fractionation. This study was carried out to provide an insight into the relationship between the performance of PILs in the two respective processes and their liquid structure. It was found that a persistent sulfate/sulfuric acid/water network structure was retained through the transition from sulfuric acid to PILs, even in the presence of 2 moles ( B 17 wt%) of water. Hydrogen sulfate PILs have the propensity to incorporate water into hydrogen-bonded anionic chains, with strong and directional hydrogen bonds, which essentially form a new water-in-salt solvent system, with its own distinct structure and physico-chemical properties. It is the properties of this hydrated PIL that can be credited both for the good performance in esterification and beneficial solvent/antisolvent behaviour in biomass fractionation


Introduction
Protic ionic liquids (PILs) derived from sulfuric acid and amines combine excellent performance across a number of chemical processes, with inherently lower costs than comparable aprotic ionic liquids.Even among PILs, these ionic liquids are amongst the least expensive since both sulfuric acid and triethylamine are cheap commodity chemicals.2][3][4] Ionic liquids formulated from di-and tri-alkylamines with excess sulfuric acid were postulated to be even cheaper, 5 dispensing with the still-popular misconception that ionic liquids are, generically, too expensive for mainstream industrial use. 6][9][10] In esterification reactions, equilibria are strongly shifted towards the products because phase separation of esters from the acidic PIL phase is enhanced when compared to use of concentrated sulfuric acid. 10][13] In both of these examples, enhanced performance arises from doping of the simple hydrogensulfate salt, [Hbase][HSO 4 ], either with an excess of acid or with water, respectively, exemplifying a ''4th generation'' of ionic liquids, 14 where molecular dopants are incorporated into the ionic liquid matrix, at quantities that preserve the key characteristics of an ionic liquid system (that is, without turning it into a concentrated solution of ions) while enhancing physico-chemical characteristics, viz.increased acidity, lowered viscosity or favourably modified phase behaviour.There are two characteristic features of these doped PILs: simplicity of the preparation, and potential complexity of the speciation/liquid structure of the resulting three-component mixtures that requires investigation and understanding.

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There has been a history of debate surrounding the speciation in ionic liquids generated from proton transfer reactions, especially those with non-stoichiometric compositions, which could be described as network vs. cluster models.In 1981, Evans, Arnett and co-workers postulated that ethylammonium nitrate exhibited 3D hydrogen bonded network similar to that of water. 15The liquid network structure of ethylammonium nitrate was only confirmed, through far-infrared spectroscopy and DFT calculations, in 2009. 16This was followed by direct liquid structural investigations using neutron [17][18][19][20] and X-ray 21 scattering that identified nanostructured polar/non-polar domains generated by the extensive hydrogen bonding interactions in the polar regions.Similar structure is also present in PILs formed from other primary alkylamines bearing a C n alkyl chain (n = 2-4) and anions including hydrogen sulfate, thiocyanate, nitrate and formate. 22Weaker acids add further complexity of incomplete proton transfer.For example, mixtures of pyridine and acetic acid were shown to feature a hydrogen-bonded network, which consisted of charge-neutral molecules of acetic acid, with free pyridine sitting in pockets of this network, unprotonated even with a large excess of acid. 23n the other hand, in PILs synthesised from amines and excess of hydrogen halides (w acid 4 0.5, X = F À , [24][25][26][27][28][29] Cl À , 30 and Br À , 31 ) discrete dimeric and oligomeric clusters, and chains of [XÁ Á ÁHÁ Á ÁX] À have been identified, with ample crystallographic evidence in addition to liquid-phase studies.Complex anionic clusters have been proposed in other Brønsted acidic PILs. 10,32,33or example, in mixtures of amines and trifluoroacetic acid (HTFA), where w HTFA 4 0.5, was postulated to contain [TFA(HTFA) x ] À clusters. 34oping ethylammonium nitrate with water resulted in gradual altering of the liquid structure, but even upon the addition of as much as six moles of water, the structure of the hydrated PIL remained ionic liquid-like, distinctly different from homogenous solution. 35These results, along with numerous other contributions, 36 suggest that hydrated PILs form their own class of water-in-salt solvents, a sub-class of a wide family of solvent-doped ionic liquids. 14Consequently, macroscopic properties of hydrated PILs, and their liquids structures, are distinct from both the parent PIL and simple aqueous solution.
Considering the structure of PILs derived from sulfuric acid, a 3D networked structure could be postulated based on the structure of sulfuric acid.Neutron and X-ray scattering studies of H 2 SO 4 have shown that sulfate moieties have an ordered, dense network of hydrogen bonds in concentrated liquid acid, [37][38][39] in the solid state, [40][41][42][43][44] and in aqueous solution. 45,46ith increasing dilution the structure trends to resemble water, albeit with nearest neighbour intermolecular r OÁ Á ÁO bond distances ca.0.2 Å shorter than in pure H 2 O.
8][49][50] The first paper by our group reporting PILs derived from sulfuric acid 10 also postulated the existence of anionic clusters, in analogy to earlier works on trifluoroacetate ionic liquids 25 and other strongly hydrogen bonding anions.FT-IR and NMR spectroscopic studies have been used to justify these assignments, but whereas these early results do show strong interactions between molecules of H 2 SO 4 and [HSO 4 ] À , they could admittedly occur in both the cluster and the 3D network arrangement.
Here, neutron scattering has been used to compare the liquid structures of concentrated sulfuric acid and two pyridine/sulfuric acid PILs containing either excess of the acid or doped with water.[13] Experimental Materials Synthesis of ionic liquids.Three studied samples were: concentrated sulfuric acid, a Brønsted-acidic PIL formed from sulfuric acid and pyridine in 2 : 1 molar ratio, [Hpy][HSO 4 ]Á H 2 SO 4 and a hydrated PIL, formed from concentrated sulfuric acid, pyridine and water in 1 : 1 : 2 molar ratio [Hpy][HSO 4 ]Á 2H 2 O.For each composition, isotopologues containing protiated (H), deuteriated (D) or equimolar mixture of protiated and deuteriated components (H/D) were prepared.A detailed synthetic procedure can be found in the ESI.† All samples (Table 1) were homogeneous liquids at ambient temperature.
Neutron scattering experiments.Neutron scattering data from the seventeen samples (Table 1) were recorded using the SANDALS spectrometer at the ISIS Pulsed Neutron and Muon Source at Rutherford Appleton Laboratory, Oxfordshire, UK.The instrument uses neutrons over a wavelength range 0.05-4.5 Å, giving an accessible Q range of 0.1-50 Å À1 .All samples were measured in quartz cells with 30 Â 30 mm flat-plate geometry and with a path length of either 1 or 2 mm. 2 mm cells were used for samples with high deuteration levels  1) in order to avoid high levels of beam attenuation and multiple scattering.At least 1000 mA of data were collected on each sample.Prior to data collection, quartz cells filled with sample were weighed, placed in a Thermo Scientific vacuum oven (25 1C, o1 Â 10 À2 mbar, 20 min), and weighed again, to ensure tightness of the seal against leakage in the instrument vacuum.Data were collected at 25 1C, with the temperature maintained using an FP50 Julabo heating circulator.Total scattering data were reduced into a differential scattering cross section using the GUDRUN package. 51Data collected on a 3.1 mm vanadium-niobium alloy plate standard was used for calibration, while data recorded on the empty SANDALS instrument and an empty 1 mm quartz cell were used for background subtraction. 52educed data were analysed using the Empirical Potential Structure Refinement (EPSR) 53,54 software to examine the timeaveraged liquid structure.EPSR uses a Monte Carlo simulation approach coupled with Lennard-Jones potentials with atomcentred point charges, combined with basic structural information about the atoms or molecules and the total atomic densities present of the system. 53,54Differences between the experimental and simulated data sets in Q space (i.e. the empirical perturbation potential) are determined, enabling iterative refinement to generate a self-consistent fit to the scattering cross sections obtained from isotopically distinct samples.Simulations were equilibrated over ca.2000-3000 cycles before accumulating and averaging data.The EPSR refinements, in each case, were initialised using an equilibrated Monte Carlo simulation containing 500 or 1000 molecular moieties (pyridine, hydrogen sulfate, molecular sulfuric acid and water, with atomic sites labelled as shown in Fig. 1) depending on the sample.
The full set of reference potential parameters and constraints used in the EPSR simulation model, and the total number of pyridine, hydrogen sulfate and/or sulfuric acid and water and size of each simulation box, corresponding to the experimentally determined molecular densities of the fully protiated mixtures are shown in ESI.† Charges were scaled to AE0.8 e, in line with results from neutron diffraction and MD simulations of ILs. 55ere reduced charges have been shown to reproduce experimental data more effectively.This better simulates effects of electronic polarisability captured when using more expensive polarisable force fields. 56

Selection of samples
The experimental design included the study of concentrated sulfuric acid, a Brønsted acidic PIL formulated with two moles of H 2 SO 4 per one mol of pyridine, [Hpy][HSO 4 ]ÁH 2 SO 4 and a hydrated ionic liquid, [Hpy][HSO 4 ]Á2H 2 O. Adjusting for the actual water content from these idealised compositions, concentrated sulfuric acid (98%) was assumed to contain 0.2 M of either H 2 O (in H 2 SO 4 ) or D 2 O (in D 2 SO 4 ).This was confirmed by examination of the respective measured neutron scattering levels.As such, actual compositions of the examined samples used to model the neutron scattering data were: concentrated sulfuric acid (1 : 0.2, acid : water), Brønsted acidic PIL (2 : 1 : 0.4, acid : pyridine : water) and hydrated PIL (1 : 1 : 2.2, acid : pyridine : water), as summarised in Table 1.
The structure of concentrated sulfuric acid was used as a baseline to compare with the structures of both PILs.][10] The aim of this work was to elucidate its structure (3D network vs. discrete anionic clusters) and understand how differences in the liquid structure of this PIL and sulfuric acid account for their different miscibilities with the ester product.Finally, the hydrated PIL, [Hpy][HSO 4 ]Á 2H 2 O, with a 1 : 1 : 2.2, acid : pyridine : water ratio (ca.13]57 It was anticipated that particularly efficient biomass fractionation reported for this aqueous PIL composition could be tied to the speciation of water in the 1 : 1 : 2.2 mixture.

EPSR modelling and fit to experimental data
The data from sulfuric acid (samples 1-3) was modelled using molecular descriptions of the species present (H 2 SO 4 and H 2 O in a 1 : 0.2 molar ratio) -see Fig. ) have been reported as constituents of sulfuric acid hydrates, [40][41][42][43][44] descriptors were limited to the main moieties, enabling the examination of key associations in these already complex mixtures.This approach is aligned with EPSR analysis of neutron scattering data recorded for other 'neat' acids (acetic acid, formic acid), 58 in which molecular (undissociated) descriptions were used.][60][61][62][63][64][65][66] The Brønsted acidic PIL (samples 4-10) and the hydrated PIL (samples 11-17) were modelled using fully protonated Although pyridine in the presence of sulfuric acid can be considered fully protonated, based on both FT-IR spectroscopy 67 and crystallographic data for pyridinium hydrogen sulfate, 68 an alternative 'free proton' model, 23 (with discrete H + , labelled as H F ), [HSO 4 ] À and H 2 O components in the simulation box) was also examined.However, the degree of pyridine protonation found was significantly underestimated with an average NÁ Á ÁH F coordination number of only 0.2 AE 0.4 and with the maximum in the NÁ Á ÁH F correlation (corresponding to N-H bonds in pyridinium cations) overestimated at 1.6 Å.In consequence, the fullyprotonated cationic pyridinium descriptor was used.
Comparisons of experimental and simulated total structure factors, F(R), and the corresponding Fourier transforms to real space G(r) for each of the isotopically distinct experimental mixtures (Fig. 2) show the quality of fit to the experimental data.With the exception of the region at Q r 1 Å À1 , which is most susceptible to inconsistencies due to inelastic scattering contributions from hydrogen in the data, the fitted data aligns well with experiment.For the neat sulfuric acid systems (1-3, Table 1), a larger degree of scattering at low Q is present in the experimental data than is captured within the EPSR model which may reflect either incomplete subtraction of the inelastic hydrogen background from the data, or additional long range order.Similar profiles are also apparent for the fully deuteriated PILs (samples 6 and 13) suggesting that some hydrogen is present, generating inelastic scattering from the sample in this region.

Centre of mass radial distribution functions
Centre of mass (COM) radial distribution functions (RDFs) were calculated using the SHARM routines within EPSR, for each of the components in the three systems (Fig. 3).The RDFs reveal a remarkable persistence of structure, similar between the three systems:  In concentrated sulfuric acid, the {SO 4 }-{SO 4 } RDF shows a first shell correlation with a maximum at ca. 4.6 Å, with a broad second shell correlation between 6.6-11.5 Å (Fig. 3, top, blue dotted line).The corresponding water-{SO 4 } first shell correlation (Fig. 3, bottom, blue dotted line) is centred around 4 Å (minimum 5.5 Å), and the water-water RDF correlation (Fig. 3, bottom, dotted green) at 3.2 Å.This is significantly longer than water-water distances in bulk water (2.6 Å), confirming that water molecules are confined within sulfuric acid as hydrates.Further supporting this conclusion, the water-water RDF lacks a second shell peak between 4-6 Å, that would have been indicative of 'free' water.
In both PIL systems, the cation-{SO 4 } RDFs are characteristic of strong cation-anion association, typical of ionic liquids. 69The cation-anion RDFs (Fig. 3, top, green curves) exhibit a first correlation peak at 5.0-5.2Å with a shoulder at 4.2 Å, and a second broader correlation peak, indicative of the second shell, at ca. 9 Å.The shoulder at 4.2 Å reflects the oblate topology of the pyridinium cations, allowing two distinct routes to approach its centre of mass.The cation-cation first correlations (Fig. 3, top, red curves) are present at larger separations (maxima at B6 Å), followed by a second shell at 9-10 Å, overlapping with the second shell of cation-anion correlations.][73] In contrast to typical ionic liquid structure, there is very close anion-anion interaction in both PILs.The corresponding {SO 4 }-{SO 4 } associations (Fig. 3, top, blue dashed and solid curves) closely resemble sulfate associations in 'neat' sulfuric acid (first shell at 4.6 Å, second one at 6.6-11 Å).In PILs, the first correlation lengthens slightly (4.8 Å), which can be attributed to the decreasing number of acidic hydrogens in the H 2 SO  This close anion-anion interaction in both PILs results in the unusual presence of both cations and anions in the first coordination shell of {SO 4 }.Their liquid structure combines characteristics of ionic liquid (close cation-anion correlations) and of the parent sulfuric acid (sulfate-sulfate organisation).
The waterÁ Á Á{SO 4 } and waterÁ Á Á[H-Py] + RDFs (Fig. 3, bottom, blue and red lines, respectively) have their corresponding first peak correlations of 4.0 and 4.8 Å.There is no significant difference in the water association with either {SO 4 } or [H-Py] + on changing the water content of the PILs.
In The similarity of RDF correlation profiles (Fig. 3), aside from the water-water correlation, suggests essentially similar liquid structure and character of both PILs, and that the presence of the water (up to 2 moles, ca.17 wt%) does not perturb the ionion structure significantly.Moreover, the hydrogen-bonding network of sulfuric acid appears to be retained as a core structural motif in the PILs, in addition to the typical Coulombic charge screening structure usually observed in ionic liquids.
In the context of applications of Brønsted acidic PILs in esterification, it was initially assumed that phase-separation of the organic phase, that is much more efficient in Brønsted acidic PILs than in concentrated sulfuric acid, may be related to marked differences between the structures of the two liquids: H 2 SO 4 vs. [Hpy][HSO 4 ]ÁH 2 SO 4 .However, the results here suggest that there are greater similarities between the structure-defining associations within these liquids rather than significant differences that would explain the distinctly different phase behaviours of H 2 SO 4 and PILs in esterification reactions.
In contrast, the water-in-salt structure of the hydrated PIL, [Hpy][HSO 4 ]Á2H 2 O, with negatively-charged hydrogen-bonded This journal is © the Owner Societies 2023 network of hydrogen sulfate and water (with or without H 2 SO 4 present) can be expected to have distinctly different properties when compared to anhydrous [Hpy][HSO 4 ], as described in the literature. 11,74,75Furthermore, in esterification reactions, where water and ester are generated, the Brønsted acidic PIL can be envisaged to gradually bind water to form a similar hydrogen sulfate -sulfuric acid -water anionic network, in which water is bound as a hydrate and less likely to hydrolyse the ester product.
Detailed, comparative structural analysis of the three liquid systems studied in this work are provided below.

Correlation and association around {SO 4 } groups
Detailed site-site analysis of contributions to the scattering were made.The positions of first peaks within the partial RDFs of selected site-site correlations and corresponding coordination numbers, calculated to the first minima after the peak, are included within the ESI.† Fig. 4 shows oxygenÁ Á Áoxygen correlations between {SO 4 } groups, originating both from [HSO 4 ] À and H 2 SO 4 , were determined from the first peak in the site-site pRDFs.In all three systems, the 'hetero' S-OHÁ Á ÁOQS mode of correlation is dominant, appearing as a strongly defined peak in the RDF at 2.6-2.7 Å (Fig. 4, green line).In contrast, first contact correlations of S-OHÁ Á ÁHO-S and SQOÁ Á ÁOQS only occur around 3.0 Å, with a peak at ca. 5 Å, corresponding to the separation in the COM RDF.It is therefore evident that the primary mode of association between {SO 4 } groups is S-OHÁ Á ÁOQS hydrogen bonding, retained from sulfuric acid in both PILs.
Correlation distances for concentrated H 2 SO 4 are broadly consistent with the literature. 37,46,59Andreani et al. first reported data derived from direct Fourier transform of experimental X-ray and neutron scattering data, 37 with SÁ Á ÁS and OÁ Á ÁO separation distances of 5.3 Å and 2.42 Å.However, the SÁ Á ÁS separation, extrapolated from summation of intermolecular S-O and intramolecular O-O distances, appears to be over-estimated.Kameda et al. 46 subsequently reported a shorter SÁ Á ÁS correlation of 4.8 Å which is more consistent with simulation between contact pairs in concentrated H 2 SO 4 /H 2 O (4.6 AE 0.1 Å), 76 and small clusters of bulk H 2 SO 4 (4.8Å) 62 where an OÁ Á ÁO distance of 3.1 Å was also reported.Here, the first shell SÁ Á ÁS separation distance was determined as 4.6 Å, with OÁ Á ÁO correlations between OA1 and OA2 sites (S-OHÁ Á ÁOQS) at 2.6 Å, and minima after this first correlation peak at 3 Å.The {SO 4 }Á Á Á{SO 4 } coordination number (N coord = 11 AE 1) between neighbouring moieties is also in agreement with the literature (N coord = 12), 37,59 and confirms the formation of an extended, tetrahedral hydrogen bonded network linked through S-OHÁ Á ÁOQS interactions. 77n both PILs, interatomic distances for the first shell OÁ Á ÁO correlations, as well as corresponding N coord values, are very close to sulfuric acid, suggesting hydrogen bonding of similar strength across the three samples.However, the {SO 4 }Á Á Á{SO Site-site RDFs between water (Ow/Hw) and sulfate units are shown in Fig. 5.For the concentrated acid and anhydrous acidic PIL, the presence of ca.0.2 mole fraction of water leads to a broad peak in the OwÁ Á ÁO1/O2 RDFs between B2.6-3.0Å (Fig. 5, dashed lines).The HwÁ Á ÁOQS correlation at 1.8 Å (Fig. 5, solid blue lines) indicates directional hydrogenbonding retained between the acid and PILs.In the hydrated PIL, [Hpy][HSO 4 ]Á2H 2 O, the magnitude of these correlations increases, as there are more water molecules available, and less S-OH sites.Associated OwÁ Á ÁOQS correlation at 2.8 Å (Fig. 5, dashed blue line) also becomes sharper.Interestingly, the corresponding OwÁ Á ÁO(H)-S correlation at 2.8 Å (Fig. 5, dashed red line) is dramatically decreased.This indicates that the remaining available S-OH sites hydrogen bond preferentially to OQS, rather than water.In short, water in [Hpy][HSO 4 ]Á2H 2 O (absence of strong Brønsted acid) binds preferentially as HwÁ Á ÁOQS, not as OwÁ Á ÁHO-S, acting as hydrogen bond donor, rather than hydrogen bond acceptor.

Water-water association
In the low-water samples, H 2 SO 4 and [Hpy][HSO 4 ]ÁH 2 SO 4 , the OwÁ Á ÁOw distance is longer than in 'bulk' water, and the corresponding coordination number is low (N coord E 0.5), which is indicative of isolated water molecules, strongly associated with the sulfate structure.In contrast, the OwÁ Á ÁOw correlation in [Hpy][HSO 4 ]Á2H 2 O (Fig. 6, green line) has a first peak at 2.8 Å, comparable to that in pure water, and the coordination number (N coord = 4.1 AE 2) slightly lower than that of bulk water (N coord = 4.7). 78However, the O w Á Á ÁO w RDF does not have the distinctive second peak around 4.5 Å, which would have been indicative of a long-range tetrahedral order seen in bulk water, or its larger clusters.
The combination of water-like first shell and unlike-water second shell has been reported for 'bound' water in inorganic molten salt hydrates, 79 and is consistent with the [Hpy][HSO 4 ]Á 2H 2 O PIL having a water-in-ionic liquid structure, 14 but approaching the transition to a concentrated salt solution.Hydrogen bond donation from S-OH sites (O1/OA1) to the pyridinium cation is not observed.Consequently, we can see water molecules acting as hydrogen-bond donors to [HSO 4 ] À anions through HwÁ Á ÁOQS interaction, and as hydrogen-bond acceptors from [H-Py] + cations through, OwÁ Á ÁH-Py interaction.The presence and directionality of water molecules within the first shells of both {SO 4 } and [H-Py] + species demonstrates the structure-forming nature of water molecules, which contribute to the overall hydrogen bond network, reinforcing the sulfate/ sulfuric acid network and bridging cations and anions. 14,80

Conclusions
The RDFs indicate that anion-cation correlations between protonated [H-py] + cations and [HSO 4 ] À anions typical of ionic liquids (and molten salts) 69 are formed in both the 'anhydrous' acidic PIL and 'hydrated' PIL with charge screening between alternate oppositely charged ions.However, in addition to the formation of this typical cation-anion ionic liquid structure, the overall tetrahedral network structure, directed by hydrogenbonding around the {SO 4 } groups in 'neat' sulfuric acid is persistent and retained in the PILs.This is complemented by interactions with the [H-Py] + cations (as can be seen in the presence of correlation nodes between [H-Py + ] and OQS sites in 2 : 1 : 0.4 'anhydrous' acidic PIL) and increasingly, by replacement of acidic S-OH hydrogen bond donors by water molecules in the 1 : 1 : 2.2 'hydrated' PIL to retain the supramolecular {SO 4 }-network present in the parent sulfuric acid.
As such, it is clear that the 'anhydrous' acidic 2 : 1 : 0.2 PIL retains many features of concentrated sulfuric acid in terms of both acidity 10 and network structure, while simultaneously adopting the anticipated anion-cation correlation pattern characteristic of ionic liquids.Consequently, difference in phase behaviour between sulfuric acid and PILs, which result, for example, in different performances as solvents for biomass fractionation or esterification reactions, cannot be attributed directly to differences in the liquid structures as the 'anhydrous' acidic PIL strongly resembles 'neat' sulfuric acid.The addition of 2 moles of water (ca.17 wt%) provides additional hydrogen bond donation capacity to complement and replace the diminished number of S-OH hydrogen-bond donors present in the parent acid.This results in a change in the nature of the first shell hydrogen-bond donors around [HSO 4 ] À anions, but not in the overall pattern of hydrogen bonding.This 'hydrated' sulfuric acid : pyridine : water (1 : 1 : 2.2) system has an equivalent water content to that of the aqueous alkylammonium PILs reported by Hallett et al. 4,11,12,57,[81][82][83][84][85][86] as media for delignification for cellulosic biomass, with ca. 15 wt% water in the IL.It is clear that the water molecules here are present as 'bound' water participating in the ionic liquid solvation structure and not as 'free' water.That is, the system can be viewed as one with water-in-IL rather than as a concentrated IL-in-water environment.
This finding allows for certain speculations in terms of phase behaviour.In esterification reactions, where water and ester are generated, the ionic liquid gradually binds water incorporated in the hydrogen sulfate network, without the formation of 'bulk' water that can contribute to the reverse reaction of ester hydrolysis.The resulting hydrated PIL is more hydrophilic, which contributes to a lower affinity to the ester products and, in consequence, enhanced phase separation.Likewise, it appears that the ''composite'' anionic structure of water-doped PIL has a higher propensity to dissolve lignin, its structure being different from that of anhydrous IL, but unlikely to contain ''bulk'' water at the optimised ratios of 10-40% (1 : 1 : 2 composition amounts to ca. 20% water by weight).
These conclusions are aligned with the available experimental evidence, but nevertheless remain speculative.Further evidence can be provided by the study of solvation of model compounds in wet and dry PILs by neutron scattering, more advanced computational approaches, or thermodynamic studies of the energy of solvation.

Fig. 1
Fig.1Atom types used in the EPSR simulation models for the pyridinium cation, hydrogen sulfate anion, molecular sulfuric acid and water.

Fig. 5
Fig. 5 Site-site pRDFs between water Hw (solid line) and Ow (dashed line) and sulfate SQO (blue) and S-OH (red) oxygens in the three systems showing hydrogen-bond donation from the water Hw to the SQO2 oxygen (1.8 Å) and hydrogen-bond acceptance at Ow from S-OH 2.8 Å).
+ and {SO 4 }, and between [H-Py] + and water Ow, are shown in Fig. 7.In both PILs, the hydrogen-bond donating N-H site of [H-Py] + has equally close contact with hydrogen bondaccepting sites in {SO 4 } and in water.The N1Á Á ÁO2 and N1Á Á ÁOw correlations occur at 2.7 Å in [Hpy][HSO 4 ]ÁH 2 SO 4 (Fig. 7, black solid lines) and at 2.8 Å in [Hpy][HSO 4 ]Á2H 2 O (Fig. 7, black dashed lines).Carbon atoms of pyridine are separated by 3.4-3.6Å from both OQS and Ow sites; these are consistent with weak contacts at the van der Waals separation distances.
contrast, water-water RDFs show a marked difference.In H 2 SO 4 and in [Hpy][HSO 4 ]ÁH 2 SO 4 , that is with low water content, H 2 OÁ Á ÁH 2 O correlation is found at 3.2 Å, slightly more intense in [Hpy][HSO 4 ]ÁH 2 SO 4 .In [Hpy][HSO 4 ]Á2H 2 O, this distance decreases to 2.7 Å, showing both greater self-association of water molecules and a larger number of correlations, as indicated by the increase in intensity.This distance is close to H 2 OÁ Á ÁH 2 O correlation in bulk water (2.6 Å).However, lack of significant second shell correlation (4-6 Å) indicates that there are no large-size water clusters and [Hpy][HSO 4 ]Á2H 2 O retains water-in-IL rather than IL-in-water characteristics.