Solvatochromic parameters for solvents of interest in green chemistry
Philip G. Jessop,* David A. Jessop, Dongbao Fu and Lam Phan
Received 26th December 2011, Accepted 10th February 2012DOI: 10.1039/c2gc16670d
Solvatochromic data have been collected from the literature or newly measured for 83 molecular solvents,18 switchable solvents, and 187 ionic liquids that have been cited in the green chemistry literature. Thedata include the normalized Reichardt’s parameter (EN
T), the Nile red λmax, and the Kamlet–Taftparameters (α, β, and π*). Disagreements within the literature about the properties of glycerol and poly(ethylene glycol) have been resolved with new data. The switching of a switchable-polarity solvent (alsoknown as a reversible ionic liquid) by CO2 causes a significant increase in polarity/polarizability (π*) butno change in the basicity (β). A switchable-hydrophilicity solvent undergoes an even greater change inpolarity because it merges with an aqueous phase upon exposure to CO2. Trends observed from the dataof ionic liquids are presented, along with concerns about the best method for determining the Kamlet–Taftparameters.
1. Introduction
If green solvents are to be used industrially, then the solventproperties of the greenest solvents must be quantitatively known.In addition to the obvious physical properties such as meltingpoint, boiling point, and density, the solvation properties such aspolarity and hydrogen-bonding ability need to be quantifiedbefore green solvents can be compared to traditional solvents.Kamlet–Taft solvatochromic parameters are the most compre-hensive and frequently used quantitative measure of solvent
properties. They are known for a large range of traditionalsolvents but much less data is available for green solventsand it is scattered in the literature. This paper is intended torectify that situation by collecting together the literature data,adding some data measured in our lab, and offering somecommentary.
We present a tabulation of solvatochromic data, including theKamlet–Taft parameters, the EN
T parameter, and the λmax for thedye Nile red, for a range of solvents of interest in the field ofgreen chemistry. The solvents include various conventional sol-vents that have been described in the literature as being green,plus bioderived solvents, liquid polymers, fluorous liquids,switchable solvents, ionic liquids, supercritical CO2 and CO2-expanded liquids.
Philip Jessop
Philip Jessop is the CanadaResearch Chair of Green Chem-istry at Queen’s University inKingston, Canada. After hisPh.D. (British Columbia, 1991)and a PDF (Toronto, 1992), hedid contract research for theJapanese government under thedirection of Ryoji Noyori(Nobel Prize 2001). As a pro-fessor at the University of Cali-fornia-Davis (1996–2003) andsince then at Queen’s, Dr.Jessop has studied green sol-
vents and the chemistry of CO2 and H2. He also serves as Tech-nical Director of GreenCentre Canada, a National Centre ofExcellence for the commercialization of green chemistrytechnologies.
David Jessop
David Jessop is a student in theB. Sc. program at the Univer-sity of Waterloo, Ontario, study-ing both science andeconomics. He made spectro-scopic measurements for thispaper as a summer project.
Three Kamlet–Taft parameters exist: α, β and π*, whichquantify hydrogen-bond donating ability (acidity), hydrogen-bond accepting ability (basicity) and polarity/polarizability,respectively.1–4 Together these three parameters can be used topredict a wide range of observables such as rate constants, equili-brium constants, solubilities and spectral frequencies, basedupon data acquired with only a few solvents.4 Each of theseparameters is scaled to two key reference solvents, one set to avalue of 0 and the other to a value of 1. For example, π* is refer-enced to cyclohexane for a value of 0 and DMSO is given avalue of 1.3 Most solvents will have values between 0 and 1 oneach scale.
There are many dyes that can be used to determine π*, andthey do not all report identical values. The most commonly usedmethods are Kamlet and Taft’s original averaging method (theuse of 5 to 7 dyes and the reporting of the average π*)3,4 and theuse of a single dye that is considered to be representative.Laurence et al.5 proposed the use of 4-nitroanisole (OMe inScheme 1) for this purpose. The π* value is calculated usingeqn (1), derived from that in the paper by Marcus.6 Someauthors have chosen to use only the N,N-diethyl-4-nitroanilinedye (NEt2) to determine π*, which is unfortunate becausethat dye suffers from poor bandshape when used for low-polaritysolvents.7 In the tables, we indicate which dyes were used todetermine the π* values given.
π � ¼ 14:57� 4270=λmax;OMe ð1ÞThe β parameter is calculated from the λmax of 4-nitroaniline
and 4-nitrophenol (NH2 and OH in Scheme 1) using eqn (2) and(3), where δ is 1.0 for aromatics, 0.5 for polychlorinated ali-phatic, and 0.0 for all other aliphatic solvents.6 Note that theequations require prior knowledge of π*.
β ¼ 11:134� 3580=λmax;NH2 � 1:125 � π � ð2Þ
β ¼ 12:126� 3460=λmax;OH � 0:57 � π � � 0:12δ ð3ÞThe α parameter is most easily determined from peak separ-
ations in the 13C NMR spectrum of N,N-dimethylbenzamide(BA in Scheme 1) but can also be determined from 4-carbo-methoxy-1-ethylpyridinium iodide (the Z-probe, eqn (4)).6,8 Taftand coworkers assigned an α value of 0.00 to hydrocarbons,
ethers, esters, tertiary amines and N,N-disubstituted amides, aconvention which has continued ever since.
α ¼ 0:0485 � Z � 2:75� 0:46 � π � ð4Þwhere
Z ðkcal mol�1Þ ¼ 28 591=λmax;Z
In addition to the Kamlet–Taft parameters, ET(30) and Nilered data are often mentioned in the literature. ET(30) is ameasure of both polarity and acidity together,9 but is not depen-dent on polarizability.5 It is calculated from the wavelength(in nm) of maximum absorbance of Reichardt’s dye (RD inScheme 1) using eqn (5).10 The value is often normalized usingeqn (6), and the normalized data are presented here. However,ET(30) is quite sensitive to acids, so for solvents that have someacidity, Nile red or ET(33)
10 are used instead. Nile red datais reported as either the λmax (nm) or the ENR, calculated fromeqn (7).11
ETð30Þ ðkcal mol�1Þ ¼ 28 591=λmax;RD ð5Þ
Dongbao Fu
Dongbao Fu is a postdoctoralfellow at Queen’s University,funded by the NSERC Ligno-works research network. He iscurrently researching theextraction and separation ofphenols from lignin pyrolysisoil using switchable solvents.He obtained his Ph.D. inchemical engineering at ChinaUniversity of Petroleum,Beijing and had postdoctoraland research associate pos-itions at National Research
Council Canada and Agriculture and Agri-Food Canada.
Lam Phan
Lam Phan obtained a B. Sc. inchemistry at the University ofToronto. After a two-year MScdegree supervised by Dr.Jessop at Queen’s University,researching various CO2-switchable materials, Lamworked as a research assistanton a development projectfunded by GreenCentreCanada. Lam currently works
in project management for a multi-national curtainwall con-struction company.
ENRðkcal mol�1Þ ¼ 28 591=λmax;NR ð7ÞSolvents chosen for inclusion in this paper have been
described in the literature as being green or having some obviousgreen characteristics. The authors have not judged the “green-ness” of these solvents or performed a life-cycle analysis; theresult of such an exercise would depend strongly on the processfor which the solvent is being considered12 and on the availablealternative solvents. Instead, the authors simply offer a compi-lation of the solvatochromic parameters. Both literature andnewly-measured data are presented.
The following solvents are included:• The Pfizer green-list solvents:13 Water, methanol, ethanol,
1-propanol, 2-propanol, 1-butanol, t-butanol, heptane, acetone,2-butanone, ethyl acetate, and isopropyl acetate. The selection ofthese alcohols and esters was also supported by an environ-mental health and safety assessment by Capello et al.14 Relatedalcohols and esters are also included.
• Biomass derived solvents: Ethyl lactate because of the verylow toxicity, rapid metabolic hydrolysis, rapid biodegradation,and low ecotoxicity.15 γ-Valerolactone because of its low toxi-city.16 2-Methyl tetrahydrofuran because it is easier to removefrom water than diethyl ether.17 While methyl oleate and methylstearate have been evaluated as green solvents,18 the pure com-pounds are too expensive for practical use. However, methyloleate is included here as a representative example of fatty acidmethyl esters, mixtures of which are more affordable green sol-vents.19,20 Limonene has low toxicity21 and is reported to havevalue in preventing some cancers in humans.22 Other biomass-derived solvents include glycerol derivatives,5,23 eucalyptol, andpinene. A more complete listing of the properties of glycerolethers has been published.23
• Cyclopentyl methyl ether24 and dialkoxymethanes25 becausethey are ethers that do not form peroxides readily and becausethey are easy to remove from water. Acetals (glycerol formal,dialkoxymethanes, and dioxolane) have been proposed as sol-vents for paints and coatings because of their low toxicity andecotoxicity.26,27
• Cyclohexane is included as a non-neurotoxic28 alternative tohexane.
• Liquid polymers: poly(ethylene glycol) and poly(propyleneglycol) because of their low toxicity and volatility and reason-able biodegradability. Their use as solvents for catalysis has beenreported by several research groups.29–32
• CO2-related solvents. Supercritical CO2 (scCO2) because ofits low toxicity, lack of smog potential or flammability, and,most of all, because it is a recycled material.33 CO2-expandedliquids (organic solvents containing large amounts of dissolvedCO2 at elevated CO2 pressure) have many of the desirable phys-ical properties of scCO2 and minimize the use of organic sol-vents without having the high pressures of scCO2.
34 Carbonatesolvents are included because they are CO2-derived.
• Ionic liquids (ILs), because of their low volatility and nonfl-ammability.35 There is too much data on the solvatochromicproperties of ILs for them to be all included here, so the reader isinvited to see the original papers. In particular, a very recentpaper by Ab Rani et al.36 contains a valuable discussion on the
choice of dyes for determining the Kamlet–Taft parametersof ILs. The ET(30) values of many ILs were compiled byReichardt,37 while Kamlet–Taft parameters were compiled byChiappe et al.38 and Ab Rani et al.36 Chiappe et al.38 also ana-lyzed the data using principal component analysis. ET(30) valuesfor 36 guanidinium ILs were measured by Bogdanov et al.;39
only a selection are presented here. Solvatochromic parametersfor tetraalkylammonium sulfonate ILs were measured by Pooleet al. in 1989.40 There are also papers on mixtures of ionicliquids with molecular organic solvents41–43 or with water.44,45
More data for ionic liquids can be found in literature com-pilations of ET(30),
10,38,46,47 Nile red,48 and Kamlet–Taftparameters.38,46,47,49
• Fluorous liquids, because of their nonflammability.50
• Switchable solvents, because they offer the potential of redu-cing the amount of solvent used in a process and the energyused in separations.51
• Piperylene sulfone is included because it is a DMSO-mimicthat can be removed at much lower temperatures thanDMSO.52,53
2 Results and discussion
Solvatochromic data for molecular solvents, switchable solvents,and ionic liquids are shown in Tables 1–3.
2.1 Molecular solvents
In an earlier paper,12 one of us commented that there are manyproblematic solvents for which a green substitute is unavailable.Considering only the Kamlet–Taft parameters (not boiling pointor other properties), there is no obvious green substitute formethylene chloride, for example (α = 0.13, β = 0.10, π* =0.82).6 As shown in Fig. 1 (upper graph), there is no greensolvent near that position (shown as an X in the lower rightcorner). Commonly known molecular solvents, as a group, havea wide range of properties, encompassed by the dashed lines inFig. 1. This range was identified by plotting the most commonsolvents on a graph similar to Fig. 1 and then drawing thedashed lines around the area populated by data points (see ref.12). Molecular solvents of interest in green chemistry, becausethere are fewer of them, cover a smaller range of properties (dotsin the figure). There are no aprotic greener solvents that are high-polarity/low-basicity, high-polarity/high-basicity, or low-polarity/high-basicity. Among the protic solvents, there are very few oflow basicity. For applications requiring such solvents, processchemists and formulators will need to choose solvents that arenot particularly green. The only solvents that approach these setsof properties are eucalyptol, dibutoxymethane (DBM), and poly(ethylene glycol) (PEG). To assist in the identification of poten-tially greener options, we have collected the literature data forgreener molecular solvents and measured the properties of others(Table 1).
Solvents that have been studied by more than one researchgroup often have conflicting data. For example, the value ofβ for water varies greatly between reports, for reasons that areunknown. For most solvents, we put the generally accepted
values in Table 1. However, we investigated two disagreementsin particular: PEG and glycerol.
There is some disagreement about the π* value of PEG.57
Singh and Pandey,71 using NEt2 dye only, report that the π*
values of PEG-200, -400, and -600 are 0.91, 0.87, and 0.84,respectively, showing that the π* decreases with increasing MW.However, Kim et al.,70 using an average of five dyes, report a π*for PEG-200, -300, and -400 to be 0.98, 1.00, and 1.08. Not
a The dye used to measure π*. “ave” = the average value obtained from several dyes. μ = estimated from the dipole moment using eqn (8). Otheracronyms defined in Scheme 1. Literature estimates are shown in parentheses. bAssumed value. cMeasured using ET(33) and corrected to the ET(30)scale. d This work. e This work; measured using N,N-dimethylnitroaniline and converted to the π*(OMe) scale using the equation π* = 0.0215λmax −7.6533 (determined by plotting the data for hydrocarbons from Laurence et al.5). f This work; measured using Z-probe.6,8 g This work; calculated froma π* of 1.07 and the ET(30) value of Marcus et al.60 h This work; measured using the NMR method. i The value of 0.586 given in Reichardt10 is atypographical error. j See text.
only are Kim’s numbers higher but the trend with MW isreversed. Their value for PEG-400 is artificially high because ofan outlier result with the dye 4-ethylnitrobenzene; excluding thatoutlier, the average π* is 0.96. We find that the π* for PEG-400is 0.91, which is between the values of Singh and Kim.
The literature also contains disagreement about the propertiesof glycerol. Marcus60 made a rough estimate of 0.62 for the π*of glycerol using eqn (8), where μ is the dipole moment inDebye. This π* indicates that glycerol is much less polar thanethylene glycol or even acetone. Salari et al. reported a dramati-cally different π*, using OMe, of 1.04,66 indicating that glycerolis more polar than ethylene glycol and almost as polar as water.Abbott et al. used NH2 (which is normally used to determine β)to determine a π* of 0.96. We performed 9 measurements usingOMe at 3 different concentrations and found a π* of 1.07. The αvalue of 1.21 for glycerol calculated by Marcus (based upon eqn(9) and his rough π* estimate) seems too high; ethylene glycol isonly 0.90.4 In a subsequent paper, Marcus54 used the 13C NMRspectroscopic method with N,N-dimethyl- and diethyl-benza-mide to determine an α value of 1.06, but that calculation againrelied on the erroneous π* estimate of 0.62. Salari66 andAbbott65 obtained lower α values of 0.93 and 0.88 using Reich-ardt’s dye and their values of π*. We calculated the α twice,based upon our π* of 1.07; using Reichardt’s dye we found an αof 0.90 while using the NMR method we found an α of 0.80.While these two methods do not give identical answers (whichis unsurprising), we suggest that the preponderance of evidencesupports an α of approximately 0.9 and does not support eithervalue suggested by Marcus.
π � ¼ 0:03þ 0:23μ ð8Þ
α ¼ 0:0649ETð30Þ � 2:03� 0:72π � ð9ÞFor several other alcohols, the accepted π* (and hence α)
values were estimated in the literature60 from the dipole momentusing eqn (8) and therefore can not be considered reliable
until π* has been actually measured. These include 1-hexanol,1-octanol, and a number of alcohols not shown in Table 1. An αvalue for 1-hexanol was been measured61 and differs greatlyfrom the estimate. 1-Octanol has, fortunately, been studied indetail by Dallas et al.,63 who determined all three Kamlet–Taftparameters by multiple methods. π* is 0.53 by NEt2 and 0.47 byOMe, β is 0.86 (NH2) or 0.96 (OH), and α is 0.70 (RD), 0.73(bis[α-(2-pyridyl)benzylidene-3,4-dimethylaniline]bis(cyano))iron(II), 0.78 (phenol blue), or 0.82 (NR). This study shows thatthere is significant variation in the value of each parameterdepending on which dye is used to measure it and that themeasured values of π* differ from the estimate.
Because methylenes and methynes with multiple electronega-tive substituents can have elevated α values, we measured the αvalues of diethoxymethane and dibutoxymethane using the 13CNMR/N,N-dimethylbenzamide method54 but found them to beessentially zero (−0.01 and −0.04, respectively). PPG-1200,despite the fact that it has two hydroxyl end groups, gave anessentially zero value.
While the solvents in Fig. 1 are shown as single points(representing observations at standard temperature and pressure),in truth their properties are functions of temperature. This isparticularly true for superheated and supercritical solventsbecause of the greater temperature range. They therefore appearas arcs (Fig. 2). Lu et al.,56 studying water and ethanol, foundthat increasing temperature causes the π* to decrease for bothsolvents, but only in the case of ethanol does the β valuechange significantly. For supercritical CO2, π*, β and EN
T
all increase with pressure. Increasing temperature causes π*and β to decrease74,108–110 and has little effect on the EN
T ofscCO2.
111
CO2-expanded liquids34 have solvatochromic parameters thatare functions of both temperature and CO2 pressure. Thepressure-dependent data77 for two expanded liquids are shown asarcs in Fig. 2. For both acetone and methanol, increasingpressure lowers π* and β, while α is unaffected for methanol andincreases slightly for acetone.
Table 2 Solvatochromic parameters for CO2-triggered switchable solventsa
A switchable solvent, unlike other solvents, has two sets of sol-vatochromic parameters, one set for each of its two forms (for areview of switchable solvents, see Jessop et al.51). Three classesof switchable solvents are presented in Table 2.
First, a switchable polarity solvent (SPS) is a solvent thatchanges its polarity when CO2 is added or removed from thesystem. While there are several such systems, two kinds of SPSare presented here. Certain liquid secondary amines, such asNHEtBu, react with CO2 according to eqn (10), creating an ionicliquid and therefore a significant polarity increase. Similarly, acombination of an organic base, such as an amidine, with analcohol reacts with CO2 to give a more polar ionic liquid (eqn(11)). The solvatochromic data in the switchable solvents litera-ture data is primarily Nile red data (Table 2) but we presentKamlet–Taft parameters for one SPS (Fig. 2 and Table 2). Theπ* of the equimolar mix of DBU (1,8-diazabicyclo[5,4,0]undec-7-ene) and 1-propanol is 0.71 according to the OMe dye (0.80according to NMe2), falling closer to the value of DBU (0.79,measured with OMe) than the value of 1-propanol (0.52).4 Asexpected, the π* value increases significantly when the DBU–propanol mixture is treated with CO2, changing to 0.98 (more
polar than ethylene glycol and similar to many ionic liquids)regardless of whether the OMe or NMe2 dye is used. The βvalue of an amidine-containing SPS cannot be determined usingthe dye 4-nitrophenol because the dye is fully deprotonated bythe amidine. For this reason, 4-nitroaniline was used, along withthe π* from NMe2. The resulting β values were high and nearlyidentical for both the neutral (1.04) and ionic liquid (1.00) formsof the SPS. The β is high, no doubt due to the strong basicity ofthe amidine DBU and possibly also the good hydrogen-bondaccepting ability of the alkylcarbonate anion. The inertness of4-nitroaniline in the SPS is uncertain. There is no other literaturefor the β of alkylcarbonate salts but a high value is reasonablebecause acetate ILs have β values around 1. Despite the presenceof 50 mol% alcohol in the SPS, the α value is negative (with orwithout CO2), which is a curious result; the literature onlyreports zero or positive α values, although many of those zerovalues were arbitrarily assigned.6,57,60 We presume that the extre-mely low α is due to the effect of the strongly basic amidine onthe probe molecule and/or hydrogen-bonding between thealcohol and the amidine. The α value of dried DBU alone is−0.28 (the π*(OMe) is 0.79).
A similar series of SPS has been developed using amidine–amine mixtures (eqn (12)),112,113 but the polarity was measured
a ave = π* reported as the average of values from several dyes. Other acronyms defined in Scheme 1. bMeasured using ET(33) and corrected to theET(30) scale.
using the DAPNE dye and is not directly comparable to the othersolvatochromic scales discussed here. Nevertheless, the λmax
data show that the addition of CO2 to the mixture of amidine andamine (R = hexyl) increases in polarity from 423 nm (similar totoluene) to 438 nm (between acetone and DMF). A change from424 nm to 443 nm (approaching DMF in polarity) occurs whenL-valine methyl ester was used as the amine.
ð10Þ
ð11Þ
ð12Þ
The second class of switchable solvents is the switchablehydrophilicity solvents (SHS), such as CyNMe2.
114,115 In oneform, an SHS is largely immiscible with water, so that the SHSand water form two liquid phases. In the other form, the SHSis miscible with water and the two phases therefore merge.CyNMe2, for example, forms a biphasic mixture with waterwhen CO2 is absent. Even though the liquid amine phase iswater-saturated, the polarity is only moderate. However, whenCO2 is present, the SHS is converted to its water-miscible form(eqn (13)) and the amine and aqueous phases merge, resulting ina significant increase in π* and α and a decrease in β (Table 2and Fig. 2). Removal of CO2 reverses the process.
ð13Þ
The third class of switchable solvents is called “switchablewater”, meaning an aqueous solution of switchable ionicstrength.116,117 It is a solution of an amine or polyamine, such asMe2N(CH2)4NMe2. In the absence of CO2, the solution has alow ionic strength due to a low degree of protonation of theamine by water. In the presence of CO2, the solution has a highionic strength because the amine has been converted to the bicar-bonate salt (eqn (14)). Despite the great change in the ionicstrength, there is little change in the solvatochromic parametersof the aqueous solution; only β changes, dropping slightly whenCO2 is added.
ð14Þ
2.3 Ionic liquids
Of the 187 ionic liquids in Table 3, 135 have data for all threeKamlet–Taft parameters. The abbreviations for the ions aredefined in Schemes 2 and 3.
Fig. 1 The aprotic (above) and protic (below) molecular solvents ofinterest in green chemistry, plotted according to their β and π* values.The solvents include alcohols ( ), ethers ( ), esters ( ), hydrocarbons( ), and others ( ). The X in the lower right corner of the upper figurerepresents the position of CH2Cl2, which while not a green solvent isshown for comparison. The β values of limonene and p-xylene are esti-mates because of the lack of published values. The areas enclosed by thedashed lines represent the range of properties that are possessed by theset of commonly-known solvents, adjusted from ref. 12 to include thepropanediols.
Fig. 2 The β and π* values of a switchable polarity solvent (DBU–PrOH, green squares), switchable-hydrophilicity solvent CyNMe2 (redsquares), CO2-expanded acetone (green circles), CO2-expanded metha-nol (purple),77 superheated water (brown), superheated ethanol (tan),supercritical CO2 (35 °C, hollow circles)74,108,109 and ionic liquids (bluediamonds).
Among the protic ILs, those with a hydroxyl group in one ofthe ions have much higher α values than those containing a pro-tonated amine cation. Among the technically “aprotic” ILs, thelowest α values are 0.31 for the liquids containing piperidiniumand DABCO-derived cations and the highest values are up to
0.80 for the pyrrolidium ILs. The 2-methylimidazolium ILs haveα values of 0.38–0.45, lower than the values for the imidazoliumILs with a hydrogen atom in that position (0.48–0.67).
It is evident from Fig. 2 that the ionic liquids cover only asmall part of the range of possible solvent properties. While con-ventional solvents span 0 to over 1 in both β and π*, ILs onlycover the right part of the diagram (0.81 ≤ π* ≤ 1.20 with oneexception) and especially the lower right part (β < 0.7). Even theILs with octyl or decyl chains have π* values above 0.8. Theoutlier, having a π* value of 0.73, is [MeOctPyrr]NTf2 but onehas to wonder whether the value is correct given that it is so verymuch lower than the values for [MeHexPyrr]NTf2, [MeBuPyrr]-NTf2 or any other known IL. The IL with the highest π* value(1.20) is [MeGlyMor]N(CN)2,
38 thanks to its glyceryl group; allglyceryl ILs38 have very high π* values except for [Glymim]Cl(which is, oddly, reported to be less polar than the hexyl analog[hmim]Cl).
A plot of the β and π* values for the ILs (Fig. 3), colourcoded to indicate the anion, reveals the extent to which anionchoice affects β and π*. For example, ILs with the NTf2
− anion(bistriflamide) are generally less polar/polarizable than thosewith the N(CN)2
− anion. The graph also indicates that the βvalue is strongly controlled by the choice of anion, with the βvalues increasing in the following trend (with average β valuesshown), though there is some overlap in the ranges involved.
NTf2,PF6,BF4,OTf,NðCNÞ2,NO3,RSO4,Cl,OAc
0:24 0:29 0:36 0:41 0:52 0:56 0:73 0:93 0:99
Nevertheless, there is some vertical variation within the graph,meaning that the choice of cation has some weaker effect on β.Other anions having very high β values88 are MeHPO3
−,Me2PO4
−, glycinate36 and to a lesser extent EtSO4−.
Scheme 2 The structures and abbreviations of the cations within theionic liquids. “Gly” = –CH2CH(OH)CH2OH.
Scheme 3 The structures and abbreviations of the anions within theionic liquids.
Fig. 3 Ionic liquids containing the more common anions plottedaccording to their β and π* values and categorized by the choice ofanion. For parameters for which more than one value has been pub-lished, an average of the published values is presented.
Applications of ILs that require a high β include the dissolutionof acetylene118 and cellulose.88
Comparisons of this type can make it easier to identify ques-tionable outliers. For example, all but two of the dicyanamideILs have β values in the narrow range 0.49–0.60. The two excep-tions are [bmim]N(CN)2, which is reported to have a β of 0.71,making it the most basic of all of the dicyanamide ILs, and[emim]N(CN)2, which, according to Zhang et al.83 has a β of0.35, which would make it by far the least basic of all of thedicyanamide ILs. It is unlikely that the [bmim]+ and [emim]+
cations would have such large and opposite effects on the basi-city; more likely one or both data points is erroneous.
Looking at those ILs for which more than one paper hasreported values, it is evident that there is a great deal of inconsis-tency. The pyrrolidinium ILs, for example, show inconsistencyin their EN
T and α values. Although one would not expect the[1-methyl-1-alkylpyrrolidinium]NTf2 ILs (where the alkyl chainranges from propyl to octyl) to differ greatly in their α values,the published values vary randomly between 0.41 and 1.00; thelatter value in particular is in a range normally seen for ILs withalcohol pendant groups or with protonated amine cations. Onecould speculate that the highest α and EN
T values are due to watercontamination. It is known44 that 2 wt% water in an IL can artifi-cially boost the α value of an IL by 0.2 units without signifi-cantly affecting the β and π* values.
Many authors in the ionic liquid field have chosen to useNEt2 to determine π* and ET(30) to determine α. Over 90% ofπ* data for ionic liquids were determined with NEt2, while thatdye is not nearly as commonly used for molecular green solvents(Table 1), where the averaging method and OMe are more oftenchosen. Ab Rani et al.36 argued recently that NEt2 should con-tinue to be used for IL studies because most of the IL literatureto date has used that dye. While that makes sense for the IL com-munity, it is unfortunate for comparisons with molecular sol-vents. Laurence et al.5 showed that NEt2 is unsuitable formolecular solvents because of the poor lineshape of the band inlow-polarity solvents, a problem that is not encountered withOMe. Thus the two fields disagree on the best dye to choose,which makes comparisons rather difficult.
Whether a single π* dye is a suitable substitute for an averageof several dyes is a complicated question. Fortunately, Pooleet al.40 and Ab Rani et al.36 have published enough data that onecan compare π*(NEt2) and π*(OMe) to π*(ave) determined froma common set of four dyes (OMe, NEt2, N,N-diethyl-3-nitroani-line and N-methyl-2-nitroaniline). For eight tetraalkylammoniumalkylsulfonate dyes,40 on average, π*(NEt2) is equal to π*(ave)
while π*(OMe) is too high (Table 4). In contrast, for sevenimidazolium ILs and one pyrrolidinium IL,36 the situation isreversed; π*(OMe) is equal to π*(ave) while π*(NEt2) is high.Assuming that the molecular solvents community will continueto use OMe and the IL community will continue to use NEt2,anyone trying to do solvent comparisons should keep in mindthat the polarities of imidazolium ILs (but not tetraalkylammo-nium sulfonates) are being over-reported compared to molecularsolvents.
The calculation of α from the ET(30) value (via eqn (9)) haslimitations because the ET(30) dye is bleached by acidic com-pounds and because the ET(30) value is mathematically muchless sensitive6 to α and more sensitive to π* than the NMRmethod with N,N-dimethylbenzamide. Thus, errors in π* arecarried through to the calculation of α, as we saw earlier in thecase of glycerol. For example, an error of 0.06 in the π* valueused in the calculation causes the α to be off by 0.04 if ET(30) isused but only by 0.01 if the NMR method is used. The NMRmethod does not, however, work well for aromatic ILs becauseof peak overlap. Ab Rani et al.36 proposed that ET(30), becauseit is zwitterionic, is also affected by Coulombic interactions withILs and not with molecular solvents; this is another reason whya molecular probe like N,N-dimethylbenzamide would be prefer-able in order to obtain α values for ILs that can be comparedwith those for molecular solvents.
Experimental methods
Glycerol formal, isopropyl acetate, pinene, cyclopentyl methylether, 2-methyl tetrahydrofuran and PEG were purchased fromSigma-Aldrich and dried with anhydrous Na2SO4 to a watercontent of <1000 ppm. 1-Propanol (anhydrous), glycerol (<0.1%water), limonene (<0.1% water), ethyl lactate (0.26% water),PPG (0.14% water), γ-valerolactone (0.20% water) and all of thedyes were used as received from Sigma-Aldrich. Diethoxy-methane (>99% purity, <0.1% water) and dibutoxymethane(>98.5% purity, <0.2% water) were used as received fromFutureFuel Chemical Company. DBU (Aldrich, 98% purity) wasrefluxed over CaH2 and distilled under vacuum. To get rid of theremaining trace water, CO2 was bubbled through the DBU andthe resulting precipitate (DBU bicarbonate salt) was removed byfiltration in a glove box.
For measurements of π* and β, the dye was dissolved in thesolvent at two to three concentrations typically ranging from10−5 to 10−4 M and three samples were prepared for each con-centration (6–9 samples in total). Absorbance was measuredwith an Agilent 8453 UV-visible spectrophotometer at roomtemperature. The Kamlet–Taft α values were calculated from the13C NMR spectra of N,N-dimethylbenzamide using the methodof Schneider et al.54 except for the α value of ethyl lactate whichwas determined using the Z-probe.6,8
Conclusions
The selection of a greener alternative solvent to replace a solventof concern requires that the solvent properties of greener solventsbe known. To that end, solvatochromic data for 83 molecular sol-vents, 18 switchable solvents, and 187 ionic liquids have been
Table 4 Average deviations of π*(OMe) and π*(NEt2) from π*(ave)a
for three classes of ILs36,40
Ionic liquidsDeviation ofπ*(NEt2)
bDeviation ofπ*(OMe)b
8 Tetraalkylammoniumsulfonates
0.00 (−0.04 to+0.05)
0.14 (0.00 to+0.36)
7 Imidazolium ILs 0.11 (+0.09 to0.14)
0.00 (−0.01 to+0.03)
[MeBuPyrr]NTf2 0.14 0.00
a π*(ave) calculated from OMe, NEt2, N,N-diethyl-3-nitroaniline andN-methyl-2-nitroaniline. bRange shown in parentheses.
collected from the literature or newly measured. The solventsinclude solvents described as greener in the literature, solventsthat are bioderived, liquid polymers, supercritical CO2, CO2-expanded liquids, switchable solvents, fluorous liquids, andionic liquids. No comment is given on whether these solventsare, or are not, relatively green for any application; that questionis left to future study.
Literature disagreements about the properties of glycerol andpoly(ethylene glycol) have been resolved with new data.
The switching of a switchable-polarity solvent (an equimolarmix of DBU and 1-propanol) by CO2 causes a significantincrease in polarity/polarizability (π*) but not basicity (β). The αvalue of the mixture, which one would expect to be well abovezero due to the presence of 1-propanol, is below zero, presum-ably due to the strong basicity of the amidine and hydrogen-bonding between the alcohol and the amidine. A switchable-hydrophilicity solvent undergoes an even greater change inpolarity because it merges with an aqueous phase upon exposureto CO2. The solvatochromic data for “switchable water” (anaqueous solution of switchable ionic strength) do not change sig-nificantly when CO2 is added.
Analysis of the reported properties of ionic liquids revealsboth structural trends and possible inconsistencies in some of thedata. Of more concern is the fact that the molecular solventscommunity and the ionic liquids community are using differentdyes for the measurement of π*, so that the data are not directlycomparable. The IL community has selected NEt2, which isinappropriate for lower-polarity molecular solvents. The differ-ence in π* values obtained by OMe, NEt2, and the averagingmethod appears to depend on the cation of the ionic liquid.Reported π* values for imidazolium ILs are over-reported rela-tive to molecular solvents on the π*(OMe) scale. The determi-nation of α values requires that the π* is known, but systematicand random errors in the π* value will significantly affect the αvalue much less if N,N-dialkylbenzamide rather than ET(30) isused to determine α.
Acknowledgements
The authors gratefully acknowledge the financial support ofthe Natural Sciences and Engineering Research Council andLignoworks – The NSERC Biomaterials and ChemicalsStrategic Network. PGJ thanks the Canada Research ChairsProgram and the Killam Trusts/Canada Council for the Arts.
Notes and references
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