896
J. Phys. Chem. B 2008, 112, 896-905
Protic Ionic Liquids: Physicochemical Properties and Behavior as Amphiphile Self-Assembly Solvents Tamar L. Greaves,† Asoka Weerawardena,† Irena Krodkiewska,† and Calum J. Drummond*,†,‡ CSIRO Molecular and Health Technologies (CMHT), Bag 10 Clayton, Vic 3169, Australia, and CSIRO Materials Science and Engineering (CMSE), PriVate Bag 33, Clayton MDC, Vic 3169, Australia ReceiVed: August 23, 2007; In Final Form: October 18, 2007
The physicochemical properties of 22 protic ionic liquids (PILs) and 6 protic molten salts, and the selfassembly behavior of 3 amphiphiles in the PILs, are reported. Structure-property relationships have been explored for the PILs, including the effect of increasing the substitution of ammonium cations and the presence of methoxy and hydroxyl moieties in the cation. Anion choices included the formate, pivalate, trifluoroacetate, nitrate, and hydrogen sulfate anions. This series of PILs had a diverse range of physicochemical properties, with ionic conductivities up to 51.10 mS/cm, viscosities down to 5.4 mPa‚s, surface tensions between 38.3 and 82.1 mN/m, and densities between 0.990 and 1.558 g/cm3. PILs were designed with various levels of solvent cohesiveness, as quantified by the Gordon parameter. Fourteen PILs were found to promote the selfassembly of amphiphiles. High-throughput polarized optical microscopy was used to identify lamellar, hexagonal, and bicontinuous cubic amphiphile self-assembly phases. The presence and extent of amphiphile self-assembly have been discussed in terms of the Gordon parameter.
Introduction Protic ionic liquids (PILs) are a class of ionic liquids (ILs) which are formed through proton transfer from a Brønsted acid to a Brønsted base. Like other ILs, they are defined by having a melting point less than 100 °C. PILs have not received a particularly large share of the literature on ILs.1 This is despite their having many useful properties and potential applications,1 often arising from their protic nature, including as self-assembly media,2-6 reaction media and catalysts for organic reactions,7-9 biological applications,10-13 as proton conducting electrolytes for polymer membrane fuel cells,14-16 and as explosives.17-19 Significant studies have been conducted on the structureproperty relationships for aprotic ionic liquids (AILs), including a broad range of anions and cations, particularly imidazolium based cations and fluorinated anions.20-24 In comparison, little has been done on PILs.1,25-27 We have previously investigated a series of protic ionic fused salts, including many PILs, containing a primary ammonium cation with a simple carboxylate or inorganic anion. The effects of structural changes such as hydroxyl groups, alkyl branching, and increasing alkyl chain lengths on the thermal and physicochemical properties26 and on their ability to promote self-assembly of amphiphiles5 were investigated. The PILs used in this current investigation were designed using previously identified structure-property relationships for PILs,26 and extending the structural features to include secondary and tertiary ammonium cations, along with methoxy groups within the cation. We are particularly interested in the behavior of PILs as amphiphile self-assembly solvents, and a consideration in the design of this new series was to produce PILs which were * To whom correspondence should be addressed.
[email protected]. † CSIRO Molecular and Health Technologies (CMHT). ‡ CSIRO Materials Science and Engineering (CMSE).
E-mail:
capable of promoting the self-assembly of amphiphiles, and which had higher Gordon values. The Gordon value gives a measure of the cohesiveness of liquids, and hence an indication of the driving force for self-assembly. The Gordon value is given by eq 1, where γ is the air-liquid surface tension and Vm the molar volume.2
G ) γ/Vm-1/3
(1)
Previously, it was identified that the presence of hydroxyl groups in PILs led to higher surface tensions, and hence higher Gordon values.5 In this current series of PILs, the cations were selected to investigate the effect of multiple hydroxyl groups, and of methoxy moieties upon the Gordon value, and consequently upon their ability to promote the self-assembly of amphiphiles. A series of 28 protic fused salts were prepared and characterized, with the cations and anions used shown in Figures 1 and 2, respectively. Of the 28 protic fused salts, 22 were PILs, and to our knowledge, it is the first time that many of these have been reported in the literature. Structure-property correlations were developed for this series, and where appropriate compared with previously reported PILs.5,6,26 The ability of these new PILs to promote the self-assembly of the amphiphiles hexadecyltrimethylammonium bromide (CTAB), myverol 18-99K, and phytantriol was investigated and the liquid crystal phase (LCP) behavior reported. Investigating amphiphile self-assembly by varying PIL properties is a different strategy to varying the molecular structure of the amphiphiles.28 Experimental Section The amines used were methylamine (40% in water, Fluka Chemika), ethylamine (70% in water, Fluka Chemika), ethanolamine (>99%, Fluka Chemika), dimethylamine (40% solution in water, Sigma-Aldrich), trimethylamine (45% solution in water, Fluka), diethylamine (99.5%, Aldrich), triethylamine
10.1021/jp0767819 CCC: $40.75 Published 2008 by the American Chemical Society Published on Web 01/01/2008
Protic Ionic Liquids
J. Phys. Chem. B, Vol. 112, No. 3, 2008 897 TABLE 1: Aqueous pKa Values of the Acid and Amines Used pKa
pKa Acids
PV F TFA
5.03 3.75 0.5
MA DMA TMA EA DEA
10.64 10.72 9.74 10.75 10.98
N HS
-1.5 -3
TEA EOA DEOA TEOA
10.76 9.44 8.88 7.77
Amines
TABLE 2: Solid State Thermal Properties of the Fused Salts, Including Their Glass Transition Temperature, Tg, Devitrification Temperature, Tc, Unassigned Phase Transition Temperature, Tp, and Melting Point, Tm (All Temperatures Are in °C; the Enthalpies (kJ/mol) of the Phase Changes for Tc, Tp, and Tm Are Contained in Parentheses, with Positive Values Corresponding to Exothermic Transitions and Negative Values to Endothermic Transitions) Figure 1. Structures and abbreviations of the cations used: methylammonium (MA), dimethylammonium (DMA), trimethylammonium (TMA), ethylammonium (EA), diethylammonium (DEA), triethylammonium (TEA), ethanolammonium (EOA), diethanolammonium (DEOA), triethanolammonium (TEOA), 2-hydroxy-1,1-bis-hydroxymethyl-ethylammonium (TRIS), 2-methoxy-ethyl-ammonium (2MEAF), and 2-(2hydroxy-ethoxy)-ethyl-ammonium (22HEEA).
Figure 2. Structures and abbreviations of the anions used: formate (F), trifluoroacetate (TFA), nitrate (N), hydrogen sulfate (HS), and pivalate (PV).
(45% solution in water, Fluka), 2-hydroxy-1,1-bis-hydroxymethyl-ethyl-amine (>99%, Aldrich), 2-methoxy-ethyl amine (>98%, MERCK), 2-(2-hydroxy-ethoxy)-ethyl-amine (98%, Aldrich), diethanolamine (MERCK, was distilled, fraction collected at 122 °C and 6.8 × 10-1 mmHg), and triethanolamine (BDH, was distilled, fraction collected at 152 °C and 2 × 10-1 mmHg). The acids used were formic acid (98%, Ajax Chemicals), pivalic acid (puriss, Fluka), nitric acid (69% in water, Merck), sulfuric acid (>96%, Biolab), and trifluoroacetic acid (99%, Aldrich). The amphiphiles used were hexadecyltrimethylammonium bromide (CTAB) (KODAK, U.S., used as received), myverol 18-99K (approximately 70% monoolein, gift from Bronson and Jacobs, supplied by Quest International, melting point 35 °C), and phytantriol (96% purity, Aldrich, melting point 5-10 °C, boiling point 130 °C). The water contents of the PILs, as determined from Karl Fischer titration, were all less than 1% (see Table 2). The reported salts were prepared by dropwise addition of acids to the equimolar quantities of the respective amines, at 0 °C or below, depending on the freezing point of the reaction mixture during synthesis. In cases where one of the substrates, or the reaction product that formed, was a solid, solvents were applied to make the reactants miscible. Excess solvent was removed on a rotary evaporator, and final traces of water were
fused salt
water (%)
EAFc DEAF TEAF EOAFc DEOAF TEOAF MAN DMAN MAFc DMAF MAHS DMAHS EANc DEAN TEAN EOANc DEOAN TEOAN EAHSc EOAHS TEOAHS 22HEEAF 22HEEAN 22HEEAHS 22HEEATFA 2MEAF 2MEAN TRISF TRISN EAPV EOAPV MATFA EATFA EOATFA EAAc EOAAc
0.38 0.44 0.19 0.55 0.60 0.44 0.40 0.42 0.42 0.41 0.48 0.56 0.22 0.13 0.84 0.72 0.42 0.32 0.31 0.62 0.55 0.66 0.34 0.54 0.71 0.58 0.71 0.68 0.44 0.20 0.72 0.45 0.51 0.34 0.12 0.47
Tga -106 -109 -119 -85 -78
-114 -114 -97
-82
Tcb
Tpb
-51 (59) -74 (4.3)
-71 (17) -33 (2.7)
-15 (-61) 4 (-2.6)
81 (-8.7)d 26 (-4.8) -13 (86) -38 (-0.7)
-52 (6.3)e -56 (29) -44 (-5) 63 (-6.4)d 7 (-6.1) -40 (103)
-84 -40 (70) -88 -63d -17 (13.9)f -78 -79 -70 -64 -98 -49 -40 (-0.4)g 94 (-0.9) -42f,h 11 (9.8)i -71 (-1.9)d 67 (-3.3)d
-67
-37 (90)
Tmb
67 (-26.7) 111 (-3.8)d 80 (-7.3) -13 (-2.7) 93 (-28.0) -21 (-5.4)e 9 (-101) 105 (-6.5)d 115 (-5.9) 51 (-100) 72 (-26.8) 80 (-26.2) 40 (-88) 53 (-691)
37 (-13.5) 109 (-26.4) 60 (-17.2) 122 (-4.9)d 103 (-8.1)d 61 (-12.8) -80 (-2.7) 58 (-10.0) -20, -10 (1.1)j 89 (-23.1) 1 (-24) 87 (-203)
a Tg from DSC traces recorded at 5 °C/min. b Tc, Tp, and Tm from DSC traces recorded at 2.5 °C/min. c Values from ref 26. d Tm and Tp distinguished using thermograph (visual) measurements. e Peaks poorly formed at 2.5 °C, so values from 5 °C/min scan. f Only observable at 10 °C/min and not at 2.5 or 5 °C/min. g Only observable at 5 °C/min and not at 2.5 or 10 °C/min. h Only observable after preheating to above Tm, with the disorder of the liquid state thought to be responsible for the formation of the glassy state on cooling. i Only observable after preheating sample to above Tm, scan rate 5 °C/min. j Only observable after preheating sample to above Tm, scan rate 10 °C/min.
removed by lyophilization. The solvents used were methanol (in preparation of EATFA, EOATFA, DMAF, TEAF, DEAN, DEOAF, TEOF, 2MEAN, and 22HEEAN), ethanol (in prepara-
898 J. Phys. Chem. B, Vol. 112, No. 3, 2008 tion of TEOAHS, DEOAN, TEOAN, 22HEEATFA, and 22HEEAHS), chloroform (in preparation of TRISF, 2MEAN, 22HEEAF, and TRISN), and butanone (in preparation of EAPV and EOAPV). Fresh samples were used for all of the experiments in order to minimize possible complications caused by amide formation and moisture uptake. The thermal and physicochemical characterization techniques used on the protic fused salts were previously described.26 The penetration scans were conducted as previously described for myverol and phytantriol5 and for CTAB.6 The combination of trimethylamine with formic acid produced trimethylammonium formate (TMAF), which was extremely odorous, and this PIL was not characterized. PIL Design. The cation and anion combinations for designing the PILs took into account previous structure-property relationships identified for PILs.26 There are many anions and cations available, which means a vast number of PILs could be prepared. Consequently, it is essential to have structure-property relationships to aid in the prediction of which ions will lead to the desired properties for specific applications. Generally for ILs, small cations with high asymmetry lead to low Tg values,25 with many factors important for achieving low Tm, mainly having steric hindrance and minimizing hydrogen bonding.29 Low densities tend to be the result of bulkier cations and anions, with hydroxyl groups significantly increasing density, generally due to hydrogen bonding.26 Previously, it was found for primary ammonium cations and/ or carboxylate anions that increasing the number of methyl groups in the alkyl chain led to lower glass transition temperatures, greater viscosities, and lower surface tensions. The same trends were observed for Tg and η, when a hydroxyl group was present on the cation or anion, with the hydroxyl groups having a much larger effect than increasing the number of methyl groups.26 Carboxylate anions tend to have the lowest thermal stability due to amide formation.26,30 The ionicity of the PILs will be affected by how completely the proton is transferred from the acid to the base, the degree of aggregation, and complexation of the ions. The aqueous pKa values for the precursor acids and bases have been reported as being predictive of the behavior of the acids and bases in PILs.27,31 In particular, a ∆pKa value greater than about 8-10, where ∆pKa ) pKa(base) - pKa(acid), has been reported to produce PILs with close to ideal ionicity.27,31 The aqueous pKa values for most of the precursor acids and bases used are given in Table 1. It is evident that the pKa values of the amine bases have little variation in comparison to the acids. The inorganic acids of nitrate and hydrogen sulfate both have sufficiently low pKa values to have ∆pKa > 8 for all of the amines. Choice of Cation. While primary ammonium cations have been well represented in PILs, there has been very little use of secondary and tertiary ammonium cations. Consequently in the current work, dimethyl-, trimethyl-, diethyl-, triethyl-, diethanol-, and triethanolammonium cations were chosen to observe the trends associated with increased substitution of alkyl and alkylhydroxyl chains of the ammonium cation. Previously, the presence of hydroxyl groups, on either the cation or anion, has led to highly beneficial properties, especially in producing PILs which are good self-assembly media. Following on from that, cations were included which contained more than one hydroxyl group, such as the 2-hydroxy-1,1-bishydroxymethyl-ethyl-ammonium (TRIS) cation, which contains three hydroxyl groups within a branched structure, as well as the diethanol- and triethanolammonium cations. In addition, the
Greaves et al. TRIS and triethanolammonium cations have a similar structure which further enables the effect of increased substitution to be explored. Last, primary ammonium cations containing methoxy moieties were included, namely, 2MEA and 22HEEA, where the latter also has a hydroxyl group present. The methoxy containing PILs were chosen to see whether the oxygen within the chain led to similarly good properties as hydroxyl groups. Non primary amine cations were also chosen to explore the steric impact on amide formation in carboxylate systems. The chemical structures of the cations used are represented in Figure 1. The MA, EA, and EOA cations have been widely used in PILs, and hence were included to allow good comparisons of the different anions to previous PILs. Choice of Anion. The anions used in this investigation are shown in Figure 2 and include two organic anions, pivalate and formate, two inorganic anions, nitrate and hydrogen sulfate, and one fluorinated anion, trifluoroacetate. The formate anion has previously been shown to lead to PILs with many particularly good properties, such as high ionic conductivities, low viscosities, and generally low melting points.26,27,30,32 However, there is a very high tendency for ammonium cations and formate to form amides.26 Unfortunately, the amide formation can occur from ambient temperatures, and hence limits their usability. In this current investigation, the formate anion was used, since it enabled the effects of the cation to be compared against the large library of other PILs containing formates. The pivalate anion was selected as a potential candidate to replace the formate anion. It was thought that the highly branched nature, see Figure 2, might decrease the amide formation, while the small size might maintain the beneficial formate properties. The high pKa and high symmetry of the pivalate anion was not desirable, but it was still thought to be an interesting anion to trial. The nitrate and hydrogen sulfate anions were previously shown to lead to good PILs, with high thermal stability, fairly low melting points, and good ionic conductivities. This investigation used them to extend the library of PILs containing these inorganic anions, and to enable further comparisons between them, organic and fluorinated anions. The effect of fluorinated anions was investigated using trifluoroacetate. This specific fluorinated anion was selected due to its hydrocarbon analogue, acetate, previously being used in the creation of PILs. Thermal and Physicochemical Results. A series of 28 protic fused salts were prepared from the cations and anions described above. Of these, 22 were classified as PILs, due to having melting points below 100 °C, and 6 were classified as protic fused salts, due to having higher melting points. Their solid state and liquid state thermal properties are reported in Tables 2 and 3, respectively. The boiling points, Tb, in Table 3, were tentatively assigned for the PILs. The column labeled “amide” states the boiling point of the amide, where reported in the literature, which could form between the cation and anion of the protic fused salt. Unlike many AILs, these PILs have a nonnegligible vapor pressure, with some losing weight from ambient temperatures, as shown by thermogravimetric analysis (TGA). The temperature where weight loss began is included in Table 3, with “Amb” used to refer to those salts where this occurred from ambient temperature. Some physicochemical properties of the 11 salts which were liquid at room temperature are reported in Tables 4 and 5. Some of the PILs previously reported by our group26 were integral to the identification of the trends in the following discussion, and
Protic Ionic Liquids
J. Phys. Chem. B, Vol. 112, No. 3, 2008 899
Figure 3. Tg versus density. Region A denotes PILs containing the formate anion, and region B denotes those with the HS anion. The two points not in region A or B have the TFA anion (+) or N anion (4).
TABLE 3: Liquid State Thermal Properties of the Fused Salts, Measured from the DSC Scans Taken at 20 °C/min (Amide Boiling Points Are Included Where Available for the Amide Which Would Result between the Cation and Anion; the Temperature Where Weight Loss Began Is Included TWL, Where “Amb” Indicates the Salt Lost Weight from Ambient Temperature; all Temperatures Are in °C) fused salt
T1
T2
EAFa DEAF TEAF EOAFa DEOAF TEOAF MAN DMAN MAFa DMAF MAHS DMAHS EANa DEAN TEAN EOANa DEOAN TEOAN EAHSa EOAHS TEOAHSc 22HEEAF 22HEEAN 22HEEAHS 22HEEATFA 2MEAF 2MEAN TRISF TRISN EAPV EOAPV MATFA EATFA EOATFA EAAa EOAAa
180 126 202 192 182 197 269b 210b 182 150-180 258b 348 242b 194b 226b
195 194
245b 260, 264b 323 310 170 143 278 185 242 130 202b 182 240b 132 211 249 253 247 156 210
T3
amide 195 170-179
290 295 260 277 198 352
198 152 362
272 320 258 189
318
320 272 174 220 250
244
174
248
286 205 156-157, 157-160 194-234
189 275
189 210
TWL Amb Amb Amb 50 70 Amb 200 160 Amb Amb 240 Amb 220 160 140 230 245 190 Amb Amb Amb 260 Amb 190 50 Amb 150 230 50 60 130 Amb 170 70 Amb
a Exothermic peaks, all others are endothermic. b Multiple peaks from 170 to 360; the three peaks listed are the main ones. c Values from ref 26.
hence, the thermal and physicochemical properties of EAF, EOAF, EAA, EOAA, EAN, and EOAN have been included in the following tables and figures where appropriate. The general trends identified from the data due to changing the substitution of the ammonium cation and for the different anions are summarized in Table 5. The effect of substitution
Figure 4. Arrhenius behavior of the PILs at room temperature scaled against Tg.
for the series of MAF, DMAF and EAF, and DEAF and TEAF were grouped separately to EOAF, DEOAF, and TEOAF due to some differences in their behavior. It is important to note that TEOAF was a solid at room temperature, and since TMAF could not be characterized, that made TEAF the only tertiary ammonium cation in these series. The physicochemical data from Table 4, combined with the Tg values from Table 2, were used to construct a plot of Tg versus density, shown in Figure 3. Distinct regions were observed for the PILs containing the F anion and for those containing the HS anion. The Arrhenius plot in Figure 4 has been scaled against Tg and shows that these PILs all occupy a similar region, and all have good fragility, which is consistent with other PILs which have been reported.26,32 Hence, their viscosity is likely to decrease with increasing temperature at a greater rate than that expected by the Arrhenius relationship. There was little difference in the refractive index between the PILs, with the values ranging from 1.41 to 1.48, which is consistent with their similar densities and atomic compositions. The molar refractivity, which is directly related to the polarizability of a material, showed a tendency to increase with increasing substitution. A model was previously proposed relating the liquid-vapor surface structure of PILs containing primary ammonium cations with carboxylic or inorganic anions.26 This model is extended here to include secondary and tertiary ammonium cations, and fluorinated anions. For the previous PILs, we have reported the highest surface tension was 65.0 mN/m for ethanolammonium formate (EOAF).26 For this series of PILs, the surface tensions were generally higher, with EOAHS having the highest reported value for a PIL of 82.1 mN/m, which was higher than the value for water (72 mN/m) and those for DEOAF, 22HEEAN, and DMAHS, having values greater than 60 mN/m. The basis of the surface tension model was that the charged group of the ions will direct itself toward the bulk of the liquid, with the hydrocarbon chains toward the air. Surface tension will be decreased by higher surface packing efficiency and increased by higher cohesiveness of the PIL. Many of these new PILs which were liquid at room temperature had relatively low viscosities, with DMAF, DEAF, and TEAF having viscosities less than 10 mPa‚s. Liquid Crystal Phase Behavior of Amphiphiles in PILs. The liquid crystal phase (LCP) behavior of myverol 18-99K and phytantriol was investigated in the PILs which were liquid at room temperature, and the phases are reported in Tables 6 and 7, respectively. The phase behavior for CTAB also included PILs which had Tm values less than 80 °C, and the CTAB-
900 J. Phys. Chem. B, Vol. 112, No. 3, 2008
Greaves et al.
TABLE 4: Physicochemcial Properties of the Fused Salts Which Were Liquid at Room Temperature (Protic Ionic Liquids), Including Density, G, Viscosity, η, Liquid-Vapor Surface Tension, γLV, Refractive Index, nD, Ionic Conductivity, K, Molecular Weight, Mw, Molar Volume, Vm, Molar Refractivity, MR, Parachor Parameter, P, Gordon Parameter, G, and Molarity, Mol. (All Measurements Were Made at 25 °C) fused salt
F (g/cm3)
γLV (mN/m)
η (mPa‚s)
nD
κ (mS/cm)
Mw (g/mol)
Vm (cm3/mol)
MR (cm3/mol)
P
G (J/m3)
Mol. (g/dm3)
EAFa DEAF TEAF EOAFa DEOAF MAFa DMAF DMAHS EAHSa EOAHS 22HEEAF 22HEEAN 22HEEAHS 22HEEATFA 2MEAF
1.039 0.990 1.028 1.184 0.988 1.087 1.046 1.558 1.438 1.407 1.172 1.305 1.388 1.343 1.105
38.5 38.3 42.5 65.0 63.4 43.1 44.2 69.5 56.3 82.1 52.2 61.8 57.7 40.1 42.7
32 5.4 5.8 220 494 17 8.1 120 128 309 61 281 1310 1010 38
1.4344 1.4264 1.4298 1.4705 1.4806 1.4336 1.4147 1.4426 1.4489 1.4578 1.4600 1.4659 1.4684 1.4162 1.4472
12.16 13.13 13.05 3.40 0.77 43.8 51.10 8.10 4.40 4.90 2.54 1.12 0.32 0.36 4.59
91.11 119.16 147.22 107.11 151.16 77.08 91.11 143.16 143.16 159.16 151.16 168.15 203.22 219.16 121.14
87.69 120.37 143.21 90.46 153.00 70.91 87.10 91.89 99.56 113.12 128.98 128.85 146.41 163.19 109.63
30.33 41.04 49.13 33.25 57.14 24.49 29.06 32.24 35.31 40.73 46.61 47.01 53.63 54.60 38.77
218.4 299.4 365.6 256.9 431.7 181.7 224.6 265.3 272.70 340.5 346.6 361.2 403.5 410.5 280.2
0.866 0.775 0.812 1.448 1.185 1.041 0.997 1.540 1.215 1.698 1.032 1.223 1.094 0.733 0.891
11.40 8.31 6.98 11.05 6.54 14.10 11.48 10.88 10.04 8.84 7.75 7.76 6.83 6.13 9.12
a
Values from refs 5 and 26. Measurements from these references were made at 27 °C.
TABLE 5: Summary of the General Trends Observed for Increasing the Number of Alkyl Chains on the Cation (e.g., Series of EAF, DEAF, and TEAF) and Similarly for Increasing the Number of Alkylhydroxyl Chains (e.g., Series of EOAF and DEOAF) (The Effects on the Thermal and Physicochemical Properties Are Summarized as Increasing (Inc), Decreasing (Dec), No Significant Change (NC), or Ambiguous (A); Note That Some of the Data Did Not Fit These General Trends)
Tg Tm Tb Td F γLV η nD κ MR Vm Mol. G P Agg
increased number of alkyl chains
increased number of alkylhydroxyl chains
NC-Dec Inc Inc A Dec NC Dec Dec Inc Inc Inc Dec Dec Inc Dec
Inc a Incb A Dec NC Inc Inc Dec Inc Inc Dec Dec Inc Dec
a Only TEOAF had a T b m value from this series. Secondary > tertiary > primary.
TABLE 6: Regions of Stability for the Liquid Crystal Phases of Myverol 18-99K in the PILs water29 a DMAF DEOAF 2MEAF 22HEEAF 22HEEAN DMAHS EOAHS 2MEAN
isotropic
lamellar
inverse hexagonal
20-86
20-48 17-43 22-61 22-35 8-47 23-42b
84-98
31-63 26-59 28-47
TABLE 7: Regions of Stability for the Liquid Crystal Phases of Phytantriol in the PILs water30 DMAF DEAF DEOAF 2MEAF 22HEEAF 22HEEAN DMAHS EOAHS 2MEAN
isotropic
lamellar
inverse hexagonal
22-48
22-35 4-18 4-22a 8-42 10-31a
44-60
28-42
8-17 6-20 13-43 10-79 37-41
a
Poorly developed texture. Assigned as lamellar but could possibly be inverse hexagonal.
TABLE 8: Regions of Stability for the Liquid Crystal Phases of CTAB in the PILs hexagonal water31 DMAF DEAF DEOAF 2MEAF 22HEEAF 22HEEAN DMAHS EOAHS EATFA TEOAF 2MEAN DEOAN TEOAN MATFA
isotropic
lamellar
25-90
40-185
70 to >107
88 to >107 91-104
71 to >106 67 to >107b 54 to >105b
78-106
50 to >200 88 to >107 98 to >104a 88 to >107 88-104 83 to >106 82 to >106
79-96 74 to >106 73 to >107 114 to >132
89-96 80-106 88-107 116-132 94 to >108
88 to >106 92 to >124 81-102 87 to >107 c 83 to >108b
a Very weak phase. Poorly developed texture. b Poorly developed texture. Assigned as lamellar but could be hexagonal. c Evidence of very weak lamellar phase texture (see Figure 5).
22-51 24-52 37-46
a Monoolein-water investigation, where monoolein is the main constituent of myverol 18-99K. b Poorly developed texture. Assigned as lamellar but could possibly be inverse hexagonal.
PIL LCPs are reported in Table 8. The water-amphiphile LCP temperature ranges have been included in Tables 6-8 for comparison. Representative phase textures are shown in Figure 5.
The LCPs seen for myverol 18-99K and phytantriol in the PILs were an isotropic phase (considered to be bicontinuous cubic), lamellar, and/or inverse hexagonal. For CTAB-PIL systems, the phases were lamellar, isotropic (considered cubic), and hexagonal. The isotropic band was assigned as bicontinuous cubic on the basis of the rheology of the phase and its position in the concentration gradient sequence. Frequently, there were fewer phases observed for the amphiphiles in the PILs compared to those in water, with only six
Protic Ionic Liquids
J. Phys. Chem. B, Vol. 112, No. 3, 2008 901
Figure 5. Penetration scans showing the LCPs for the PIL-surfactant systems. The neat PIL is on the left and the neat surfactant on the right, except for (i) where the highest surfactant concentration is in the center of the image. The phases are described from left to right. The arrows indicate the interface of cubic and neat PIL for the myverol 18-99K and phytantriol surfactant systems. (a) Myverol 18-99K in 22HEEAF at 26 °C: neat 22HEEAF, isotropic (cubic) and lamellar. (b) Myverol 18-99K in DMAHS at 44 °C: neat DMAHS, inverse hexagonal. (c) Myverol 18-99K in 22HEEAN at 41 °C: neat 22HEEAN, isotropic (cubic), lamellar (or inverse hexagonal), melted myverol. (d) Phytantriol in DMAHS at 31 °C: neat DMAHS, inverse hexagonal, liquid phytantriol. (e) Phytantriol in DEOAF at 28 °C: neat DEOAF, isotropic (cubic), lamellar, liquid phytantriol. (f) Phytantriol in 2MEAN at 37 °C: neat 2MEAN, lamellar, liquid phytantriol. (g) CTAB in EATFA at 94 °C: neat EATFA, lamellar. (h) CTAB in TEOAN at 132 °C: neat TEOAN, hexagonal, isotropic (cubic), CTAB crystals. A lamellar batonette feature within the CTAB crystals is circled. (i) CTAB in 2MEAN at 98 °C: from outside to center, the phases are hexagonal, isotropic (cubic), lamellar.
of the PIL-CTAB systems containing all three phases which were seen in the aqueous system. Of the PILs which were liquid at room temperature, only TEAF, 22HEEATFA, and 22HEEAHS did not support LCPs with any of the amphiphiles, and DEAF did not with myverol 18-99K. The differences between the PILs and other solvents, such as water, capable of promoting the self-assembly of amphiphiles have been previously discussed, including generalized phase diagrams for the PIL-amphiphile systems.5,6 The behavior of these new PIL-amphiphile systems was consistent with the phase diagrams. There were some differences in the thermal ranges of these PIL-amphiphile systems compared to those previously reported. The most significant example was the EOAHS-phytantriol system reported here which had a stable inverse hexagonal phase from 10 to 79 °C, where previously the maximum temperature for this phase was 67 °C for ethylammonium glycolate (EAG)-phytantriol, and is considerably higher than that in water (maximum stability 60 °C). Previously, all of the PILs which had a LCP with CTAB had a lamellar phase, as just lamellar, or lamellar, cubic, and hexagonal. For these new PILs, the lamellar phase was mostly present, except for DMAHS, EOAHS, and only with extremely weak texture for TEOAN. DEOAF led to the greatest phase diversity, forming isotropic (cubic) phases with all three surfactants. In particular, the DEOAF-myverol 18-99K system had a stable isotropic phase up to 68 °C which was higher than the 59 °C for EOAA which was the previous maximum seen for PIL-myverol 18-99K
systems, though still less than aqueous systems where this cubic phase is stable up to 86 °C. The Gordon parameter has previously been used to give an indication of the driving force of solvents toward amphiphile self-assembly,2 with ethylammonium butyrate (EAB) with a G value of 0.576 J/m3 having the lowest reported value of a solvent capable of promoting the self-assembly process.5,6 These new PILs which were liquid at room temperature had G values between 0.733 J/m3 for HEEATFA and 1.698 J/m3 for EOAHS, where previously the highest reported G value for PILs was 1.215 J/cm3 for EAHS.6 Discussion Stability at Room Temperature. The amount of amide formation detected for the protic fused salts containing the formate or pivalate anions is shown in Table 9. No amide formation was observed after 15 months for the two salts containing the pivalate anion; however, this could be due to the slow reaction kinetics, since these were solids. No amide formation was observed for the TEAF, TEOAF, and TRISF, whereas the other formate containing PILs all had some amide present. Most likely, the steric hindrance from the branching and tertiary substitution prevented the amide formation. The steric hindrance for DMAF and DEAF compared to MAF and EAF did not reduce the amide formation but rather led to a greater rate of amide formation. The PILs and molten salts containing the TFA anion appeared to have no amide
902 J. Phys. Chem. B, Vol. 112, No. 3, 2008
Greaves et al.
TABLE 9: Amount of the Fused Salts Remaining after Long Term Storage at Room Temperature EAPV EOAPV MAFa DMAF DMAF EAFa DEAF TEAF DEOAF TEOAF TRISF 2MEAF 22HEEAF 22HEEATFA MATFA EATFA EOATFA a
salt remaining (%) 100 100 23 37 8 75 75 100 60 100 100 29 20 100 100 100 100
time (months) 15 15 54 6.5 12.5 50 15 15 14.5 12.5 14 14 14 15 15 17 17
Figure 6. Walden plot of the PILs, where Λ is the equivalent conductivity and η-1 is the fluidity. The solid line represents “good” ionic liquid behavior, whereas the PILs are classified as “poor” ionic liquids according to the classification scheme devised by Angell et al.31,32
Values from ref 26.
formation. However, this could in part be due to slow reaction kinetics, since MATFA, EATFA, and EOATFA were all solid at room temperature, and while 22HEEATFA was liquid at room-temperature, it had a very high viscosity. Aggregation and Ionicity. An important characteristic of the PILs is the degree of proton transfer from the acid to the base, which influences their ionicity. For an ideal PIL, there would be complete proton transfer, though it is expected that there will be some nonionic acid and base species present, though previously these have not been detected.33 If the proton transfer process does not result in the vast majority of the species being present as ions, then the resulting salt is not categorized as an ionic liquid. The ionicity of the PILs is complex and depends upon how completely the proton is transferred from the acid to the base, aggregation, and complexation of the ions. Previously, we were unable to find any evidence of molecular species (precursor acid or base) present in the PILs.33 It has been proposed by Kohler et al. that PILs formed from primary amines with carboxylic acids form aggregates of the ion species.34,35 An indication of the ionicity of the PILs was determined using a Walden plot of log(Λ) against log(fluidity), where Λ is the equivalent conductivity (conductivity per mole of charge) and fluidity is the inverse viscosity.32 It is reported that greater differences between the aqueous pKa of the acid and base, ∆pKa, will lead to greater proton transfer.31 The Walden plot for these PILs is given in Figure 6, with the solid line representing good ionic behavior. According to this plot, these PILs had fairly poor ionicity, which could be due to complexation, aggregation, or incomplete proton transfer from the acid to the base. According to the Walden plot, DEAF and TEAF had the least ionic nature, due to being furthest from the “ideal” line, whereas EOAHS and DMAHS had the highest ionicity. The excess boiling point, ∆Tb, has been reported as providing an indication of how complete the proton transfer is within the fused salts, and hence indirectly providing information about the ionicity.27,32 ∆Tb is defined as how much higher the boiling point of the protic fused salt is compared to the average of the boiling points of the acid and amine precursors (for stoichiometric salts), with ∆Tb ) 0 if there is no proton transfer. The plot of ∆Tb against ∆pKa is shown in Figure 7. There is a rough monotonic trend of increasing ∆Tb with ∆pKa. It is difficult to determine how much of an influence the proton transfer from the acid to the base has upon the ionicity,
Figure 7. Excess boiling point, ∆Tb, versus ∆pKa. Region A denotes PILs containing the formate or pivalate anion, and region B denotes those with the TFA anion.
TABLE 10: Aggregation Number, Mvisc/M, of the PILs PIL
Mvisc/M
PIL
Mvisc/M
PIL
Mvisc/M
MAFa DMAF EAFa DEAF TEAF EOAFa
7.0 5.7 6.2 4.4 3.6 5.8
DEOAF EAHSa EOAHS EANa EOANa DMAHS
4.4 4.3 4.0 5.2 4.9 4.2
2MEAF 22HEEAF 22HEEAN 22HEEATFA 22HEEAHS
4.8 4.0 3.8 3.2 3.4
a Physicochemical data from ref 26 used to calculate the aggregation.
and hence upon the interpretation of conductivity and ionicity data. Other processes within the PILs, such as aggregation and ion complexation will also lead to ∆Tb > 0. The amount of aggregation of the cations and anions was estimated using a method described by Kohler et al.34 The method utilizes the assumption that the viscosity was related to the number of ions (or aggregates of ions) to calculate the molecular mass, Mvisc. The degree of aggregation was calculated from the ratio of Mvisc/ M, where M is the molecular mass of the 1:1 PIL. The aggregation numbers are given in Table 10 and range from 3.2 to 5.7, suggesting that there may be a significant degree of aggregation occurring. There were only three stoichiometric acid-base combinations investigated by Kohler et al. consisting of TEATFA with an aggregation number of 1.8, dibutylammonium butyrate with an aggregation number of 2.3, and butylammonium butyrate with an aggregation number of 4.2.34 Our data were consistent with these results, with comparable aggregation numbers, and a
Protic Ionic Liquids general trend that increasing the substitution of the ammonium cation led to lower aggregation numbers. Effect of Substitution of the Ammonium Cation. To aid in the discussion, a summary of the main trends has been given in Table 5. This table contains structure-property trends for increasing substitution of alkyl and hydroxyalkyl substituents on the ammonium cations. The effects of increasing alkyl substituents using MAF, DMAF, EAF, DEAF, and TEAF are given in the first column, and the effects of increasing substitution of hydroxyl containing substituents for EOAF and DEOAF, in the second column. The main difference between the alkyl and hydroxyl substituents was that increasing the number of alkyl chains on the ammonium cation caused the viscosity to decrease, and subsequently the ionic conductivity to increase, whereas the opposite trend occurred for alkylhydroxyl chains. For example, the viscosity decreased for MAF to DMAF and EAF to DEAF, while it increased for EOAF to DEOAF. Increasing the number of alkyl chains present on the cation (i.e., going from primary to secondary or tertiary) probably led to more symmetrical cations, since all of the chains were the same, thus decreasing the viscosity. The opposite trend for increasing the number of alkylhydroxyl chains (DEOAF compared to EOAF) probably resulted from the increased interactions between ions due to the increased number of hydroxyl groups, such as through hydrogen bonding. The ionic conductivity is governed by the number of free ions present and how easily they migrate. The ability of the ions to migrate is strongly related to the viscosity, with a rapid decrease in the conductivity with increasing viscosity. There are multiple factors which impact the number of free ions, such as the degree of proton transfer from the acid to the base and the amount of aggregation present. From these PILs, it appeared that increasing the substitution of the ammonium cation generally increased Tm, where the melting point reflects the ability of the ions to pack in crystalline lattices. This was most evident for EAN, DEAN, and TEAN with Tm values of 9, 105, and 115 °C, respectively, showing that EAN has a particularly low melting point compared to its more substituted versions. The exception to this trend was that MAN had a higher Tm value than DMAN. It is quite possible that the small MA cation was not behaving in a consistent fashion with the other cations. In comparison to DEAN and TEAN, EAN has a surprisingly low Tm value. The density showed a strong dependence on the degree of substitution of the ammonium cation, clearly following the series of primary > secondary, with some evidence that the tertiary substituted PILs have an intermediate density. The surface tension showed negligible change due to changes in the direct substitution on the nitrogen. Therefore, the expected decrease due to the increased amount of hydrocarbon present through increasing the substitution was balanced by a competing increase in the surface tension. Most likely, the increased substitution is causing an increase in the ionic radius of the cation; thus, while there are more hydrocarbon per ion, there is a greater ionic separation, and hence overall less hydrocarbon present per unit area at the surface. Increasing the number of alkyl substituents on the ammonium cation decreased the Gordon parameter and the diversity of LCPs observed in the PILs. For example, DMAF and DEAF formed fewer LCP phases than had previously been seen for MAF and EAF, while TEAF did not support the formation of any LCPs. Similar trends were observed with EAHS and DMAHS. In contrast, increasing the number of hydroxyl containing substit-
J. Phys. Chem. B, Vol. 112, No. 3, 2008 903 uents, for DEOAF compared to EOAF, decreased the Gordon parameter, yet it led to remarkably similar phase behavior and temperature ranges for DEOAF compared to EOAF. Methoxy and Hydroxyl Containing Cations. In our earlier work, it was found that the presence of a hydroxyl group on either the cation or anion led to PILs supporting a greater diversity of liquid crystal phases. A simple quantitative way to observe this was through the increase to the Gordon parameter (G) with the addition of a hydroxyl group. For example, the Gordon parameter of EOAF was 1.448 J/m3, compared to only 0.866 for EAF J/m3. To extend on our earlier work, other hydroxyl containing cations as well as methoxy containing cations were trialed. In general, the PILs which had cations that contained a hydroxyl group had a significantly lower ∆Tb value, and hence probably poorer ionicity, such as in the case of EOATFA, where ∆Tb was 126 °C, compared to 208 °C for EATFA. The sole exception to this was EAPV and EOAPV, where ∆Tb was slightly higher for EOAPV. The PIL EOAHS was designed to have a high Gordon paramter, and hence anticipated to be a good self-assembly media. Previously, the highest Gordon value had been obtained for EAHS, with a value of 1.215 J/m3, and which was a good self-assembly medium.5,6 The addition of the hydroxyl group increased the Gordon value to 1.698 J/m3. However, lower phase diversity was observed for all three amphiphiles trialed in EOAHS compared to EAHS, though the LCPs of phytantriol and CTAB were supported over greater temperature ranges. In this series of PILs, cations containing methoxy groups were also explored, where the methoxy group has the ability to hydrogen bond accept but unlike hydroxyl groups cannot hydrogen bond donate and accept. The effect of the methoxy group was investigated by comparison of PILs containing the methoxy cations 2MEA or 22HEEA to the analogous EOA cations previously reported.26 Only the 2MEA cation with the formate anion was liquid at RT (2MEAF). It had a moderately high Gordon value of 0.891 J/m3, showing that the methoxy group increased G but not by as much as a hydroxyl. The methoxy group within the alkyl chain caused an increase in the surface tension, though not as large as what the hydroxyls cause. From the behavior of 2MEAF with the amphiphiles, it appears that the methoxy group improves the ability of the PIL to promote self-assembly compared to a similar length alkyl chain with no methoxy group (e.g., propylammonium formate or butylammonium formate) but not to the same extent as having a hydroxyl group present (e.g., 2-hydroxylpropylammonium formate). This is consistent with less hydrogen bonding interaction in methoxy containing PILs compared to hydroxyl containing PILs. The 22HEEA cation was interesting in that the PILs formed from it with the anions F, N, TFA, and HS were all liquid at room temperature. All four of these PILs displayed similar thermal phase behavior with a glass transition between -64 to -79 °C, and no other solid state thermal peaks. The viscosity of 22HEEAF was fairly low at 61 mPa‚s, whereas the other three had higher viscosities, with the TFA and HS versions having very high viscosities, greater than 1000 mPa‚s. The ionic conductivities were fairly low, with 22HEEAF having the highest value at 2.54 mS/cm, reflecting its lower viscosity. The four 22HEEA containing PILs had relatively high G values, which normally predicts that they would be good as self-assembly media. Despite the high G values, only 22HEEAF and 22HEEAN supported the formation of LCPs; however, since
904 J. Phys. Chem. B, Vol. 112, No. 3, 2008 22HEEAHS and 22HEEATFA have very high viscosities, it is possible that LCPs may have formed if the systems were left longer. Increasing the number of hydroxyl groups was investigated using the cations DEOA, TEOA, and TRIS. Of the PILs made using these cations, only DEOAF was liquid at room temperature, and it had a lower surface tension and lower G value than EOAF. Hence, increasing the number of hydroxyl groups through addition of ethanol groups on the ammonium cation did not increase G. The cation TRIS was selected due to it being a primary ammonium analogue of the tertiary TEOA cation. The TRIS cation led to salts with high melting points, well above room temperature, probably due to the high degree of symmetry of the TRIS ion; consequently, the PILs containing the TRIS cation were not tested as self-assembly media. The melting points of TEOAF, DEOAN, and TEOAN were all below 80 °C, which was sufficiently low for them to be trialed as self-assembly media for CTAB, but not for myverol 18-99K or phytantriol. Interestingly, DEOAF and DEOAN showed comparable phase diversity and thermal stability ranges to what had been seen in EOAF, while TEOAN supported the LCPs at higher temperatures. Overall, it has been shown that the presence of methoxy and hydroxyl groups can generally be used to increase the Gordon parameter of a PIL (except for higher substituted cations), but unfortunately, that alone is not sufficient to produce PILs which are good self-assembly media. The Gordon parameter should still be considered a very useful parameter in designing PILs as self-assembly media, but its limitations need to be recognized. Anion Trends. Formate Vs PiValate Anions. The small, highly branched pivalate anion was not a good alternative to the formate anion. The pivalate anion led to EAPV and EOAPV having improved thermal stability at ambient temperatures than the corresponding formates but was more than compensated for by melting points greater than 100 °C. The highly symmetric nature of the pivalate anion was the likely cause of the high melting points; hence, potentially a small, branched and asymmetric carboxylate anion could lead to a useable replacement. As shown in Table 9, there was no amide detected in the pivalate samples after 15 months; however, this is possibly only due to a slow reaction because the samples are solid. TFA Vs Acetate Anions. From structure-property relationships developed for AILs,23 it was expected that the fluorinated TFA anion could lead to improved properties through the highly electronegative fluorine atoms distributing the anionic charge, thus decreasing the interactions between the cation and anion and causing less ionic association.21 PILs were prepared using the TFA anion with the MA, EA, EOA, and 22HEEA cations. The EATFA and EOATFA salts were compared to the previously reported hydrocarbon analogues with acetate of EAA and EOAA.26 Unfortunately, there was insufficient data to obtain a meaningful comparison between TFA and acetate. The only PIL containing TFA which was liquid at room temperature was 22HEEATFA, indicating the tendency of higher melting point for PILs containing TFA. The 22HEEA cation did have a slightly lower amount of aggregation with TFA compared to the other anions, which suggests there may be less ionic interactions. On the favorable side, TFA did appear to increase the thermal stability of the salts with the Tb and Td values of EATFA 100 °C higher than those of EAA and the Tb and Td values of EOATFA 37 and 55 °C higher than those of EOAA, respectively.
Greaves et al. Inorganic Anions. The protic fused salts containing the N or HS anion generally had higher melting points and greater thermal stability compared to the other PILs. The N containing salts had no weight loss as measured by TGA until 140260 °C, depending on which cation it was combined. In comparison, the HS salts tended to lose mass from low temperatures through evaporation but otherwise had the highest thermal stabilities before undergoing decomposition processes. All of the protic fused salts which contained the nitrate anion underwent exothermic decomposition (exploded). The decomposition of MAHS involved an exothermic process followed by two endothermic processes. Interestingly, the HS and N containing PILs appear to have generally lower levels of calculated aggregation than the other PILs, as shown in Table 10, and a good range of properties. The PILs with the N or HS anions that were liquid at room temperature were 22HEEAN, DMAHS, EOAHS, and 22HEEAHS. This effectively prevented trends to be obtained from their physicochemical properties. It was observed, though, that the HS anions led to the highest densities of these PILs, from 1.338 to 1.558 g/cm3, indicating very efficient packing of the ions in the liquid state. Interestingly, EOAHS had a very high surface tension of 82.1 mN/m, which led to the highest Gordon parameter reported for a PIL of 1.698 J/m3. The liquid crystal phase behavior of the three amphiphiles in EOAHS had surprisingly low phase diversity but very good thermal stability compared to the other PILs, which can be associated with the high G value. Concluding Remarks This series of protic ionic liquids and protic fused salts extends what has been previously reported26 for salts containing primary ammonium cations. Previously, structure-property relationships were proposed for protic ionic liquids containing primary ammonium cations. In this study, those relationships were extended to include a wider range of primary ammonium cations as well as secondary and tertiary ammonium cations. In addition, the previously studied carboxylate, nitrogen, and hydrogen sulfate anions were further utilized and extended to include pivalate and trifluoroacetate anions. Interestingly, the cation 2-(2-hydroxy-ethoxy)-ethyl-ammonium (22HEEA) which contains a methoxy and hydroxyl group was noted to be particularly good for forming low-melting-point PILs, and was the only cation which formed a PIL liquid at room temperature with each of the anions used in this study. Unfortunately, the other properties of 22HEEA were not as remarkable, with only 22HEEAF and 22HEEAN capable of supporting self-assembly, while 22HEEATFA and 22HEEAHS had particularly high viscosities. Of these new PILs, 14 were found to be capable of supporting amphiphile self-assembly with CTAB, myverol 18-99K, or phytantriol. Increasing the substitution of the ammonium cation had a detrimental effect on the diversity of liquid phases, with the exception of EOAF and DEOAF where the additional ethylhydroxyl group caused very little change. Two of the most interesting PILs of this series were DEOAF for its characteristics as a self-assembly media, leading to good phase diversity with the amphiphiles used, and DMAF which had a particularly high ionic conductivity of 51.10 mS/cm, but both were susceptible to amide formation. It was clearly shown that PILs could be designed to have high Gordon values; however, this did not always correspond to increased liquid crystal phase diversity or thermal ranges of phase stability. Consequently, while the Gordon parameter gives
Protic Ionic Liquids a measure of the driving force of the solvent as a promoter of self-assembly of amphiphiles, there are definitely other factors which affect their behavior. Acknowledgment. C.J.D. is the recipient of an Australian Research Council (ARC) Federation Fellowship. This work was also partially supported by an ARC Discovery Project, DP0666961, grant. References and Notes (1) Greaves, T. L.; Drummond, C. J. Chem. ReV., published on line Dec. 21, 2007, DOI: 10.1021/cr068040u. (2) Evans, D. F. Langmuir 1988, 4, 3-12. (3) Araos, M. U.; Warr, G. G. J. Phys. Chem. B 2005, 109, 1427514277. (4) Spicer, P. T.; Small, W. B.; Lynch, M. L. WO066014-A2, 2002. (5) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. J. Phys. Chem. B 2007, 111, 4082-4088. (6) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Langmuir 2007, 23, 402-404. (7) Zhao, G. Y.; Jiang, T.; Gao, H. X.; Han, B. X.; Huang, J.; Sun, D. H. Green Chem. 2004, 6, 75-77. (8) Janus, E.; Goc-Maciejewska, I.; Lozynski, M.; Pernak, J. Tetrahedron Lett. 2006, 47, 4079-4083. (9) Lansalot-Matras, C.; Moreau, C. Catal. Commun. 2003, 4, 517520. (10) Garlitz, J. A.; Summers, C. A.; Flowers, R. A.; Borgstahl, G. E. O. Acta Crystallogr., Sect. D 1999, 55, 2037-2038. (11) Lau, R. M.; Sorgedrager, M. J.; Carrea, G.; Van Rantwijk, F.; Secundo, F.; Sheldon, R. A. Green Chem. 2004, 6, 483-487. (12) Angell, C. A.; Wang, L. M. Biophys. Chem. 2003, 105, 621-637. (13) Pernak, J.; Goc, I.; Fojutowski, A. Holzforschung 2005, 59, 473475. (14) Noda, A.; Susan, M. A. B. H.; Kudo, K.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2003, 107, 4024-4033. (15) Sun, J. Z.; Jordan, L. R.; Forsyth, M.; Macfarlane, D. R. Electrochim. Acta 2001, 46, 1703-1708. (16) Angell, C. A.; Xu, W.; Belieres, J.-P.; Yoshizawa, M. Patent WO114445, 2004.
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