Chapter 20, HSP for ionic liquids (How to Assign HSP to New Materials)

There is a popular notion that one contribution to saving the planet is to use ionic liquids as “green solvents”. This chapter describes a work in progress and, because of this, is in a different format. We would love to provide the HSP community with a definitive list of ionic liquid HSP, but the data are simply not there to produce such a list. We hope that this chapter will stimulate debate within the ionic liquid community and, especially, create more datasets from which HSP can be derived.

This chapter is written in a form that is intended to serve as a guide for determining the HSP of given chemicals using ionic liquids as examples. The reason for this choice is that ionic liquids combine special properties in one molecule and there is a need to discuss methods to measure or estimate their HSP. Relevant aspects of the HSP methodology are presented and then applied to data from the literature to estimate the HSP for several ionic liquids. A well-chosen ionic liquid can be a reaction medium for polymerization or dissolve a polymer, and later be recycled. There is a lot of interest in the 1,3-dialkylimidazolium-based ionic liquids as solvents in the polymer industry. In addition to pure scientific interest, there are also uses or potential uses for ionic liquids as plasticizers, ionic polymer liquids, and in separation processes. Ionic liquids, usually amine salts with organic acid moieties, have been used in the coatings industry for many years. It is thought instructive to see what these uses have been and what they may tell about the current usage of these special liquids in polymers.

A major application in coatings is to have “ionic” moieties as local regions covalently bonded on polymers, such as water-reducible acrylics, that are not in themselves soluble in water. The acrylic acid additions are generally neutralized with amines. This improves the stability of the polymer micelles in water since the organic salts as such are not soluble in the polymer micelle, and necessarily reside at the water/polymer interface in a stabilizing function, enhanced by the electrostatic repulsion in the water with its high dielectric constant.  In contrast to this, great care is taken to remove all water in studies of the solvent properties of ionic liquids for polymers.

In other cases amine salts provide good pigment dispersion stability since the energy relations of the local “ionic” regions are similar to those of especially inorganic pigment surfaces. Lecithin, for example, is a traditional dispersing agent of this kind that is used in solvent based systems with the “ionic” portion being attached to a pigment surface with nothing in the system to being able to dissolve this essentially permanent anchor away from its desired location. Anionic (or cationic) surfactants are also used for dispersing pigments, often in connection with traditional nonionic surfactants. As is well known, these have hydrocarbon tails and water soluble heads in the same molecule. Sodium salts of organic acids are frequently used in such applications. One can therefore expect relatively high HSP for the heads of such salts, matching to some extent the HSP of water or pigment surfaces. This has been confirmed earlier in terms of what are now called HSP (1).

 Another study of some interest involved a base phthalocyanine pigment that was surface treated in different ways to give quality differences labeled as +++, ++ and down to ---. These characterizations were found from measurement of electrophoretic mobility in a given solvent. Those pigments with a strong “+” characterization suspended primarily in solvents with high δ_H while those with strong “-“ character suspended primarily in solvents with high δ_P. These pigment suspension results in a set of standard solvents provided data for HSP assignment of these same surface treatments. Those pigments with little or no character of the “+” or “-“ scale did not readily suspend in any solvents, making a characterization with HSP difficult.

Consideration has been given in the above to the “ionic” character of local regions of larger molecules or at surfaces. The focus of current interest is solubility where it is the HSP of the whole “ionic” molecule that has interest. The amine salts discussed above have never been thought of or used as solvents, although they may have acted in a swelling capacity at times without this being recognized.  Table 1 shows experimental results from page 337 of the second edition of the Hansen handbook for reference. A suspicion developed over the years but never proven is that larger metal ions increase the δ_D parameter more than smaller ones, without any significant effect on the δ_P and δ_H. Among the organic liquids the larger moieties, such as Cl, Br, and aromatic rings, for example, do give higher δ_D.

 

Salt	D	P	H	R
Mg(NO3)2.6H2O	19.5	22.1	21.9	13.2
Formic acid/DMEOH	17.2	21.5	22.5	-
Acetic acid/DMEOH	16.8	19.8	19.8	-

Table 1‑1 HSP for given salts. All units are MPa^½. DMEOH is dimethyl ethanolamine.

 

How can HSP be assigned to ionic liquids?

 

Solubility relations – what do they dissolve or do not dissolve?

A list of what a solvent dissolves or does not dissolve is important to assign its HSP. The HSP of the solvent should be such that they are located within the HSP sphere for solubility of those solutes they dissolve, and outside of such HSP spheres for those solutes that they do not dissolve. Interpretation of data reporting what they dissolve is (usually but not always) straightforward, whereas the meaning of data for what they do not dissolve is often complicated by the effect of their molar volumes. This non-solubility data is also significant since this helps to eliminate large regions of HSP space from consideration in assigning HSP when using this methodology.  When it is reported that an ionic liquid dissolves 3% cellulose of a given degree of polymerization at 70°C, insight is given into its HSP, but not necessarily to precise values. All three of the HSP must be very high compared with other liquids. The HSP of cellulose are not determined with accuracy due to the lack of data for its solubility in organic solvents. In general ionic liquid solubility data does not include results for polymers for which the HSP polymer sphere characterization is available. This prevents the satisfactory use of many of the available solubility data. There is also considerable variation in the HSP data for acrylic polymers, and solubility also depends to a certain extent on the molecular weight of the polymer, with more solvents for low molecular weights. Larger V may prevent solution that would otherwise be inferred by the HSP alone, so care is also required here. Lower molecular volume ionic liquids are generally preferred in practice, all else (HSP included) being equal. The HSP of given ionic liquids can either be too high or too low for a given use, but the lower molecular volume species can be expected to produce lower viscosity room temperature liquids, which seems to be the goal for many purposes.

 

Comparison with solubility/miscibility profiles of reference liquids

The approach used by Hansen to assign HSP to amine salts of organic acids in (1) was to determine their miscibility is a series of well chosen solvents, just like in a traditional determination of the HSP of a solute. The miscibility profiles of the acids and bases, as well as given solvents with much higher HSP were also determined. When the miscibility profile of a given salt was found to be the same as that of a given reference solvent, the salt was assigned the HSP of that solvent. This procedure has the potential for some modest error due to possible errors in the HSP of the reference solvent as well as the fact that other HSP values, close to those of the reference solvent, could also give the same miscibility profile. The boundary solvents found most useful to show differences in behavior, and therefore to allow such characterizations, were 2-ethyl butanol, n-amyl alcohol, 2-nitropropane, nitroethane, aromatic solvents, aliphatic solvents, n-butyl acetate, and ethyl acetate. Other solvents were also tested but were not found useful for this purpose, since it is differences in behavior that are interesting in these procedures. The amine/organic acid salts with acetic acid, for example, tend to have HSP in the range of [17.1, 22.5, 23.3] MPa^½ giving a δ_t equal to 36.6 MPa^½. These HSP are used in the following to derive group contributions for amine/organic acid salts in connection with a molar volume, V, for the n-butyl amine/acetic acid salt approximately equal to 139 cc/mole. The salt of dimethyl ethanol amine with p-toluene sulfonic acid was assigned HSP equal to [17.2, 21.5, 22.5] MPa^½ giving a δ_t equal to 35.6 MPa^½ by the same method. V here is estimated as 216 cc/mole. These estimates of molar volume include measured shrinkage of about 10% compared with the sum of the molar volumes of the reactants. These group contributions are useful for estimates of the HSP of related salts. In the case of the salt of methanesulfonic acid/dimethyl ethanolamine a solid was formed that was not soluble in any of the above test solvents indicating still higher values, although the molar volume of this salt is also lower. This would lead to higher HSP for the same cohesive energy of the salt moiety.

 

HSP relative to the solid/liquid boundary at room temperature

Another comparison can possibly be made by recognizing that there is a boundary between solids and liquids in HSP space. This boundary is not sharp, but can provide information about the approximate values expected for a solid with a melting point at room temperature, for example. Examples of interest for comparison here are given in the following Table 2.

 

Solvent	MPt °C	D	P	H	V
Dimethyl Sulfoxide	18.5	18.4	16.4	10.2	71.3
Sulfolane	27.6	17.8	17.4	8.7	95.5
Dimethyl sulfone	109	19.0	19.4	12.3	75.0

Table 1‑2 Comparison of melting point and HSP for reference solvents.

 

HSP for amine/COOH salts

The HSP given above for these salts on average can be used to derive group contributions for use with other salts. These can be found from the equation where the units of the solubility parameters are MPa^½ and the V are in cc/mole, as the cohesion energy attributable to the given group:

GC_i = (δ_i)²V

For an average of organic amine/acid salts (using n-butyl amine/acetic acid as a model):

GC_D = (17.1)²(139) = 40,645 joules

 GC_P = (22.5)²(139) = 70,368 joules

GC_H = (23.3)²(139) = 75,462 joules

The sum of these contributions is 186,475.

These GC can then be used to estimate HSP for amine salts of given V, or perhaps for segments of molecules where molecular breaking gives a V that can be used to find the local HSP, since this may also be of interest. These GC are the cohesive energy due to the respective types of interaction, dispersion (D), dipolar (P) and hydrogen bonding (H). The HSP methodology is quantitative to the extent data permit assigning accurate values.

 

HSP for dimethyl ethanolamine/p-toluenesulfonic acid salt

The same general equation given above can also be used for this salt. Corrections for the alcohol group are included as 2900 J for the polar contribution and 19,400 J for the hydrogen bonding contribution.

GC_D = (17.2)²(216) = 63,901 J

GC_P  = (21.5)²(216)  - 2,900 = 96,946 J

GC_H = (22.5)²(216)  - 19,400 = 89,950 J

Even though the HSP are similar to those above, these group contribution values are higher because the V of the salt is larger. The total cohesive energy found by this method is 250,797 joules.

 

HSP for other ionic liquids

Methods of characterization

Assigning reasonably accurate HSP to other ionic liquids is difficult since no complete systematic studies of their behavior have been performed that would have allowed this. Information in references (2-13) has been used to try to estimate HSP for representative ionic liquids for which the very limited data allow this. As stated above, these estimates are probably more indicative than precise. Strictly theoretical studies can always be questioned, and the experimental studies are not sufficiently complete. A problem with experimental determination of the HSP of liquids in general is that they are usually miscible in most of the test solvents, and this does not provide sufficient differentiation for an accurate assignment, if any assignment at all. Calculations based on group contributions or other methods of estimation available with the HSPiP software/eBook are likewise a possible approach, but nothing really replaces experiments, if they are possible-

 

Inverse Gas Chromatography (IGC)

A most promising methodology to avoid the traditional problems of assigning HSP to liquids is inverse gas chromatography (IGC), assuming a sufficiently broad selection of test solvents is used to generate the data. The liquids are borne on substrates as thin films. Most IGC studies do not include liquids with high δ_P, in the faulty belief that alcohols are sufficient for this purpose.

 

The intrinsic viscosity method

The intrinsic viscosity method used in (6) appears to be a simple and reliable way to determine the HSP of liquids that are not readily characterized by traditional methods. Higher intrinsic viscosity is associated with a closer HSP match, and viscous test liquids can also be included. The method involves extrapolating the intrinsic viscosity (viscosity of solution/viscosity of solvent minus 1) of very dilute solutions to zero concentration. Capillary tube viscosimeters are usually used. A suitable range of solvents is also required for this method. Suggestions for added test solvents of this kind are easily found with the HSPiP software using the HSP estimates for some ionic liquids that follow

 

Reaction rate method

A new method to assign HSP is to study the reaction rate in a large number of liquids for the [4+2]-cycloaddition of singlet oxygen with 1,4-dimethylnaphthalene and derivatives (10,14). This method is very promising, giving results that appear to be correct because of agreement with other methods of characterization. The reaction rates reported in (14) could be used with the HSPiP software to find given HSP characterizing what is presumed to relate to the solubility of an intermediate in the reaction as discussed below in detail. These HSP are also in agreement with the HSP of commonly used reaction solvents, being only very slightly higher. The solvents used in (14) appear to be adequate but it might be wise to supplement them with others in the region of interest.

 

Nomenclature

There is a special nomenclature that is used to describe the ionic liquids. This is briefly explained for those ionic liquids discussed in this chapter in the following. The cation is usually put in brackets with lower case letters. The anion is not always put into brackets, and generally follows usage any chemist will recognize.

 

im          1,3-dialkylimidazolium

m             methyl

e              ethyl

b              butyl

h              hexyl

o, oc        octyl

dd            dodecyl

[bmim] stands for 1,3-butyl methyl imidazolium, for example.

 

The ionic liquids for which HSP are specifically estimated are:

[bmim]Cl, [bmim]PF₆, [omim]PF₆, and [bmim]BF₄.

 

Review of the content of the references

Winterton (2) provides an extensive review of ionic liquids with data of the kind that are useful to assign HSP to given ionic liquids if there is enough diversity in the results. There is also a brief discussion of solubility parameters and an indication that fresh insights might be found by application of solubility parameter methods. This report is an attempt to do just that. There is also mention of work that estimates the Hildebrand parameter, δ_t, from intrinsic viscosity measurements in different liquids and reaction rate constants. When analyzing data for test liquids one should recognize that assigning HSP outside the range of an average of these is not possible unless some method is used to extrapolate into this region, such as is done with the HSPiP software.

Kubisa (3) mentions a long list of solutes where ionic liquids have been shown to be good solvents including inorganic and metalorganic compounds, enzymes, and polymers such as methacrylates (in particular). The main kinds of ionic liquids discussed are [bmim]PF₆, probably because of its low melting (glass transition) point of  -61°C and  [bmim]BF₄. There is little help in assigning HSP to these, although the qualitative solubility of a series of methacrylate polymers in [bmim]PF₆ is given. This allows a potential check on HSP estimates for this ionic liquid.

Kim et.al. (4) describe studies of the solubility and reactions of polyepichlorohydrin in particular, but provides solubility data for seven polymers in three ionic liquids [bmim]Cl, [bmim]BF₄, and [bmim]PF₆. This is the kind of data that is desired, but the present data are of limited value since they are taken at 70°C, and there is only one polymer for which HSP have been determined and one (cellulose) for which estimates of the HSP can be made with considerable uncertainty. The number of useful systems that show differences in behavior, and their HSP relations with these, are the important data that determine the accuracy of HSP assignments. These solubility data must also be considered with caution since they are based on the stability of a solution in the ionic liquids. This solution was made by including a good, volatile solvent that was subsequently evaporated to form the solutions that were then observed for a long time (weeks).

Vygodskii et.al. (5) report numerous polyamide and polyimide polymerization reactions with different 1,3-dialkylimidazolium-based ionic liquids. These can provide different molecular weight polymers, with those with highest molecular weight being those where the dialkyl groups are C4, C5, and C6 aliphatic chains with Br as counterion. The solvents that can otherwise be used in the reactions with greater or lesser success are N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), nitrobenzene (NB), and m-Cresol (MC). One anticipates from these data that the HSP of these ionic liquids will be within the HSP region defined by these liquids, or perhaps have slightly higher HSP approaching the solid region, and this indeed turns out to be the case as seen below. This HSP region is often referred to as the reaction solvent region, in agreement with some of the uses of the ionic solvents. The molecular solvents may require more extreme conditions than for the ionic liquids, but they are more or less successful as solvents in the cases reported. The solvent used for the intrinsic viscosity measurements on thoroughly dried polymers to find an indication of their molecular weights was concentrated sulphuric acid in all cases but one (where NMP was used).

Lee and Lee (6) measured intrinsic viscosities for a number of ionic liquids in up to 15 different solvents, but only reported the intrinsic viscosity data for two liquids and up to 9 solvents in a figure that is not easy to interpret. A table with the full data would have been extremely helpful to evaluate the quality of the estimate, and probably even to estimate the HSP using the HSPiP software. The Hildebrand parameters discussed in the above for organic amine/acid salts are 36.6 MPa^½ for n-butyl amine/acetic acid and 35.6 MPa^½ for dimethyl ethanol amine/p-toluene sulfonic acid. The test liquids that best matched the miscibility profile of the n-butylamine/acetic acid salt were ethanolamine (δ_t=31.7) and triethanolamine (δ_t=36.6), with the latter being accepted as correct. These values are somewhat higher than those reported by Lee and Lee for 8 of the more frequently mentioned ionic liquids having cations of the 1-alkyl-3-methylimidazolium type. These are in the range of 25.4 to 29.8 MPa^½. It is stated that the intrinsic viscosity method is reliable since, in control experiments with a set of well-selected solvents, it accurately found δ_t for dipropylene glycol (δ_t=26.4) and 1,3-butanediol (δ_t=29.0). These have viscosities similar to some ionic liquids, and δ_t very close to those of the ionic liquids. This speaks well for the intrinsic viscosity method in itself, but care must be taken since solutes may have δ_t and HSP that are out of the range attainable by the test solvents. This article also gives the molar volumes and densities for the ionic liquids studied. This is a very useful help for those who may continue this study. The δ_t reported in this article for [bmim]PF₆ (29.8 MPa^½) and [omim]PF₆ (27.8 MPa^½) have been used as a reference values in the following. 

Weingärtner (7) does not report any data useful for present purposes, although the review covers other aspects of ionic liquids very well.

Derecskei and Derecskei-Kovacs (8) give molecular modeling calculations of the densities and cohesive energy density for [emim][bmim][C₆mim] all with anions [CF₃C0₂], [(CN)₂N] and [N(SO₂CF₃)₂] abbreviated [Tf₂N]. These agree reasonably well with “experimental” values. This data is worthy of comparison with other approaches for finding the cohesive energy density and total solubility parameter. There is also a split of the total solubility parameter into “Coulombic” and “H-bonding” contributions that is difficult to use in the present context.

Xu, et.al. (9) provide glass transition temperatures for a number of ionic liquids in a plot showing a minimum at about 250 cc/mole. These temperatures are from about 0°C down to about -100°, making direct relations to room temperature behavior rather remote.

Swiderski et.al. (10) determined cohesive energies and Hildebrand solubility parameters for a number of common [bmim] ionic liquids that appear to agree well with other estimates in the literature. The anions were [BF₄], [SbF₆], [PF₆], [(CF₃SO₂)₂N], [CF₃SO₃]. [bm₂im][(CF₃SO₂)₂N] was also studied. The methodology was a previously determined procedure and equation developed using 28 solvents that related molecular oxygen reaction rates with 1,4-dimethylnaphthalene to the Hildebrand parameters (14). The δ_t found for [bmim][BF₄] and [bmim][PF₆] were 31.6 MPa^½ and 30.2 MPa^½ in good agreement with the values found by Lee and Lee (6), and have been used as reference values below. This method appears to be quite reliable. Correlation with HSP rather than the lesser meaningful δ_t would have been preferred, at least in the present context. 24 of the solvents used in (14) were correlated with HSP using the HSPiP software with the following results:

 

Rate x10-4 M-1s-1	δD	δP	δH	R0	Fit	G/T
k>86	21.47	15.16	17.44	13.9	0.964	4/24
k>65	19.06	16.46	15.07	11.2	0.936	5/24
k>40	20.41	15.99	14.63	12.7	0.976	6/24
k>26	20.73	16.17	14.02	13.4	0.999	8/24
k>7	21.02	15.85	15.70	17.5	1.000	18/24
Average	20.5	15.9	15.4	-	-	-

Table 1‑3 HSP correlations of the rate constant data given in (14). G is the number of “good” solvents out of a total number, T.

With this positive result, it would seem logical to extend this method with solvents having higher HSP, perhaps at the expense of some having lower HSP to lessen the work. The results found by this method have been incorporated into the estimates for the HSP of the ionic liquids analyzed below.

The average of the HSP correlations given in Table 3 is in the higher HSP region of the HSP sphere defined by typical reaction solvents such as DMSO, DMF, DMAC, and NMP. This can be seen from the figure on page 278 in the handbook. This is a little closer to the solid region than any of these molecular liquids. This clearly points to a generality involving the enhanced solubility of an intermediate species. The HSP for the ionic liquids discussed in detail below somewhat higher than this likewise just above the HSP of the typical reaction solvents. This gives added support to this conclusion. 

 

Mutelet et.al. (11) used inverse gas chromatography (IGC) in an attempt to evaluate both the Hildebrand and Hansen solubility parameters. Data and results are reported for [mocim][Cl], [emim][(CF₃SO₂)₂], and [bmim][PF₆]. The authors report differences in values of the Hildebrand parameters found in the literature for [bmim][PF₆] depending on method of determination that vary from 22.02 calculated in this work, a Monte Carlo simulation giving 26.70, and other experimental data giving 18.60, all in J^½cm^-3/2. This study is not useful to assign HSP since the solvents used did not sufficiently cover the required HSP space (the same old story). There is a marked lack of solvents with high δ_P, since the authors erroneously assumed alcohols covered the highly “polar” liquids.

What can be gained in this context from the 1969 article by Hansen (12) is that HSP could be assigned to proteins (zein from corn) and inorganic salts, and that mixtures of inorganic salts in given liquids could dissolve certain polymers such as Nylon 66 (15% calcium chloride in methanol) and zein (10% calcium nitrate in gamma butyrolactone, 20% magnesium nitrate in 1-butanol). The “ionic” moiety need not be chemically attached to the organic solvents. The nitrates and chlorides are those types most readily soluble in organic solvents. The nitrate and chloride entities have considerable HSP leverage in “salts” to reduce their HSP. This statement is based on the fact that the HSP of the nitro compounds and chlorinated solvents are well removed from regions where all three HSP are high. These anions reduce HSP in an inorganic salt to a greater extent than other common anions thus allowing organic solvents to dissolve them.

 

Polymers used in this study to help assign HSP to ionic liquids

Sheets were made to note potentially relevant information for each ionic liquid for which there was hope for a HSP characterization. It became evident that such would be impossible for most of the ionic liquids at this time (September 2009), even those discussed in many sources. This is primarily because the solvents used to generate the data or methodology used did not sufficiently cover the required HSP space. The data related to polymerization are also of such a nature that it can be difficult to assess just how to use it. Miscibility relations are reported for some monomers and many polymers. Most of the polymers discussed in this literature have not been assigned HSP. The miscibility of the resulting polymers in these polymerization studies depends on a number of factors such as the temperature prevailing at the time of judgment.

Table 4. HSP relations for polymers of significance in assigning HSP to the ionic liquids. Units for δ are MPa^½.

 

Polymer	δD	δP	δH	R0
PMMA #66	18.64	10.52	7.51	8.59
PAN #183	21.7	14.1	9.1	10.9
PS (p.102-G, Handbook)	21.3	5.8	4.3	12.7
Dextran C (Cellulosea)	24.3	19.9	22.5	17.4
PEG-3350	17.3	9.5	3.3	10.0
PEO #209 (heated solns.)b	21.5	10.9	13.1	15.9

Table 1‑4 HSP relations for polymers of significance in assigning HSP to the ionic liquids. Units for δ are MPa^½.

^a The HSP for soluble cellulose are thought to be (much) higher in all three parameters, with a correspondingly larger R₀ that would not encompass what dissolves Dextran C. Dextran is an amorphous polymer resembling amorphous cellulose in structure. This estimate is used for guidance only with something reported as a solvent for cellulose expected to have a relatively low RED, and non-solvents for cellulose may still have RED somewhat lower than 1.0 based on this correlation. 

^b This correlation with heated solutions is based on very old so-called Solvent Range data and is highly questionable. It is included here for the sake of completeness, and because some data used for comparison also involve heated solutions.

Among the numerous HSP correlations for PMMA its solubility relations are thought best represented by correlation #66 in the Hansen handbook. The PAN correlation is thought to be quite good. There are numerous correlations for PS in the handbook, with this one being based on the authors own experience with a very high molecular weight type suitable for injection molding. Solvents that only swell this PS may well dissolve lower molecular weight polystyrenes.

The solubility of cellulose is so important commercially as well as to assigning HSP that an extended discussion is given in the following. Hansen has made estimates of the HSP for cellulose using noncrystalline Dextran C as a model for soluble cellulose (13). Dextran A with a molecular weight of 200,000 to 275,000 dissolved in the same solvents as Dextran C with a molecular weight of 60,000 to 90,000, but with obvious differences in viscosity (12). The Dextran C model is also thought useful in the present context. It is based on solubility in ethylene glycol, glycerol, formamide, ethanolamine, and dimethyl sulfoxide. Later the HSP assigned to lignin monomers placed them on the boundary of this correlation (13). N-methyl morpholine-N-oxide [19.0, 16.1, 10.2, V:97.6] also conformed to these HSP as a good solvent (RED:16.7/17.4) for soluble (amorphous) cellulose. The HSP of cellulose will be higher than the values for Dextran C, but presumably have the same relative values and with a radius of interaction that is larger than that for Dextran C but not encompassing the liquids mentioned above.

There are two HSP sets for PEG that have not been published until this time. These are for PEG-3350 solubility and PEG-200,000 for swelling and solution. The HSP for the higher molecular weight species were not used here but are given for reference as [17.0, 11.0, 5.2, R:8.2]. To avoid future misunderstandings it should be noted that chloroform and trichloroethylene were the only solvents dissolving the higher molecular weight PEO, and that the lower molecular weight PEO also dissolved in three solvents (water, formamide, and methanol, the only alcohol to dissolve) with RED greater than 1.0 and molar volumes less than 41 cm³/mole. The PEO data must be considered with care since this polymer forms special structures that may significantly affect solubility results.

 

HSP assignments to ionic liquids

It has only been possible to assign HSP to the four ionic liquids listed in Table 5 in spite of all the data in the literature. The methods discussed above will hopefully be used in the future to improve this situation. The data used for assigning the respective HSP are discussed in the following. Fortunately these four ionic liquids are representative and their HSP can be used with care together with other data, especially the Hildebrand parameters given in (6,10), to estimate HSP for others.

Ionic liquid	δD	δP	δH	δt	V, cc/mole
[bmim]Cl	19.1	20.7	20.7	35.0	175.0
[bmim]PF6	21.0	17.2	10.9	29.3	207.6
[omim]PF6	20.0	16.5	10.0	27.8	276.0
[bmim]BF4	23.0	19.0	10.0	31.5	201.4

Table 1‑5 Estimated HSP for given ionic liquids, MPa^½.

 

[bmim]Cl

[bmim]Cl is interesting because at 70°C it dissolves 3% cellulose having a degree of polymerization of 223 (molecular weight about 36,000) as described above (4), although the solutions were observed for weeks after initially being helped with a volatile solvent. Other related ionic liquids produce swollen solutions with this cellulose.  Other polymers [bmim]Cl dissolve include poly(vinyl phenol), poly(vinyl alcohol), poly(epichlorohydrin), and smaller amounts of poly(chloromethyl styrene). It does not dissolve poly(ethylene glycol) (PEG) or poly(methyl hydrosilane). HSP are only available as estimates for one of these, PEG.

Using the interactions with cellulose modeled as described above and PEO gives what might be considered as a minimum HSP estimate for [bmim]Cl as [19.1,13,13, δ_t:26.5]. The RED to Dextran C is 15.7/17.4 and to PEO is 10.9/10.0. If one uses 75,000 J as a group contribution, derived from the δ_P and δ_H from the n-butyl amine/acetic acid salt data, the estimate is [19.1, 20.7, 20.7, δ_t:35.0]. The HSP could easily be higher than these the currently accepted values. There are no data we are aware of at the present time to eliminate this uncertainty. Based on the estimated HSP for the following ionic liquids, there is no apparent need to require that δ_P be equal to δ_H. RED values in parenthesis for the polymers of significance here are Dextran C (0.61) and PEO #209 (0.82) and PEG-3350 (2.10). PMMS (1.94), PS (1.78), and PAN (1.32) are predicted as being not soluble.  The HSP assignment fits the data available, but other HSP can also do this, so adjustments may be required in the future.

 

[bmim]PF₆

[bmim]PF₆ has been assigned a Hildebrand parameter equal to 30.2 MPa^½ by the oxygen reaction rate method (10) and 29.8 MPa^½ by the intrinsic viscosity method (6).  The δ_t estimated here equal to 29.3 MPa^½ is considered close enough to these. The HSP estimates are [21.0, 17.2, 10.9] MPa^½. [bmim]PF₆ is plasticizer for PMMA (2). PAN is soluble in the presence of unknown amounts of sulfolane, which also is a solvent for PAN, reducing the value of this finding in the present context. Cellulose is stated as being insoluble while PEG maintained solubility at 33% for a prolonged time after an initial solution was made in a volatile solvent (4). Low molecular weight PS is also soluble in this ionic liquid (2). There is considerably more information on polymer solubility in the literature, but the solubility relations of the polymers involved have not been characterized by suitable HSP studies. The solubility relations  (RED) for the polymers listed in Table 4 are PMMA (1.03), PAN (0.35), PS (1.04), Dextran C (Cellulose) (0.78), PEG-3350 (1.31), and PEO #209 (0.42). These data support [bmim]PF₆ as a potential plasticizer for PMMA, as being soluble in PAN/sulfolane, and being able to support low molecular weight PS. The RED for the Dextran C correlation is higher than that for [bmim]Cl but still indicates a potential interaction is not far off, and the PEO/PEG data support any conclusion desired. The data all appear to be in reasonable agreement with the assigned HSP.

 

[omim]PF₆

The only help in determining the HSP for [omim]PF₆ is that low molecular weight PS is soluble and there is a higher solubility of PMMA than in [bmim]PF₆. This is reasonable enough for the increase in length of a side chain from butyl to octyl. The estimated δ_t is 27.8 MPa^½ in complete agreement with the identical value found from intrinsic viscosity measurements (6). The RED numbers for PMMA (0.82) and PS (0.98) are in agreement with all of these data as well.

 

[bmim]BF₄

[bmim]BF₄ has been assigned a δ_t of 31.6 MPa^½ based on the oxygen reaction rate method. This is matched by the δ_t of 31.5 found from the HSP [23.0, 19.0, 10.0] that also are supported by what few data are available. PMMA is not soluble (RED=1.47), polyimides and polyamides are compatible in agreement with chemical resistance correlations in the handbook, and PEG is miscible in the heated sample experiments reported in (4). The RED numbers here are 0.58 for the correlation with heated solutions of PEG and 1.63 for the PEG-3350 correlation. These HSP could be confirmed by further testing, such as RED for PAN being 0.515, or perhaps using other polymers with known HSP solubility correlations.

 

Discussion of the HSP assignments

The HSP assigned to the four ionic liquids in Table 5 appear to be reasonable. They are presumably not precise but there is general agreement among the available independent methods of assigning Hildebrand parameters with these HSP. It is also significant that these HSP are slightly higher in all three parameters compared with the HSP region defined by the usual reaction solvents, DMSO, DMF, DMAC, and NMP. This is clearly seen in the figure on page 278 of the handbook. One of the major uses of ionic liquids is as a reaction solvent. The HSP correlation found for reaction rate data in (14) are also close to the HSP of the reaction solvents and ionic liquids, giving a convergence of HSP for all of these.

These HSP are somewhat dependent on how one evaluates the data, but values near [20.4, 16.0,14.6] are a reasonable average. There is obviously a region of HSP at high values of δP and δH not too different from the HSP of the common reaction solvents that is characteristic of the positive solubility effects in this reaction as well.

The reaction rate constants are for the [4+2]-cycloaddition of singlet oxygen with 1,4-dimethylnaphthalene and derivatives in different solvents. This finding is considered significant in itself, in addition to the help this technique can give for future HSP assignments of ionic and other liquids of interest.

 

There are now three methods that can be used to assign HSP to ionic liquids in addition to the traditional ones based on what is miscible with what. This offers great promise for the future, but in each case careful selection of test solvents is mandatory for an accurate HSP assignment. Where HSP spheres are calculated, it is boundary solvents that have greatest interest, while in the more direct methods, it is suggested that solvents with HSP closer to those of the sample be used.

Conclusion

HSP have tentatively been assigned to [bmim]Cl, [bmim]PF₆, [omim]PF₆, and [bmim]BF₄ based on solubility relations and proposed Hildebrand parameters that were found in the literature. The solubility relations with given polymers aid in arriving at the tentative assignments, but useful data are surprisingly few. Three other methods are suggested as offering great promise for determining the HSP of such solutes. These are IGC, intrinsic viscosity (6), and oxygen reaction rate measurement (10,14). In each of these a series of test solvents is used to determine interactions with the liquid sample. Liquids are difficult to characterize with HSP by more traditional methods. The intrinsic viscosity is highest in the best solvent(s), as is the reaction rate constant in the oxygen reaction rate method. The HSP of a significant number of test solvents should be close to the (estimated) HSP of the liquid being studied in all of these methods, whereas in a traditional solubility parameter study based on solubility, it is the boundary solvents for the HSP sphere that are more important. Boundary solvents can also become important for these newer methods, however, if the data are entered directly into the HSPiP sphere optimizing program as done in the above. This more direct analysis is particularly promising, also for finding additional test solvents that help define the situation more accurately.

It may be that one or more of these methods will also allow greater insight into the region where materials are solids and HSP is very much higher than for test liquids. HSP characterizations of materials in this region are very uncertain being based on extrapolations using techniques and equations that have been found very useful in the liquid region, but lack studies providing convincing proof for their use in the solid region.

References:

1.     Hansen, C.M., Einige Aspekte der Säure/Base-Wechselwirkung (Some Aspects of Acid/Base Interactions), Farbe und Lack, Vol. 83, No. 7, 595-598 (1977).

2.     Niel Winterton,  Solubilization of polymers by ionic liquids, J. Mater. Chem., 16, 4281-4293 (2006).

3.     Przemyslaw Kubisa, Application of ionic liquids as solvents for polymerization processes, Prog. Polym. Sci. 29, 3-12 (2004).

4.     Byoung Gak Kim, Eun-Ho Sung, Jae-Seung Chung, Seung-Yeop Kwak, and Jong-Chan Lee, Solubilization and Polymer Analogous Reactions of Polyepichlorohydrin in Ionic Liquids, Journal of Applied Polymer Science, 114, 132-138 (2009).

5.     Yakov S. Vygodskii, Elena I. Lozinskaya, and Alexandre S. Shaplov, Macromol. Rapid Commun. 23, No. 12, 676-680 (2002).

6.     Sang Hyun Lee and Sun Bok Lee, The Hildebrand solubility parameters,  cohesive energy densities and internal energies of 1-alkyl-3-methylimidazolium-based room temperature ionic liquids, Chem. Commun., 3469-3471 (2005).

7.     Hermann Weingärtner, Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversies, Angew. Chem. Int. Ed. 47, 654-670 (2008)

8.     Bela Derecskei and Agnes Derecskei-Kovacs, Molecular modeling simulations to predict density and solubility parameters of ionic liquids, Molecular Simulations, 34, Nos. 10-15, September-December, 1167-1175 (2008).

9.     Wu Xu, Emanuel I.  Cooper, and C. Austen Angell, Ionic Liquids: Ion Mobilities, Glass Temperatures, and Fragilities, J. Phys. Chem. B, 107, 6170-6178 (2003).

10.  Konrad Swiderski, Andrew McLean, Charles M. Gordon and D. Huw Vaughan, Estimates of internal energies of vaporization of some room temperature ionic liquids, Chem. Commun., 2178-2179 (2004).

11.  Fabrice Mutelet, Vincent  Butet and Jean-Noël Jaubert, Application of Inverse Gas Chromatography and Regular Solution Theory for Characterization of Ionic Liquids, Ind. Eng. Chem. Res. 44, 4120-4127 (2005).

12.  Hansen, C.M. The Universality of the Solubility Parameter, Industrial and Engineering Chemistry Product Research and Development, 8, No. 1, March, 2-11 (1969).

13.  Hansen, C.M. and A. Björkman, The Ultrastructure of Wood from a Solubility Parameter Point of View, Holzforschung, 52, 335-344 (1998). 

14.  Jean-Marie Aubrey, Bernadette Mandard-Cazin, Michael Rougee, and René V. Benasson, Kinetic Studies of Singlet Oxygen [4 + 2]-Cycloadditions with Cyclic 1,3-Dienes in 28 Solvents, J. Am. Chem. Soc. 117, 9159-9164 (1995).

 

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