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Article Cite This: ACS Omega 2019, 4, 9400−9406
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Hexadecyl-Containing Organic Salts as Novel Organogelators for Ionic, Eutectic, and Molecular Liquids Cesar N. Prieto Kullmer, Daniel Ta, Christian Y. Chen, Christopher J. Cieker, Onofrio Annunziata, and Sergei V. Dzyuba* Department of Chemistry and Biochemistry, Texas Christian University, Fort Worth, Texas 76129, United States
Downloaded by 193.56.75.43 at 03:23:47:534 on May 30, 2019 from https://pubs.acs.org/doi/10.1021/acsomega.9b00038.
S Supporting Information *
ABSTRACT: The incorporation of a hexadecyl group on imidazolium, pyridinium, and pyrrolidinium scaffolds produces low-molecular-weight ionic organogelators that can gel several types of ionic liquids, deep eutectic solvents (DESs), and several molecular organic solvents. Minimum gelator concentrations fall in the 0.9−15.0% (w/v) range, with the lower end of the gelator concentrations observed in the gelation of DESs. On the basis of polarized optical microscopy, differential scanning calorimetry, and X-ray data, crystallization of these salts appear to produce high-surface-area crystals, which generate sufficiently stable three-dimensional networks that are capable of trapping the solvent molecules. Importantly, the nature of the fluid component of the gel appears to have a profound effect on the morphology of the crystallized organogelators. On the other hand, the organogelators appeared to modulate phase transitions of the liquids.
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transitions of these gels were noted in ca. 20−40 °C range as determined by both ball-drop method and differential scanning calorimetry (DSC). However, the ability of a very simple, normal alkyl chain containing 1-alkyl-3-methylimidazolium-based ILs to act as LMWGs toward ILs, for example, has not been explored. Yet, it could be argued that the simple preparation of LMWGs is one of the main advantages in terms of the potential applications and utilization. Furthermore, especially in the case of imidazolium-based salts, the ability to generate a series of gelators via a relatively straightforward and facile anion metathesis reaction significantly increases the versatility and scope of LMWGs. Here, we report a novel ability of cetylcontaining organic salts, namely, 1-hexadecyl-3-methylimidazolium [C16-mim], N-hexadecylpyridinium [C16-py], and Ncetyl-N-methylpyrrolidinium [C16C1-pyrr] (Figure 1A), to gel several representative classes of ILs (Figure 1B), deep eutectic solvents (DESs; Figure 1C), and organic molecular solvents. It should be noted that [C16-mim]-based salts were previously used either as surfactants or as a part of the gel composites.18−22 In addition, the cetyl-containing, CO2Hfunctionalized imidazolium salt [N-C16,N′-CO2H-im]Br was used to gel a binary (H2O/DMSO) mixture at 20 wt %, whereas [C16-mim]Br, which was used as a control, failed to induce the gelation.23 Furthermore, betaine-ester imidazolium bromide, that is, [C1,(CH2CO2)C16-im]Br (at 21.9−49.3 wt % concentration), was used for gelation of wet CHCl3; the MGC for [C1,(CH2CO2)C16-im]PF6 and [C1,(CH2CO2)C16-im]BF4 salts was above 50 wt %.24 Also, [C16-py]-based salts were used
INTRODUCTION The self-assembly of liquids into gels, that is, solid-like structures that possess some fluidity within the solid network, presents an interesting, intriguing, and useful paradigm.1−3 Gel-based systems have shown to be viable for various catalytic, energy-related, and environmental processes.1−3 Although numerous types of molecules (e.g., small molecule, oligomer, polymer, inorganic materials, etc.) have been used to induce the gelation of various types of fluids, it could be argued that the use of low-molecular-weight gelators (LMWGs) provides maximum flexibility in regard to the manipulation of the structures of the gelators and gels as well as the properties of the gels for a better fit for potential applications.3−5 Because of the tunability of their physical properties via straightforward structural and functional manipulations of the cationic and anionic counterparts, ionic liquids (ILs) are unique, versatile materials that are of interest for many applications.6,7 Imidazolium-based ILs are among the most versatile and widely used types of ILs, and they have been utilized in a number of gel-based systems.8,9 Several LMWGs featuring the imidazolium moiety along with either long dialkyl substituents or dicationic imidazolium-based salts have been reported to gel aqueous and organic solvents as well as ILs.10−17 Specifically, didodecylimidazolium salts with di- and tricarboxylate-containing anions were able to gel dimethyl sulfoxide (DMSO) as well as several imidazolium-, pyrrolidinium-, and ammonium-based ILs with a minimum gelator concentration (MGC) of 1.2−5.0% (wt).10 In addition, noctyl- and n-dodecyl-containing bis-imidazolium-based gelators were shown to gel not only imidazolium-based ILs but also alcohols, such as ethanol, propanol, octanol, and glycerol, with MGC in 2−20% (w/w) range.11−17 In general, gel-to-sol © 2019 American Chemical Society
Received: January 5, 2019 Accepted: May 9, 2019 Published: May 29, 2019 9400
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Table 1. MGC and Melting Points of Various Gels Using [C16-mim]Br as Gelatora entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Figure 1. Structures of compounds used in this work: (A) Nhexadecyl-containing imidazolium, pyridinium, and pyrrolidinium bromides used as LMWGs; (B) imidazolium, phosphonium, pyridinium, and pyrrolidinium ILs, where X is an anion, e.g., NO3, PF6, BF4, NTf2, etc.; and (C) DESs.
15 16 17 18 19 20 21 22 23 24 25 26
as a component of more complex gelation systems and in the gelation of biphasic systems.25−27
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RESULTS AND DISCUSSION As with many cases involving LMWGs, we serendipitously discovered that [C16-mim]Br could gel [C4-mim]Br at 5.1% w/ v concentration (Table 1, entry 1). Interestingly, neither [C12mim]Br nor [C10-mim]Br gelled [C4-mim]Br at the same concentration (data not shown), which suggested that the cetyl substituent on the imidazolium core could be required to induce the gelation, and thus we decided to investigate the gelating ability of [C16-mim]Br toward other ILs. Depending on the length of the 1-alkyl substituent on 1-alkyl-3methylimidazolum bromide, that is, [Cn-mim]Br, with n = 5−9, the gelating ability of [C16-mim]Br was diminished as the length of the 1-alkyl substituent on the IL increased (Figure S1). Only [C5-mim]Br and [C6-mim]Br formed gels, albeit at a higher concentration of the gelator than in the case of [C4mim]Br (Table 1, entries 2 and 3); the rest of the ILs in this series remained fluid upon addition of 10% w/v of [C16-mim] Br even after 48 h (Figure S1). As bromide-containing imidazolium ILs are highly viscous, hygroscopic oil-like materials and are mainly used as precursors for the synthesis of other ILs, we evaluated other [C4-mim]based ILs using [C16-mim]Br as the gelator (Table 1, entries 3−11). In the concentration range of 5−15% w/v, [C16-mim] Br was able to gel PF6-, BF4-, and NO3-containing ILs (Table 1, entries 4−8). On the other hand, [C4-mim]TFA, [C4mim]NTf2, and [C6-mim]NTf2 did not undergo gelation even in the presence of 15% w/v of [C16-mim]Br (Table 1, entries 9−11). Overall, there appeared to be an intricate balance between the length of the alkyl substituent and the nature of the anion that modulated the gelation process of imidazoliumbased ILs. In order to determine whether other types of ILs could be gelled using [C16-mim]Br as an LMWG, we examined the gelation of phosphonium, pyridinium, and pyrrolidinium ILs, namely, [P66614]Cl, [C4-py]NO3, and [C4C1-pyrr]NTf2 (Table
MGC, % w/v (mp, °C) ILs [C4-mim]Br [C5-mim]Br [C6-mim]Br [C4-mim]PF6 [C6-mim]PF6 [C4-mim]NO3 [C6-mim]NO3 [C4-mim]BF4 [C4-mim]TFA [C4-mim]NTf2 [C6-mim]NTf2 [P66614]Cl [C4-py]NO3 [C4C1-pyrr]NTf2 DESs L-Pro/OA L-menth/C11H23CO2H EG/ZnCl2 Molecular Solvents toluene dioxane chlorobenzene EG methanol acetone DMSO chloroform pyridine
5.1 (36) 10.0 (26)b 10.0 (36)b 6.0 (41) 5.0 (30) 10.6 (27) 11.0 (28) 5.5 (47) c c c
10.0 (37) 4.4 (29) c
1.0 (37) 0.9 (41) 1.1 (38) 9.9 (35) 9.9 (41) c c c c c c c
a
Average melting points, determined using the inverted vial method after 12−24 h. bLonger (>24 h), nonconsistent gelation times were noted. cSolution, no gelation upon addition of at least 15% w/v of [C16-mim]Br; TFA = CF3CO2, NTf2 = N(SO2CF3)2, and EG = ethylene glycol.
1, entries 12−14), respectively. Although the gelation of phosphonium and pyridinium ILs was observed, no gel formation was noted in the case of [C4C1-pyrr]NTf2 (Table 1, entries 12−14, respectively). Next, the scope of fluids that could be gelled by [C16-mim] Br was expanded to DESs, which have recently emerged as a relatively inexpensive alternative to ILs.28,29 It appeared that commonly used DESs, such as L -Pro/OA, L -menth/ C11H23CO2H, and ethylene glycol (EG)/ZnCl2 (Figure 1C), could be gelled by [C16-mim]Br (Table 1, entries 15−17). It is of interest to note that the MGC values for gelation of DESs were several-fold lower than those for ILs (Table 1). Finally, the ability of [C16-mim]Br to gel molecular organic solvents was evaluated (Table 1, entries 18−20). Out of the solvents tested, only gelation of toluene, dioxane, and chlorobenzene formed gels with ca. 10% w/v of the gelator (entries 17−20). Notably, unlike previous examples of imidazolium-based organogelators, which were able to gel DMSO and some alcohols and glycerol,10,12,15 [C16-mim]Br was unable to induce gel-like formation of alcohols (entries 21 and 22). However, it is of interest to point out that although EG did not undergo gelation even when 15% w/v of [C16mim]Br was used (Table 1, entry 21), EG/ZnCl2 was gelled in the presence of a relatively low amount of [C16-mim]Br (Table 9401
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Figure 2. POM images of gels of ILs (A−D), DESs (E,F), and molecular solvents (G,H) with minimum [C16-mim]Br concentration (see Table 1). Top images: Polarized light exposure; bottom: direct field exposure. White scale bar is 20 μm.
the fluid from the solid components. 1H NMR of both phases (fluid and solid/crystal) revealed that the solid component was primarily composed of [C16-mim]Br (small amounts of Lmenth/C11H23CO2H were expected, as complete removal of the DES without compromising the nature of the solid phase was not possible), whereas the fluid component was L-menth/ C11H23CO2H (Figure S2). On the other hand, well-defined, relatively large crystals with similar morphologies were observed for toluene and dioxane gels (Figure 2G,H, respectively). Crystalline-like structure of [C16-mim]Br in various fluids was further confirmed by powder X-ray diffraction (XRD) measurements (Figure S3). The diffractograms of L-menth/ C11H23CO2H, [C4-mim]PF6, and dioxane gels exhibited peaks which were indicative of some degree of order. Notably, however, the crystal structure of the gelator in these fluids was found to be distinct form that observed in the case of the neat gelator (Figure S3). This was also supported by distinctly different crystalline morphologies of [C16-mim]Br that were observed by POM in these liquids (Figure 2B,E,G). Such structural organization of the gelator in the gels, likely coupled with high surface area of the crystals of the gelator, as evident from POM images, might be responsible for the induction of the gel formation of these liquids. Overall, based on POM images and XRD data, it appeared that gelator was present in a crystalline form in the gel, and the specific shape, size, and thus the morphology of its crystals depended on the environment. Next, DSC studies were conducted to assess the effect of [C16-mim]Br on the phase transitions of the liquids (Figures 3 and S4−S21, Table S1). In general, the nature of phase transitions of the liquids (ILs and DES) appeared to be largely unaffected by [C16-mim]Br, while additional transitions related to the melting and crystallization of the gels were noted. For example, in the case of [C4-mim]NO3 (Figure 3A), only cold crystallization (Tc at 22 °C) was noted in the presence of the gelator (bright blue curve) as well as the melting of the gel (Tm) at 27 °C; yet, the glass transition of this IL was unaffected during the heating and cooling cycles. In contrast, in the case of [C4-mim]PF6 IL (Figure 3B), a glass transition (Tg) at ca.
1, entry 17). [C16-mim]Br was insoluble in water, hexane, and cyclohexane. Subsequently, the gel phases were examined using polarized optical microscopy (POM), and the morphology of the gels was found to be strongly dependent on the nature of the fluid (Figure 2). It should be noted that the choice for POM was driven by its least invasive nature, as compared to scanning electron microscopy and transmission electron microscopy, for example, which require removal of the fluid prior to analysis. It has been suggested that the removal of ILs, which is typically done by using a solvent which is miscible with IL, for example, but immiscible with the gelator, might compromise the structure of the gelator.30 Considering that the ILs (and DESs) and the gelator [C16-mim]Br possess slightly similar solubility in a range of solvents, removal of ILs (or DESs) from the gel without disrupting the structure of the gelator would not be feasible. On the other hand, POM allowed for the assessment of the morphology of the gels without perturbing the overall structure of the gels. In the case of ILs, POM images showed optical anisotropy, which could be attributed to the formation of ordered crystalline structures; however, distinct morphologies were observed (Figure 2A−D). Specifically, for [C4-mim]PF6 gel (Figure 2A), platelet-like structure with widths on the order of 10−100 μm were noted. For [C4-mim]NO3 gel (Figure 2C), clusters of rodlike crystalline fragments with sizes similar to those shown for [C4-mim]PF6 gel were observed. On the other hand, needlelike small crystals with thicknesses on the order of 1 μm were observed in the case of [C4-mim]BF4 and [P66614] Cl (Figure 2B,D, respectively). Distinct morphologies were also noted for DES gels, with Lmenth/C11H23CO2H exhibiting relatively large crystalline structures (Figure 2E), while needlelike microcrystals were observed in the L-Pro/OA solvent (Figure 2F). In addition, because L-menthol is known to form large crystals, it was possible that the addition of [C16-mim]Br to this DES induced the crystallization of methanol, which in turn caused gelation. To evaluate this possibility, the gel was broken up with a spatula, followed by brief vortexing, and centrifuged to separate 9402
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morphology of the resulting gels appeared to be dependent on the anion of the gelator. POM images of the gels (Figure S22) revealed fairly large platelet-like structures for X = Br and PF6 and crystalline, rodlike structures for X = BF4 (Figure S22). On the other hand, for X = NTf2 and TFA, amorphous morphologies were observed (Figure S22). Furthermore, DSC measurements demonstrated that these gels were fairly similar to the gel of [C4-mim]PF6 formed with the aid of [C16-mim]Br (Figure 3A), that is, the disappearance of the glass transition, disappearance of the cold crystallization upon heating, with the appearance of the cold crystallization upon cooling was observed in all cases, regardless of the nature of the anion of the gelator (Figure S23). Finally, in order to probe the importance of the cation on the gelation, we prepared N-hexadecylpyridinium bromide, [C16-py]Br, and N-hexadecyl-N-methylpyrrolidinium bromide, [C16C1-pyrr]Br (Figure 1A), and investigated their ability to gel various liquids (Table S3). We found that both [C16-py]Br and [C16C1-pyrr]Br were able to gel ILs, DESs, and dioxane (Table S3). Although preliminary in scope, the results indicated that the gelating ability of the gelators containing aromatic moieties, that is, [C16-mim]Br and [C16-py]Br, was slightly superior to the aliphatic counterpart, that is, [C16-pyrr] Br (Table S3). It is also of interest to point out that as compared to dipyrrolidinium-based organogelators,13 the gelation ability of [C16-pyrr]Br gelator toward ILs was comparable in regard to MGC as well as gel-to-sol phasetransition temperature. Arguably, these results further supported a notion that gelation of various fluids, including ILs, could be achieved with structurally simple and easily accessible gelators.
Figure 3. DSC traces (heatingred; coolingblue) of liquids (pale lines) and respective gels (bright lines); the traces were artificially offset to illustrate the differences. (A) [C4-mim]NO3 and [C4mim]NO3 + [C16-mim]Br (10.5% w/v); (B) [C4-mim]PF6 and [C4mim]PF6 + [C16-mim]Br (6.0% w/v). Heating/cooling rates were 10°/min. Samples were heated to 70 °C (segment not shown) and subsequently cooled (blue traces) and heated (red traces). Tg midpoint of a glass transition; Tc and Tmmaximum and minimum of the transition.
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−80 °C was observed during both cooling and heating cycles. The heating curve (pale red) also exhibited a Tc at −26 °C, followed by melting (Tm) at 12 °C. On the contrary, in the presence of the gelator, several crystallizations occurred upon cooling, that is, crystallization of the gelator (16 °C) and then the IL (−17 °C). This suggested that the gelator-induced crystallization of the IL and, as a result, glass transitions were not observed upon either cooling or heating. Interestingly, upon heating (Figure 3B, bright red), a complex phase behavior was observed at around 0 °C: an overlap of exothermic and endothermic peaks suggested that as the crystal was redissolving, and another type of crystal was forming. This indicated that the crystalline phase formed in the presence of [C16-mim]Br was distinct from the crystalline phase that was forming upon heating neat [C4-mim]PF6. Such a phase behavior was also noted in a number of other cases (Figures S4−S20, Table S1). Arguably, these preliminary studies indicated that [C16-mim]Br might be used to modulate the phase transitions of some ILs over a wide range of temperatures. In order to determine whether the anion of the cetylcontaining imidazolium salts had an effect on gelation, we prepared a set of [C16-mim]X, where X = PF6, NTf2, TFA, and BF4 and evaluated their ability to gel [C4-mim]PF6 (Table S1). No significant variations in the MGCs or in melting points of the gels were noted upon changing the anion of the [C16mim]-based salts (Table S2). This result was not surprising considering the common ion effect. Because the amount of the gelator (and hence the amount of the anion associated with [C16-mim]-cation) was significantly lower than the amount of PF6 anion of [C4-mim]PF6, the gelation would be predominately governed by the [C16-mim]-cation. However, the
CONCLUSIONS In conclusion, imidazolium-, pyridinium-, and pyrrolidiniumbased salts containing n-hexadecyl groups were found to be viable LMWGs that were capable of gelling various types of liquids, including ILs, DESs, and molecular solvents. The gelation ability of the n-hexadecyl-containing salts appeared to stem from crystallization of these salts upon cooling, which produced relatively small crystals with high surface. These crystals of the gelator molecules may span throughout the mixture, generating a sufficiently stable network that significantly reduces the fluidity of solvent because of trapping of the solvent molecules within their three-dimensional networks. The crystalline network of these novel gelators appears to induce the gelation of the investigated fluids similarly to fibrous aggregates of common organogelators. In addition, when compared to other more structurally complex imidazolium-based organogelators, both mono- and dicationic ones,10−17 it appeared that the reported novel gelator 1hexadecyl-3-methyl-imidazolium bromide, for example, exhibited a similar minimum gelation concentration range [0.9−11.0% (w/v)] as well as the gel-to-sol phase-transition temperature range (ca. 25−40 °C). Overall, current results indicate that structurally minimalistic, easily accessible organic salts could be used to impart gel-like properties to various types of fluids.
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EXPERIMENTAL SECTION Materials and Methods. All chemicals and solvents were from commercial sources (Aldrich, TCI, Acros), they were of highest grade possible, and used as received. 1H, 19F, and 31P
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and oxalic acid dehydrate were mixed in 1:1 molar ratio. Lmeth/C11H23CO2H:36,37 L-menthol and dodecanoic acid were mixed in 1:1 molar ratio. ZnCl2/EG:38,39 ZnCl2 and ethylene glycol were mixed in 1:4 molar ratio. Imidazolium, pyridinium, and pyrrolidinium ILs as well as the corresponding gelators were made according to literature procedures (Scheme S1).40−47 [P66614]Cl was a commercial sample from TCI. All IL samples were dried for several hours under vacuum prior to the addition of the gelator. All ILs were purified as follows: ILs were dissolved in CH2Cl2, followed by filtration to get rid of inorganic impurities. Next, ILs were repeatedly treated with charcoal in EtOH at elevated temperatures followed by filtration and removal of EtOH in vacuo (for an azeotropic removal of residual water). Finally, the ILs were dried under vacuum for 2−4 h.
NMR spectra were recorded on a Bruker (400 MHz) spectrometer, and the chemical shifts were reported in ppm (δ) downfield from tetramethylsilane in CDCl3 or residual solvent peak in acetone-d6 or DMSO-d6. Preparation of Gels and Gel Melting Temperature Measurements. Gelation tests were performed by the inverted vial method. Briefly, a screw-cap vial was charged with the appropriate amount of organogelator and the appropriate liquid (typically 0.5 mL). The vial was capped and heated using a heat gun until a homogeneous solution was obtained. The vial was allowed to cool to room temperature, and after 12−24 h, it was inverted to check for any fluids sliding down the wall. No fluid sliding down the vial’s wall was considered as a positive test for gelation. Melting points were determined by using test-tube-tilting method.31 Briefly, a vial containing the gel was inversely/vertically (or sideways, i.e., horizontally) immersed into a water bath and the temperature was raised at 1°/min. The Tgel was recorded as the temperature when the gelled mass started to flow downward. Averages of at least two independent measurements (the vial with the gel was positioned vertically and horizontally during the melting temperature measurement) are reported; the standard deviation (±SD) was less than 2° in all cases. Phase Transitions of the Gels and Fluids. Phase transitions were measured using METTLER TOLEDO DSC 1 instrument operated under N2 atmosphere. Samples (5−25 mg in Al pans, Mettler Toledo) were heated from 25 to 70 °C at 10°/min (segment 1), then cooled from 70 to −120 °C (segment 2), and then heated to 70 °C at 10°/min (segment 3). Segments 2 and 3 were plotted in figures of the main text and the Supporting Information. The exact conditions are specified in the figure captions. For all ILs and DESs, the samples were prepared in Al pans with a hole in the lids; for toluene and dioxane, DSC samples were prepared in Al pans without a hole in the lids to prevent evaporation of the solvent. Morphological Study of the Gels. POM images were obtained using a light microscope (Axioskop 40, Zeiss), and images were taken by a digital camera (AxioCam MRc, Zeiss) and processed using a manufacturer-provided software (AxioVision AC 4.5, Zeiss). Samples were pipetted from the vials using plastic disposable pipettes and placed in between glass slides for visualization. Powder XRD of the Gelator and the Gels. The samples were analyzed using the Shimadzu XRD-7000 in the Center for Environmental, Forensics, and Material Science at the University of Texas at Arlington. A sample was transferred onto aluminum plates containing 25 mm diameter divots. Care was taken to ensure that the sample was solely contained within the divots and maintained an approximately even surface with the surrounding sample plate. Once loaded in to the XRD, the samples were rotated at 6 rpm and ran from a 2θ of 2°−50° at a step size of 0.02° and scan speed of 2°/min with an X-ray (Cu Kα source = 1.5406 Å) accelerating voltage of 40 kV and current of 30 mA. Scans were obtained in approximately 30 min. After the scans were collected, they were processed using the MDI Jade 9 software package. Synthesis of Gelators, ILs, and DESs. DESs were prepared according to modified literature procedures.32−39 Briefly, appropriate molar amounts of each compound were mixed in a screw-cap glass vial, capped, and repeatedly heated using a heat gun followed by short periods (20−30 s) of vortexing until transparent, homogeneous fluids were obtained. These liquids were used as prepared. L-Pro/OA:33−35 L-proline
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00038.
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Synthesis and characterization of ILs, additional experimental details, DSC scans, and optical microscopy images of the gels (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1 (817) 257-6218. Fax: +1 (817) 257-5851. ORCID
Onofrio Annunziata: 0000-0002-6636-4750 Sergei V. Dzyuba: 0000-0002-4835-5885 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by TCU-RCAF (O.A.), TCUDepartment of Chemistry and Biochemistry (S.V.D.), and TCU-SERC award nos. UG 170322 (C.N.P.K.), UG 180519 (D.T.), and UG 170316 (C.J.C.).
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REFERENCES
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DOI: 10.1021/acsomega.9b00038 ACS Omega 2019, 4, 9400−9406
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DOI: 10.1021/acsomega.9b00038 ACS Omega 2019, 4, 9400−9406
