Geminal Imidazolium Salts: A New Class of Gelators - Langmuir (ACS

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Geminal Imidazolium Salts: A New Class of Gelators Francesca D’Anna,* Paola Vitale, Salvatore Marullo, and Renato Noto* Dipartimento STEMBIO, Sezione di Chimica Organica “E. Paternò”, Viale delle Scienze-Parco d’Orleans II, 90128 Palermo, Italy S Supporting Information *

ABSTRACT: The gelling behavior of some geminal diimidazolium salts was investigated in solvents differing in polarity and hydrogen bond donor ability. The used salts, namely the 3,3′-di-n-decyl-1,1′(1,4phenylenedimethylene)diimidazolium dibromide [p-Xyl-(decim)2][Br]2 (1), the 3,3′-di-n-dodecyl-1,1′(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(dodecim)2][Br]2 (2), and the 3,3′-di-n-dodecyl1,1′(1,4-phenylenedimethylene)diimidazolium ditetrafluoroborate [pXyl-(dodecim)2][BF4]2 (3), differ in the alkyl chain length and in the anion properties, such as size, shape, and coordination ability. In all cases in which gelation process was observed, the obtained gels were characterized by gel melting temperature determination, resonance light scattering, and UV−vis measurements. On the whole, the investigation allowed to get information about both the thermodynamic stability and the features of the aggregates characterizing the soft materials at the equilibrium. Data collected by us point out that the used organic salts are able to behave as both hydro- and organogelators. In particular, bromide salts formed hydrogels in the presence of α-cyclodextrin allowing to hypothesize that the gelation process is favored by the formation of supramolecular assemblies. To verify this hypothesis, 1D and 2D 1H NMR measurements were carried out. Both the alkyl chain length and the anion ability to reticulate the three-dimensional network proved to be determinant factors in affecting the gelation process as well as the features of the gel phases. Finally, with the future aim to use the obtained gels as reaction media, the effect of a guest molecule such as the UV−vis active probe Nile Red was studied.

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INTRODUCTION Gels are widely used in many industrial fields, such as pharmaceutical, cosmetic, and food industry.1 They are frequently defined as two-component systems, deriving from a hierarchical organization of gelator molecules.2 In particular, as a consequence of the establishment of different supramolecular interactions, a three-dimensional fibrous network is formed. The second component, i.e. the solvent, is immobilized within this network. It is well-known that both polymers and low molecular weight organic molecules may behave as gelators.3 In this latter case, many attempts have been carried out during the years in order to understand how both solvents and gelator properties should be conjugated in order to obtain a gel phase. Obviously, this has stimulated the interest and curiosity of researchers: it is noteworthy that more than 100 papers have been published in 2011 having “gelator” as a keyword. As far as the gelator structure is concerned, different approaches have been used. Recently, Dastidar and co-workers4 have tried to understand if the structure of the gelators can be suitably designed on the grounds of criteria deriving from the knowledge of the supramolecular forces which drive the gelation processes. On the other hand, McNeil and coworkers,5 using an approach based on the determination of solubilization enthalpies, have distinguished gelators from nongelators evidencing that a complex relationship exists between molecular structure and gelation ability. On the © 2012 American Chemical Society

grounds of the thermodynamic parameters of dissolution, it has been claimed that the majority of gelators shows larger dissolution enthalpies and entropies than nongelators, consistent with the occurrence of both stronger intermolecular interactions and stricter molecular order. Furthermore, Boutellier and co-workers5c using Hansen solubility parameters for both solvents and gelators have recently proposed a method to predict the behavior of a known gelator in untested solvents. What has been argued is that small changes in the structure may induce remarkable effects on the gelation ability.6 It has been frequently stated that new gelators are serendipitously discovered. Different organic substrates have been used as gelators, including both neutral molecules and organic salts.7 Among neutral molecules, different examples of dipeptides,8 salicylanilides,9 binaphthyl-based molecules,10 urea derivatives,11 or oxadiazoles12 have been reported. On the other hand, as far as organic salts are concerned, ammonium,13 imidazolium,14 benzimidazolium,15 benzotriazolium,16 and benzylammonium17 salts have been tested. The use of organic salts as low molecular weight gelators (LMWG) might be important in view of the applications that conductive gels may have in sensors, separation membranes, and solar cells.18 Received: March 30, 2012 Revised: June 22, 2012 Published: June 24, 2012 10849

dx.doi.org/10.1021/la301319u | Langmuir 2012, 28, 10849−10859

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The aggregation process was investigated by using resonance light scattering measurements (RLS). On the other hand, gel opacity was studied by using UV−vis spectroscopy. In the case of the hydrogels, also polarimetric and NMR measurements were carried out to have further information about the nature of the gelator. Bearing in mind that many applications of the gel phases entail the presence of a second component (e.g., their use as reaction media or in drug release processes), the stability and the aggregation process of the gel formed by 2 in ethylene glycol were studied in the presence of an organic guest molecule, such as Nile Red. Finally, gels morphologies were analyzed by scanning electron microscopy (SEM).

In the past few years we have been involved in the synthesis of new geminal imidazolium salts with the main goal to use them as highly organized reaction media.19 The observation that some of these salts were able to induce gelation processes prompted us to study properties of the soft materials they were able to form. In particular, we took into account the 3,3′-di-ndecyl-1,1′(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(decim)2][Br]2 (1), the 3,3′-di-n-dodecyl-1,1′(1,4phenylenedimethylene)diimidazolium dibromide [p-Xyl(dodecim)2][Br]2 (2), and the 3,3′-di-n-dodecyl-1,1′(1,4phenylenedimethylene)diimidazolium ditetrafluoroborate [pXyl-(dodecim)2][BF4]2 (3) (Scheme 1).

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Scheme 1. Representation of the Used Gelators and Guest Molecule

RESULTS AND DISCUSSION

Gelation tests were carried out using solvents of different polarity. In particular, water, aqueous solution of α-CD, alcohols, and diols together with aprotic polar solvents, such as acetone, acetonitrile, dimethylformamide, and dimethyl sulfoxide were taken into account. In all cases in which gel formation was observed, the critical gelation concentration (CGC, i.e., the lowest organogelator concentration inducing the gel formation) and the Tgel values (i.e., the melting temperature of the gel) were determined. All gelators gave rise to stable thermoreversible opaque gels. The macroscopic behavior of the obtained materials was initially analyzed by monitoring the transition from an immobile to a mobile self-assembly state using the “tube inversion test” (Figure 1 of Supporting Information). This simple method is widely used to assess the gel formation of low molecular weight organogelators.20 Our gels remained stable for almost 4 months at room temperature. Data collected in the presence of all organogelators studied in this work, as a function of the used solvents, are reported in Table 1. In order to gain a deeper understanding of data collected, herein we will analyze them as a function of the different alkyl chain length and nature of the anion.

The used organic salts differ in the length of the alkyl chain (1 and 2) but also in the size, shape, and coordination ability of the anion (2 and 3). All these changes in the gelator structure may induce variations in the supramolecular interactions operating in the gelation processes, such as van der Waals and hydrophobic interactions as well as in hydrogen bonding. The gelling behavior of the aforementioned salts was investigated both in water and in organic solvents of different polarity and hydrogen bond donor ability. In particular, the gelation process in water solution was observed only in the presence of α-cyclodextrin. After preliminary gelation tests, we studied the thermodynamic stability of the obtained materials both as a function of the nature of the solvent used and organogelator concentration.

Table 1. Critical Gelation Concentrations and Tgel Values (K) of Gels Formed by 1, 2, and 3 in Different Solventsa [p-Xyl-(decim)2][Br]2 (1) CGCb

solvent water water/CDd water/CDe ethanol propanol 2-propanol butanol pentanol hexanol ethylene glycol glycerol acetone acetonitrile DMF DMSO

PG PG OG S S S S S PG S CP S S S S

5.0

[p-Xyl-(dodecim)2][Br]2 (2)

Tgel (K)c

305

PG OG OG S S S CP S PG OG OG OG OG S S

CGCb

Tgel (K)c

3.7 4.0

310 325

2.6 3.3 4.8 4.7

297 305f 304 306

[p-Xyl-(dodecim)2][BF4]2 (3) PG PG PG CP OG OG OG OG PG OG OG S S S S

CGCb

Tgel (K)c

3.2 4.2 2.0 4.3

315 319 332 312

4.4 3.4

345 344f

a

PG = partial gel; OG = opaque gel; CP = crystalline precipitate; S = solution. b(%, w/w, organogelator/solvent). In all cases the CGC was investigated in the range 1−6%. cTgel were reproducible within 1 K. d1 equiv of α-CD. e2 equiv of α-CD. fValues determined at 5% w/w. 10850


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Effect of the Alkyl Chain Length. First, we analyzed the ability of the two bromides differing in the alkyl chain length (1 and 2) to act as gelators. The main difference that can be immediately evidenced between the substrates is the different nature of the solvent able to induce the gel formation. Indeed, in the presence of 1 the gel formation was observed only in an aqueous solution of α-CD. On the other hand, 2 was able to act both as hydro- and organogelator. Analysis of data collected in aqueous solution allows to underline a further difference between two gelators. As a matter of fact, in both cases, in aqueous solution a gel-like precipitate was obtained. However, gel formation was induced by the addition of different amounts of α-CD. In particular, in the case of 1, a stable opaque gel was obtained using 2 equiv of α-CD. On the other hand, in the case of 2, stable opaque gels were obtained using both 1 and 2 equiv of α-CD. According to the increase in the alkyl chain length, 2 formed a hydrogel at a lower CGC. Similar behaviors as a function of the alkyl chain length have been frequently reported in the literature21 and have been ascribed to an increase in the van der Waals and hydrophobic interactions that induce a larger tendency to aggregation for the monomers. Keeping constant the α-CD/gelator ratio (2:1), the stability of the soft materials also increased on going from 1 to 2 with the alkyl chain length, as accounted for by Tgel values. On the other hand, for the same hydrogelator (2), the stability of the hydrogel was positively affected by an increase in the α-CD concentration. It is noteworthy that both the hydrogels formed by 2 showed a thixotropic behavior. Indeed, upon vigorous shaking, they were able to reform an opaque gel after storage. It is well-known that this property, sometimes characterizing gel phases, has important implications for their use. It has been frequently stated that native CDs do not undergo gelation in water. In our case this statement was further confirmed by the fact that gel formation did not occur by heating and subsequently cooling a saturated α-CD aqueous solution which has a concentration comparable to the ones used in the gel preparation (0.15 M). On the grounds of all above results, a further point to clarify was the role played by the α-CD. Indeed, this cyclic oligosaccharide might behave as an host, establishing host− guest interactions with the used organic salts or, alternatively, as a simple additive able to change properties of the solvent medium. In order to discriminate between these hypotheses, we analyzed diluted solution of both 1 and 2 in the presence of αCD, using polarimetric measurements. In all cases, the used αCD/gelator ratios were comparable to those used in the gel preparation. On carrying out polarimetric measurements, we bore in mind that the optical rotation can be significantly affected by the ionic strength of the solvent medium. Therefore, we also measured the optical rotation of α-CD solution in the presence of both inorganic and organic bromides, namely NaBr and Bu4NBr. In Table S1 of the Supporting Information, molar optical rotation (θ) and differential molar optical rotation (Δθ) are reported. In particular, the latter values represent the difference between the molar optical rotation of free α-CD and the ones measured in the presence of different bromides. As a consequence of the different water solubility of the organic salts and in order to have clear measurement samples, lower concentrations were used in the case of 2. Analysis of data reported in Table S1 allows to assess some considerations on the effect exerted by different bromides. First,

both NaBr and Bu4NBr were able to induce only minor variations in the molar optical rotation of α-CD. By contrast, addition of the geminal imidazolium salts induced in both cases a significant variation in the same parameter, which resulted more pronounced in the presence of 2. It has been frequently reported that changes in molar optical rotation of cyclodextrins, in the presence of a guest molecule, may be ascribed to conformational changes occurring in the cyclodextrin host and induced by the guest inclusion.22 As in this case the used gelators caused the highest variations, it can be argued that they were able to establish with the cyclodextrin cavity different and more significant interactions respect to the ones acting in the presence of NaBr or Bu4NBr. On the grounds of these observations, and taking into account the large hydrophobic character of the imidazolium salts used, the above results should account for the formation of a supramolecular complex. In particular, the nature of the used guests might favor the formation of pseudorotaxanes. The obtaining of thermosensitive hydrogels composed by cyclodextrin pseudorotaxanes has been reported in the literature, for example, in the case of alkylpyridinium salts.23 Physical gels based on supramolecular gelators have been recently reviewed.24 However, in most of cases, the formation of gel phase was the result of the cyclodextrin functionalization or the use of a polymer as guest molecule. Bearing in mind all the information reported above, and in order to shed light on the nature of the host−guest interactions, we analyzed diluted solutions of both 1 and 2 in the presence of α-CD, by means of 1D and 2D ROESY 1H NMR measurements. Also in this case, α-CD/gelator ratios were comparable to those used in the gel preparation. In Figure 2 of the Supporting Information, 1D and 2D NMR spectra as a function of the different gelators are reported. In all cases, as a consequence of the low solubility of the two organic salts in water solution, the NMR signals corresponding to the protons of the alkyl chain resulted quite broadened. However, the presence of α-CD improved the quality of the signals and caused for both gelators their downfield shifts without affecting the chemical shifts of all the other signals in the NMR spectra. In general, changes in chemical shift were more significant for methylene than for methyl groups (Δδ ∼ 0.06 and 0.08 ppm for methyl group of 1 and 2, respectively, whereas Δδ ∼ 0.11 for methylene groups). These results indicate that only the alkyl chain is located inside the α-CD cavity and experiences a different microenvironment. The interaction between the alkyl chain and the α-CD cavity was also confirmed by the analysis of 2D ROESY 1 H NMR spectra. Indeed, in the case of 1 (Figure 2e, Supporting Information), cross-peaks were detected between the protons in the 5-position of α-CD (H5) and the methyl group of the alkyl chain (∼0.97−3.44 ppm) and between the 3position of α-CD (H3) and the protons of methylene groups (∼1.57−3.88 ppm). On the other hand, the latter interaction was also detected for 2 in the presence of α-CD as accounted for by the cross-peak between the signal at ∼1.50 ppm and the one at ∼3.80 ppm (Figure 2f, Supporting Information). Effect of the Anion Nature. As we mentioned previously, the structure of the cation being the same, we took into account also imidazolium salts differing in the structure of the anion. In particular, we analyzed how changes in size, shape, and coordination ability, on going from bromide to tetrafluoroborate, were able to affect the gelation ability of imidazolium salts and the properties of the obtained soft materials. In 10851


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Figure 1. Plots of Tgel (K) versus the organogelator concentration (w/w, %) in ethylene glycol corresponding to (a) 2 and (b) 3 (Tgel values were reproducible within 1 K).

the presence of α-CD, gels formed by 2 in ethylene glycol and in glycerol solutions showed a thixotropic behavior. Furthermore, as far as the gelation ability of the two imidazolium salts in glycerol solution is concerned, a peculiar behavior was also evidenced. Indeed, as reported in Table 1, in both cases the gel formation was observed at comparable CGC values (∼3.4%, w/w), as confirmed by the “tube inversion test”. However, these materials were so soft that we were not able to determine Tgel values next to CGC values. Moreover, in the case of tetrafluorborate salt (3) a macroscopic change was also observed: a transparent gel was initially obtained, which became opalescent after standing at room temperature (Figure 1b of the Supporting Information). For both organogelators, in order to have a better understanding of the role played by the solvent in the gel formation process, we searched for a correlation between the gel melting temperatures and its properties, as accounted for by dielectric constants and α values. It is worth noting that this topic is quite debated in the literature, and very different results have been reported for different classes of gelators.27 We determined for both organogelators Tgel values at the same concentration (5% w/w). Data collected as a function of the solvent dielectric constant and α values are reported in Table S2 of the Supporting Information. In all cases, very poor results were obtained by attempting to correlate Tgel with the solvent parameters. Thus, according to previous reports in the literature,28 this may indicate that properties of the obtained gels do not depend only on gelator solubility but are rather affected by the gel microstructure, which often changes with solvent and gelator structure. Analysis of data collected for both organogelators (2 and 3) in ethylene glycol and in glycerol solution allows to have a better understanding of the differences induced in the gel properties by changes in the anion structure (Table S2, Supporting Information). Indeed, in both ethylene glycol and glycerol solutions, Tgel increased on going from bromide to tetrafluoroborate salt (Tgel = 300 and 350 K for 2 and 3 in ethylene glycol and Tgel = 305 and 344 K for 2 and 3 in glycerol solution). The above differences could be ascribed to the extent of cross-linking produced by the two anions, according to their

particular, the coordination ability increases on going from [BF4−] to [Br−]. Indeed, considering monocationic imidazolium salts such as 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium bromide ([bmim][Br]) as references, the β parameter (a measure of the anion ability to accept hydrogen bond) increases on going from the former to the latter one (β = 0.55 and 0.87 for ([bmim][BF4] and [bmim][Br], respectively).25 On the other hand, the above anions show also very different size ([BF4−] > [Br−]). Indeed, it has been reported that the [BF4−] ion has nearly the same radius as the [I−] ion (r = 216 and 229 pm for [I−] and [BF4−], respectively).26 Also in this case, data collected in the presence of 3 as a function of the solvent used are reported in Table 1. Analysis of data reported in Table 1 shows that, unlike 2, the corresponding tetrafluoroborate salt was able to behave only as an organogelator. Indeed, it produced a gel-like precipitate both in water solution and in aqueous solution of α-CD. Probably, this preliminary observation indicates a certain importance of the role played by anion−solvent hydrogen bond in favoring the gel formation. The analysis of data collected in organic solvents allows to highlight some differences, too. In particular, while 3 afforded clear solutions in aprotic polar solvents such as acetone, acetonitrile, DMF, and DMSO, 2 gave rise to stable opaque gels in the first two solvents. However, in the latter cases the CGC values were not significantly affected by the different polarity of the used solvent. The different nature of the anion also affected the gelling ability of two imidazolium salts in alcohol solution. Indeed, 2 was not able to give stable gels in C2−C5 alcohols. On the other hand, gelation processes were observed for the tetrafluoroborate salt in propanol, 2-propanol, butanol, and pentanol solutions. Finally, both organogelators gave rise to stable gels in glycerol as well as in ethylene glycol. In the latter solvents the CGC values increased on going from 2 to 3. Moreover, the solvent being the same, organogels formed by 3 resulted more stable than those formed by 2. From a macroscopic point of view, in analogy to what observed in 10852


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Figure 2. Plots of RLS intensity as a function of the time for 2 (a) and 3 (b) in ethylene glycol.

function of concentration were also carried out. In Figure 2, plots of RLS intensity as a function of the time for both organogelators, in ethylene glycol, at 288 K are reported. Analysis of plots reported in Figure 2 evidence that the different nature of the anion significantly affected the fibrillary network formation. Indeed, the aggregation occurred faster in the presence of 3 than in the presence of 2. Different reports in literature underlined that the gelation process is significantly affected by the rate at which the nucleation occurs.33 In our case, while 3 gave rise to an immediate nucleation (Figure 2b), in the presence of 2 an induction time was observed (∼2000 s) (Figure 2a) that slowed down on the whole the gel formation. It is noteworthy that the kinetic tracks show very different trends. Indeed, in the presence of tetrafluoroborate salt (3) and in the presence of the bromide salt (2), one and two steps were observed, respectively. In the first step an increase of the RLS intensity as a function of the time was detected, until a maximum value was reached. Subsequently, a decrease to an equilibrium value was observed. In order to verify that the observed kinetic trend was a peculiarity of the anion and that was not affected by the solvent nature, we carried out kinetic measurements also by using 1 and 2 in water solution and in the presence of α-CD. (Plots of RLS intensity as a function of the time for both gelators and at different α-CD/gelator ratio are reported in Figure 4 of the Supporting Information.) Once again, bromide salts in water solution gave rise to the gel phase formation through a two-step mechanism. Probably, in the presence of the bromide ion the obtained trend accounts for the formation of larger aggregates, which subsequently evolve into smaller ones characterizing the gel phase at equilibrium. This idea perfectly agrees with the picture frequently reported in the literature about the hierarchical organization that allows the gel phase formation through a self-assembly process giving fibers before gel formation.33 On this point, a less conspicuous but similar behavior was previously observed by Terech and coworkers studying the variation of opacity of organogels formed by 12-hydroxystearic acid, 34 which was ascribed to a contraction of the three-dimensional network proceeding through fiber stacking into bundles.

sizes. This allows to suppose the relevance of long-range interactions, which may first favor the formation of fibers and afterward promote the 3D organization of the gel, giving rise to materials having different stabilities. Interestingly, a different solvent effect was found as a consequence of different properties of the anion. Indeed, Tgel values collected in the presence of 2 increased on going from ethylene glycol to glycerol. An opposite trend was detected in the case of 3. Probably, a balance between the solvent ability to form hydrogen bonds (α = 0.90 and 1.21 for ethylene glycol and glycerol, respectively)29 and the anion ability to give a three-dimensional lattice determines the observed trends. The stabilities of the organogels formed by 2 and 3 in ethylene glycol were also analyzed as a function of the gelator concentration. In Figure 1, plots of Tgel as a function of 2 and 3 concentration are shown (Tgel values as a function of the organogelator concentration are also reported in Table S3, Supporting Information). In order to further analyze the stability of the obtained materials, we determined by using the equation previously reported30 the enthalpy variation for the gel−sol transition (ΔHf) in the two cases (plots of ln C vs 1/T are reported in Figure 3, Supporting Information). Analysis of ΔHf values evidence that the gel−sol transition is significantly affected by the nature of the organogelator. The enthalpy variation decreased from 91.5 kJ/mol down to 76.5 kJ/mol on going from 2 to 3, according to the higher hydrogen bond acceptor ability of the [Br−] anion (see β values). The formation of the self-assembled fibrillary network was also studied as a function of time. In order to investigate the nucleation and growth of the fibers network, we used resonance light scattering measurements. This technique is largely employed in order to study the aggregation processes of molecules containing chromophores in very different systems.31 It is well-known that changes in the RLS intensity can be ascribed to changes in the size of the aggregates.32 Also in this case, we took into account the organogel formed by 2 and 3 in ethylene glycol. In order to have information about the effect of the organogelator concentration on the aggregates size, studies as a 10853


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Figure 3. Plot of (a) RLS intensity and (b) absorbance values, as a function of the time, relevant to pure and two-component gel formed by 2 in ethylene glycol (7% w/w; [Nile Red] = 0.0001 M), at 288 K.

of the gel, inducing significant outcomes, for example, in drug release processes.35 On the other hand, as far as the possibility of using gel phases as reaction media36 is concerned, the knowledge of the effects that the presence of a substrate may exert on the soft materials may be quite important. As the guest molecule we chose Nile Red, due to its spectroscopic and structural properties. Indeed, it may induce changes in the interactions giving rise to the three-dimensional network of the gel, owing to its extended π-surface area and the presence of two potentially hydrogen bond acceptor sites. Furthermore, it is a typical spectrofluorimetric and UV−vis probe. In the case of 2, the study was carried out using an organogelator concentration equal to 7% (w/w) and in the presence of a Nile Red concentration, in the hot solution, equal to 0.0001 M. First, we analyzed the effect of Nile Red on thermodynamic stability of the gel, determining the Tgel values in both the pure and two-component gel. Analysis of collected values shows that the guest molecule induced only a small increase in the melting temperature of the gel at such low concentration. Indeed, Tgel changed from 304 K up to 306 K, in the absence and in the presence of Nile Red, respectively. The gelation process was followed by using RLS and UV−vis measurements. In Figure 3, plots of RLS intensity and UV−vis absorbance, as a function of the time, relevant to both pure and two component gel are reported. Comparison between the obtained results points out that the presence of the guest molecule has only a minor effect on the rate of gelation process. Indeed, both techniques suggest that only small variations occurred on both nucleation and gelation process in the presence of Nile Red. Probably, the guest molecule is able to take part in the building of the network rather than to hinder it owing to its structural features. As far as RLS measurements are concerned, the presence of Nile Red does not change the shape of the kinetic track. Interestingly, formation of two-component gel was characterized by the presence of more extended fibrillary intermediates, as accounted for by RLS intensity. However, in the gel phase, they gave rise to smaller aggregates than the ones

Analysis of data collected as a function of the organogelator concentration points out that in both cases the RLS intensity detected at the equilibrium was affected by the concentration of the salt. Indeed, the IRLS decreased on increasing in the organogelator concentration. However, this effect seems to be more apparent in the presence of 2 than in the presence of 3. As far as data relevant to [p-Xyl-(dodecim)2][Br]2 are concerned (Figure 2a), the increase in the organogelator concentration also affects the size of the intermediate aggregate. Comparison between data reported in Figure 2 shows that the organogelator concentration being the same, tetrafluoroborate gels are characterized at equilibrium by the presence of more extensive aggregates, as testified by the final IRLS values. Finally, from a macroscopic point of view, gels formed by 2 and 3 (at 5% w/w) in ethylene glycol were also analyzed by using UV−vis measurements. In this case, the variation of gel opacity at 568 nm as a function of the time was recorded. (Plots of absorbance values as a function of the time, at 288 K, are reported in Figure 5 of the Supporting Information.) By analogy to data obtained by using RLS measurements, also UV−vis investigation evidences that the gelation immediately occurred in the presence of 3. Comparison between the absorbance values detected at the equilibrium, in the two cases, accounts for a higher opacity of the gel formed by 2. Opacity can be related to the presence of polydisperse nanostructures in the system and, on the whole, to the crystallinity of the gel.34 On the grounds of our experimental data and bearing in mind the above hypothesis, a dependence of the gel opacity on the anion structure can be hypothesized. Indeed, in our case, this parameter decreases on going from bromide to tetrafluoroborate anion. Two-Component Gels. The behavior of gels formed by 2 in ethylene glycol was also studied in the presence of a guest molecule. Our initial project was to study also the effect of a guest on the gels formed by 3. However, in the latter case, the gelation instantaneously occurred and did not allow a precise determination of the whole kinetic track. The study of two-component gels may have many important implications from an applicative point of view. Indeed, the guest molecule can affect both the stability and the morphology 10854


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Figure 4. SEM images of xerogels prepared from the (a) α-CD (2 equiv) water solution of 1 (5%, w/w), (b) α-CD (1 equiv) water solution of 2 (5%, w/w), (c) ethylene glycol of 2 (7%, w/w), (d) ethylene glycol of 3 (7%, w/w), and (e) ethylene glycol of 2 (7%, w/w) in the presence of Nile Red (0.0001 M).

opacity and, consequently, in the crystallinity of the obtained gel. On the grounds of all above results, a plausible gelation mechanism can be supposed. One of the peculiarities of the

corresponding to pure gel. The effect of Nile Red on the gel morphology is also accounted for by opacity measurements. Indeed, considering the absorbance values reported in Figure 3b, the presence of Nile Red also induces a decrease in the 10855


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spherical aggregates, mostly in the range 9−11 μm (Figure 6e,f). They gave rise to a lamellar organization having a medium width of 140 nm (Figure 4d). The morphology of the xerogel formed by 2 from a ethylene glycol was also studied in the presence of Nile Red. Once again, the two-component gel showed a fibrillary organization (Figure 4c,e).

cations used is the electronic complementarity between the aromatic rings present in the structure (imidazolium and phenyl). The interaction between these aromatic moieties may favor the formation of fibrils, the entanglement of which could give rise to the formation of fibers, and consequently to the three-dimensional network needed for the obtaining of the gel phase. Then, in organic solvents π−π interactions should be the driving forces for the gelation process. Obviously, the different anion properties such as size, shape, and coordination ability can affect the distance between the interacting cations and consequently the features of the obtained gel phases. As far as data in water solution are concerned, the role played by the α-CD should be explained. Different reports have underlined that, in water solution, long alkyl chain on organic substrates are rolled up to minimize the unfavorable water/ apolar moiety interactions. Generally, cyclodextrins play a positive role on the uncoiling processes of long alkyl chains, by forming host−guest complexes able to prevent their precipitation, or slowing down the aggregates formation.37 The whole of polarimetric and NMR data collected in this work allows to confirm, also in this case, a similar behavior. Indeed, as NMR spectra seem to indicate, geminal imidazolium salts interact through the alkyl chain with the α-CD cavity. The obtained pseudorotaxanes, having the aromatic moieties exposed to the bulk solution, interact by means of complementary aromatic rings giving rise to fibrils and subsequently to formation of the fibers network. SEM Measurements. The morphology of the obtained materials was also studied by using SEM techniques. In Figure 4, SEM images obtained from the xerogels formed by the used gelators from water (5%, w/w) or ethylene glycol (7%, w/w) are reported. Analysis of the images evidences that both the alkyl chain length and the anion nature induced significant changes in the morphologies of the gels. Significant differences can be evidenced in the morphologies of the gels formed by 1 (with 2 equiv of α-CD) and 2 (with 1 equiv of α-CD) from a water solution of α-CD. In the former case the soft materials seems to have a “spongelike” morphology (Figure 6a, Supporting Information). On the other hand, the pattern for 2 is characterized by the presence of spherical aggregates uniformly distributed in the sample (Figure 6b, Supporting Information). The diameter of these spherical aggregates was mostly in the range 7−9 μm. A similar texture was previously evidenced by Sievänen and co-workers studying the morphology of the xerogels obtained from some derivatives of bile acid alkylamide.38 A deeper analysis of the xerogel obtained by 1 evidenced that the main pattern has a lamellar organization (Figure 4a). On the other hand, in the case of 2, the evaporation of the solvent may have formed pores, making the surface rough in texture (Figure 4b). For the sake of clarity, it is important to stress that we tried also to obtain SEM images of the xerogel of 2 prepared from a water solution of α-CD (2 equiv). However, the sample obtained after the shielding by gold did not allow to collect good quality images. Comparison between SEM images from xerogels formed by 2 and 3 from ethylene glycol showed that also the different nature of the anion induced significant changes in the gel morphology. In the former case a fibrillary organization (Figure 4c and Figure 6d), in which fibers appeared quite entangled and branched. A strikingly different pattern was detected in the presence of 3, characterized also in this case by the presence of

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CONCLUSIONS The whole of collected data evidence that geminal imidazolium salts are able to induce gel phases formation both in water solution and in organic solvents. Our data outline that the ability to form ionogels is significantly affected by the alkyl chain length on the imidazolium cation as well as by the nature of the anion. The formation of hydrogels was observed for bromide salts and in the presence of α-cyclodextrin. In these cases, the experimental results allowed to identify the hydrogelator as an host−guest supramolecular complex. The formation of gel phases as a consequence of pseudorotaxanes formation could offer, as suggested by a referee, a new route to prepare cyclodextrin gels in water solution. In this case, the soft material formation is favored by the intervention of small organic molecules acting as linkers among cyclodextrin cavities. This approach results simpler and more immediate than the one foreseeing the functionalization of the cyclodextrin. Furthermore, it could be quite versatile because of the structural changes to which geminal organic salts can be put. The increase in the alkyl chain length, on going from C10 to C12, was able to activate the gelling ability also in organic solvents. However, in this case, no correlation was found between solvent polarity or its hydrogen bond donor ability and organogel thermodynamic stability. The different nature of the anion was able to affect both thermodynamic and kinetic aspects of gelation process. Indeed, a larger anion size and a lower ability to form hydrogen bond induce a decrease in the thermodynamic stability and an increase in the rate of gel formation. In addition, different kinetic tracks were recorded as a consequence of the different nature of the anion. On the whole, the anion properties seem to affect the size of the aggregates present in the gel phase at the equilibrium and consequently the morphology of the obtained soft materials, as accounted for by SEM investigation. Finally, preliminary results collected in the presence of Nile Red evidence that, at low concentration, the presence of a guest molecule does not induce significant effects on both gel melting temperature and rate of gelification process in confirmation of the fact that also gel morphology stays unchanged. As a consequence, data herein reported could represent a useful support for a future application of these gels for example as reaction media.

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EXPERIMENTAL SECTION

Preparation of Gels and Tgel Determination. Gels were prepared by weighing into a screw-capped sample vial (diameter 1 cm) the amount of salt and solvent (∼250 mg) needed. The sample vial was heated in an oil bath until a clear solution was obtained. The vial was then cooled and stored at 4 °C overnight. The “tube inversion test” method was used to examine gel formation in different solvents. In order to determine the Tgel values, a lead ball (weighing less than 100 mg) was placed on the top of the gel and the vial was put into a water bath. The bath temperature was gradually increased until the gel melted (Tgel) and the lead ball started to move downward.39 Tgel values were reproducible within 1 K. 10856


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Polarimetric Measurements. Sample solutions were prepared in a volumetric flask by weighting the suitable amount of geminal imidazolium salt and α-cyclodextrin. Optical rotations were measured by using a glass polarimetric cell (light path 1 dm) and obtained as the average value over 50 readings. NMR Measurements. 1H, 13C, 19F, and 2D ROESY spectra were recorded on a 300 MHz spectrometer. 2D ROESY spectra were recorded by using a spin-lock mixing time of 300 ms and 64 scans. The temperature was kept at 300 K during measurement. RLS Measurements. RLS measurements were carried out on a spectrofluorophotometer using a synchronous scanning mode in which the emission and excitation monochromators were preset to identical wavelengths. The RLS spectrum was recorded from 300 to 600 nm with both the excitation and emission slit widths set at 1.5 nm. We chose as working wavelength the one corresponding to the intensity maximum of the obtained spectrum. Samples for a typical kinetic measurement were prepared injecting in a quartz cuvette (light path 0.2 cm) the limpid hot solution of salt. Spectra were recorded until gel formation. The gel phase obtained at the end of the measure resulted stable after the “tube inversion test”. Each kinetic measurement was carried out in duplicate, and kinetic tracks were comparable. UV−vis Measurements. Samples for a typical kinetic measurement were prepared injecting in a quartz cuvette (light path 0.2 cm) the limpid hot solution of the salt. The absorbance values were collected until gel formation was complete. The gel phase obtained at the end of the measurement resulted stable after the “tube inversion test”. In the case of two-component gel, 100 μL of a concentrated solution of Nile Red in acetone was injected in a screw-capped sample vial. After solvent evaporation under vacuum, a suitable amount of organogelator and solvent was added. The suspension was heated until a clear solution was obtained, which was than injected in a quartz cuvette (light path 0.2 cm). Thixotropy Test. A magnetic stirrer (length 8 mm, height 3 mm) was added to a preformed gel into a screw-capped vial. The sample was stirred for 5 min at 1000 rpm and left to stand at room temperature for 12 h. The stability of the obtained phase was investigated by the tube inversion test. Scanning Electron Microscopy. SEM images were recorded using an instrument with 20 kV operating voltage. The gel was placed on the stub, and the solvent was evaporated under vacuum to form a xerogel. The dry sample thus obtained was shielded by gold. In order to verify the homogeneity of the xerogel, at least 20 images for each sample were acquired.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures, gels images, table of optical rotation and differential molar optical rotation, table of Tgel values as a function of gelator concentration, NMR spectra of aqueous solution of pseudorotaxanes, plots of ln(C) vs 1/Tgel, RLS kinetic measurements of hydrogels formation, plots of opacity measurements, SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.D.), renato.noto@unipa. it (R.N.); Ph: +3909123897535; Fax: 091596825. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank University of Palermo and MUR (Prin 2008KRBX3B; FIRB 2010RBFR10BF5 V) for financial support. 10857


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Geminal Imidazolium Salts: A New Class of Gelators - Langmuir (ACS

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