Ionic liquids gels: Soft materials for environmental remediation

Journal of Colloid and Interface Science 517 (2018) 182–193

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Regular Article

Ionic liquids gels: Soft materials for environmental remediation Salvatore Marullo a,⇑, Carla Rizzo a, Nadka T. Dintcheva b, Francesco Giannici c, Francesca D’Anna a,⇑ a

Dipartimento STEBICEF-Sezione di Chimica-Università degli Studi di Palermo, Viale delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, Università degli Studi di Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy c Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze, I-90128, Palermo, Italy b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 12 December 2017 Revised 30 January 2018 Accepted 31 January 2018 Available online 2 February 2018 Keywords: Supramolecular gels Ionic liquids Dye removal Diimidazolium salts Environmental remediation Water treatment

a b s t r a c t Hypothesis: Nanostructured sorbents and, in particular, supramolecular gels are emerging as efficient materials for the removal of toxic contaminants from water, like industrial dyes. It is also known that ionic liquids can dissolve significant amounts of dyes. Consequently, supramolecular ionic liquids gels could be highly efficient sorbents for dyes removal. This would also contribute to overcome the drawbacks associated with dye removal by liquid–liquid extraction with neat ionic liquids which would require large volumes of extractant and a more difficult separation of the phases. Experiments: Herein we employed novel supramolecular ionic liquid gels based on diimidazolium salts bearing naturally occurring or biomass derived anions, to adsorb cationic and anionic dyes from wastewaters. We also carried out a detailed investigation of thermal, structural, morphological and rheological features of our gels to identify which of them are key in designing better sorbents for environmental remediation. Findings: The most effective gels showed fast and thorough removal of cationic dyes like Rhodamine B. These gels could also be reused up to 20 times without any loss in removal efficiency. Overall, our ionic gels outperform most of gel-based sorbents systems so far reported in literature. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction The growing concern for a sustainable chemical industry has led researchers to rethink fundamental aspects of the production pro⇑ Corresponding authors. E-mail addresses: [email protected] (S. Marullo), francesca.danna@ unipa.it (F. D’Anna). https://doi.org/10.1016/j.jcis.2018.01.111 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

cesses according to the principles of Green Chemistry. A key aspect of industrial sustainability involves the treatment of wastewaters. Dyes are major industry derived pollutants with an annual production of 700,000 tons [1] deriving mainly from the textile, paper and cosmetic sectors. The strategies currently used to tackle this issue, comprise ion exchange, photooxidation, electrochemical treatments and ultrafiltration [2].

S. Marullo et al. / Journal of Colloid and Interface Science 517 (2018) 182–193

Another viable strategy to remove dyes from wastewaters is adsorption, by which dye-polluted waters are treated with porous materials called sorbents. This approach is convenient in terms of lower cost and the possibility of regenerating the sorbents for multiple reuses. Ideal sorbents possess large surface area, high adsorption capacity and a porous structure. For this reason, recent years have witnessed a surge in interest in nanostructured sorbents, which fulfill the above requirements [3]. Such materials include graphene oxide [4], carbon nanotubes [5] and chitosan [6]. Another promising class of nanostructured materials for the purpose of dye removal from wastewaters comprises supramolecular gels [7]. Supramolecular gels are a class of materials originated by the self assembly of small molecules known as Low Molecular Weight Gelators (LMWGs) in solution. Small amounts of LMGWs aggregate forming a sample spanning network of fibers, which is capable of entrapping the solvent by means of capillary forces [8,9]. As a result, the solvent bulk flow is prevented and the gel is selfsupporting. Underpinned by weak non-covalent reversible interactions, supramolecular gels are in principle responsive to external stimuli and able to show self-healing capability [10–12]. They also provide a nanostructured confined environment and find applications in many fields, from tissue engineering to drug delivery and environmental remediation [7,13]. Based on which solvent is hardened, gels are distinguished in hydrogels and organogels, originated from solutions of water and organic solvents, respectively. A recent development in the field is represented by gels formed in a third category of solvents, ionic liquids (ILs) which give rise to ionogels [14]. ILs are low melting temperature organic salts and in many cases they are liquid at room temperature. Since they are practically non-volatile and generally non-flammable, they are considered more environmentally friendly than most organic solvents. Moreover, being composed entirely of ions, they provide a unique solvent microenvironment and ILs composed of aromatic ions have been defined as nanostructured materials in their own right [15,16]. In the light of this, ionogels combine the convenient properties of supramolecular gels with those of ILs. Their conductive nature leads, for instance, to their use as electrolytes for solar [17] or fuel cells, and as materials for displays [18]. The properties of ionogels are also vital for their application as lubricants [19] or all-solid supercapacitors [20]. In general, using organic salts as gelators constitutes a viable way to obtain functional materials, due to the straightforward way in which their features can be tuned by changing the ionic composition [21,22]. Consequently, employing organic salts to gel ILs can give rise to fully ionic gels. To find the best applications for these materials, it is necessary to gain a suitable understanding of their properties, structure and interactions at a molecular level. Indeed while hydro- and organogels have been deeply studied, insights into ionogels are still limited in present day literature. In the framework of our interest in studying the gelling ability of imidazolium salts [23–25], we have recently showed that diimidazolium salts can indeed act as ionogelators [25,26]. These ionogels possess in general advantageous features like the ability to self repair when destroyed by a mechanical stress or by ultrasonic irradiation [27]. Moreover, their conductivity is comparable or even higher than that of the neat ionic liquid [28]. In this context, we herein investigate the ability of diimidazolium dicarboxylate salts to act as gelators and in particular, ionogelators. The salts considered differ for the alkyl chain length on the imidazolium cations and for the isomerism in the dicarboxylate anions (Chart 1). The maleate and fumarate anions were chosen for their isomerism that, affecting the direction of hydrogen bonds, could influence the properties of the supramolecular fibrillar gel networks. Moreover, these anions derive from a naturally occurring molecule like fumaric acid, which is involved in the Krebs and urea

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metabolic cycles [29,30] and together with maleic acid can be obtained from the conversion of a renewable resource like vegetable biomass [31–33]. As for the cation structure, it has been recently demonstrated that such kind of diimidazolium ions are able to act as anti-bacterial and anti-proliferative agents, without showing significant hemolytic activity [26,34]. For each salt, we tested the gelling ability in several conventional solvents spanning a wide polarity range as well as in a number of ILs differing for the cation or the anion. When a gel was obtained, we determined the minimum amount of salt required for gelation (CGC), and the gel-sol transition temperature (Tgel). Rheological properties of each gel were studied by strain- and frequency sweep rheology measurements. We also performed the gelation kinetics by means of Resonance Light Scattering (RLS) and opacity measurements by UV–vis spectroscopy. Insights on the crystallinity and morphology of the gels were gained by X-ray diffraction (XRD) and Polarized Optical Microscopy (POM) analyses, respectively. Finally, we looked at the ability of each ionogel to remove dye pollutants from water. In particular, we tested both cationic and anionic dyes like Rhodamine B (RhB) and Methyl Orange (MO) to assess the selectivity of the ionogels towards dye adsorption (Chart 1). Then, we evaluated important parameters related to this process, such as the extent of adsorption over time, the reusability of the gels and the effect of the initial concentration of dye on the removal efficiency. Soft materials obtained show a remarkable ability to remove dyes from wastewaters and a particular selectivity towards cationic dyes. These gels retain their ability even after 20 recycles and do not exhibit saturation events.

2. Materials and methods 2.1. Materials Commercially available a,a0 -p-dibromoxylene, imidazole, 1-bromododecane, 1-bromodecane, fumaric acid, maleic acid, Amberlyte IRA-400Cl-form, were used without further purification. 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim] [PF6]), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim] [BF4]), 1-butyl-3-methyl-imidazolium thiocyanate ([bmim][SCN]), N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)im ide ([bmpyrr][NTf2]) and 1-butyl-3-methyl-imidazolium bis-(tri fluoromethanesulfonyl)imide ([bmim][NTf2]), were purchased by commercial sources. All ILs were dried on a vacuum line at 70 °C for 2 h before use, then kept in a desiccator under argon and over calcium chloride. [p-C12][Br]2 and [p-C10][Br]2 were prepared according to reported procedures [23].

2.2. General procedure for the synthesis of the diimidazolium dicarboxylate salts by anion exchange on basic resin The dicarboxylate diimidazolium salts were prepared by a modification of a reported procedure [23]. To this aim a column packed with anion-exchange Amberlite resin IRA-400 (15.9 g) was first washed with an aqueous solution of NaOH (20 mL, 10% w/v) and subsequently with water until the eluate was neutral to convert the chloride form of the resin into the hydroxide form. The suitable diimidazolium dibromide salts (0.037 mol) were dissolved in 25 mL of a methanol/water (70:30, v/v) mixture and the resulting solution was eluted onto the column, using the same methanol/ water mixture as eluent. The eluate, containing the salt in the hydroxide form, was collected in a flask containing a solution of the dicarboxylic acid in stoichiometric amount. The final product

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was washed with ethyl ether (3  30 mL) under ultrasound irradiation.

properties, such as strain and frequency sweep, were recorded at 25 °C. Rheological properties were recorded three times on three different aliquots of gels.

2.3. Preparation of gels and Tgel determination 2.8. POM measurements Gels were prepared by weighing into a screw-capped sample vial (diameter 1 cm) the suitable amount of salt and IL. The sample vial was heated in an oil bath until the whole of the gelator was dissolved. The vial was then cooled and stored at 4 °C overnight. The tube inversion test method was used to assess gel formation [35]. Tgel were determined by the falling drop method. A vial containing the preformed gel was placed upside-down in a water bath. The bath temperature was raised gradually (1 °C/min) until the gel collapsed and flow was observed [36]. Tgel values were reproducible within 1 °C. 2.4. Differential scanning calorimetry DSC measurements were carried out on a DSC Q200 calorimeter interfaced to a TA Thermal Analyst 3100 controller connected to an RCS90 cooling system. Heating and cooling cycles were done in a 50 mL/min stream of nitrogen. Samples were weighed in T-zero aluminum pans. Transition temperatures from DSC are reported at the point of maximum heat flow. For each salt, after equilibration of the sample at 25 °C, DSC measurements were performed using a temperature ramp with a rate of 20 °C/min from 25 °C to 120 °C and a cooling ramp to 0 °C. For ionogels, heat-cool cycles were carried out using a temperature ramp of 10 °C/min in the temperature range from 10 °C to 90 °C.

Polarized optical microscopy (POM) was carried out using a Zeiss Jenapol microscope equipped with a Leica DFC420 camera. The gel samples were pressed to obtain a uniform thin film on a glass slide and observed under transmitted polarized light. 2.9. XRD measurements X-ray diffraction (XRD) measurements were carried out with a PanalyticalX’Pert Pro diffractometer using Ni-filtered Cu Ka radiation and an X’Celerator detector. The scattering contribution from the solvent was subtracted from the XRD pattern of the respective gel before subsequent data analysis. 2.10. Dye adsorption tests Dye adsorption tests were carried out by placing onto the 270 mg of preformed ionogel (6.5% w:w), 500 mL of a 1  103 M aqueous solution of dye. After a given time, an aliquot of solution was withdrawn, suitably diluted, then UV–vis spectrum was recorded. Concentrations were obtained based on calibration curves obtained by measuring the absorbance of aqueous solutions of the dye at increasing concentrations.

2.5. RLS measurements

3. Results and discussion

RLS measurements were performed on a JASCO FP-777 W spectrofluorophotometer using a synchronous scanning mode with monochromators of emission and excitation preset to identical wavelengths. The RLS spectrum was recorded from 300 to 700 nm with both the excitation and emission slit widths set at 1.5 nm. The working wavelength was chosen as the one corresponding to the intensity maximum of the obtained spectrum. Samples for typical kinetic measurements of gel formation were prepared by injecting into a quartz cuvette (light path 0.2 cm) the limpid hot solution of salt in ionic liquid. Spectra were recorded until gel formation. The gel phase obtained at the end of the measurement was stable after the tube inversion test [35]. RLS spectra were not corrected for absorption of the incident and scattered light.

3.1. Gelation tests in organic solvents

2.6. Opacity measurements Opacity measurements were recorded with a Beckmann DU800 spectrophotometer. The opacity of the gel phases was determined with UV–Vis measurements as a function of time, at a wavelength of 568 nm at a temperature of 20 °C. As described for RLS measurements, samples for a typical measurement were prepared by injecting into 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 measurement was stable after the tube inversion test [35]. 2.7. Rheology measurements Rheological measurements were recorded on a straincontrolled ARES G2 by TA Instruments rheometer using a Peltier temperature controller and a plate-plate tool. Each sample was placed between the shearing plates of the rheometer. Rheological

Initially, we tested the ability of our salts to gel conventional organic solvents. The results of these investigations are reported in Table 1 and S1. All salts showed the same behavior towards a given solvent; irrespective of the concentrations used, none of the salts proved able to gel organic solvents, leading to either solutions or the formation of precipitates. In general, our salts are highly soluble in alcohols, glycols and glycerol as well as in halogenated solvents. Complete dissolution of the salts was also observed in polar aprotic solvents like tetrahydrofuran and DMF. Conversely, all salts were sparingly soluble in apolar solvents like toluene and dioxane, and in solvents of medium polarity like ethyl acetate and acetone as well as in highly polar solvents like acetonitrile. In a couple of instances, namely in DMSO and 2-methylpentanone, complete dissolution of the salts

Table 1 Gelation tests for all salts in conventional organic solvents. Soluble

Insoluble

Solution by heating and precipitates on cooling

Ethanol 1-Propanol 2-Propanol Butanol Tetrahydrofuran DMF Chloroform Dichloromethane Ethylene glycol Glycerol Octanol 1,3-Propanediol

Ethyl acetate Toluene Acetone 1,4-Dioxane Acetonitrile

DMSO 2-Methylpentanone

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S. Marullo et al. / Journal of Colloid and Interface Science 517 (2018) 182–193 Table 2 Gelation tests, concentration range explored and CGCs for the salts studied. [p-C12][Mal]

a b c

a

IL

Range

App.

[bmim][BF4] [bmim][PF6] [bmim][SCN] [bmim][NTf2] [bmpyrr][NTf2]

1–9 1–9 1–9 1–9 1–9

PG OG OG OG P

b

[p-C12][Fum] CGC

Tgel

1.9 1.0 4.6

71 40 30

c

[p-C10][Mal]

App.

CGC

Tgel

PG OG OG OG P

1.9 1.0 4.8

72 40 29

c

App. S OG OG S S

[p-C10][Fum]

CGC

Tgel

2.8 5.1

37 21

c

App. S OG OG S S

CGC

Tgelc

2.7 6.3

44 23

Investigated percentage range (w/w%). PG = Partial gel; OG = opaque gel; S = solution; P = precipitates. Tgel determined at 6.5% (w/w) of gelator. Tgel were reproducible within 1 °C.

was attained by heating at 90 °C, but upon cooling down the solutions, only precipitates could be observed. 3.2. Gelation tests in ILs As already said, we also investigated the ability of our salts to gel ILs. We used ILs differing for the anion nature and for the aromatic or aliphatic nature of the cations. In the light of the possible application of the ionogels obtained, we considered among the ILs tested [bmim][NTf2] and [bmim][PF6], previously classified as biocompatible solvents [37]. Gelation tests were carried out by heating mixtures of each salt in the examined solvent until a clear hot solution was obtained. Then, the solution was cooled at 4 °C overnight and gel formation was assessed by tube inversion methodology [35]. According to this criterion, a gel was obtained when no flow occurred upon turning the vial upside down. CGCs were determined as the minimum concentration of salt at which a self supporting gel was observed by tube inversion. In all cases, we obtained white or yellow opaque gels, which proved thermoreversible and stable for at least four months at room temperature. Results relevant to gelation tests in ILs are reported in Table 2. Examination of results reported in Table 2 highlights that two ILs, namely [bmim][SCN] and [bmim][PF6] were gelled by all salts, while [bmim][NTf2] could be gelled only in the presence of the [p-C12]-based salts. Finally, no gel formed from solutions of our salts in [bmim][BF4] and [bmpyrr][NTf2], in which cases partial gels or precipitates were observed. A closer look at the results reported in Table 2 reveals that in general the isomerism in the anion has no effect on the propensity of the salt to gel a given IL as expressed by the CGC. This finding agrees with similar observations we made for ionogels of diimidazolium salts bearing aromatic anions [27]. Comparison of the CGCs for the same salt in different ILs allows us to single out the effect exerted by the nature of the ionic liquid anion. In particular, the CGCs increase following the order: [bmim][SCN] < [bmim][PF6] < [bmim][NTf2]. This trend reverses the one relevant to the hydrogen bonding accepting ability of the anion, as expressed by the Kamlet Taft b parameter (b = 0.21, 0.24 and 0.71 for [bmim][NTf2] [38], [bmim][PF6] [38] and [bmim][SCN] [39] respectively). This indicates that for the gelators used, the presence of a stronger hydrogen bond accepting IL anion favors the establishment of the mixed solvent gelator interactions required by the aggregation events leading to the gel phase. Moreover, IL being the same, [p-C12]-based ionogels exhibit in all cases higher gelation propensity compared with the [p-C10]-based ones. Consequently, a longer alkyl chain exerts a favorable effect, thus highlighting an important role for van der Waals interactions in the gelation process, confirming a trend already observed in diimidazolium–based ionogels [27]. Then, we determined the gel-sol transition temperatures (Tgel) of the ionogels by the falling drop method [36]. For the sake of comparison, we measured the Tgel at the same gelator concentration for each ionogel (6.5 w:w%). From now on, all gel phase

characterizations are referred to this gelator concentration. The Tgel values are reported in Table 2. In general, also in this case, for a given cation and IL, the isomerism on the dicarboxylate anion barely affects the thermal stability. At variance, the alkyl chain length in the cation appears to play a major role with the [p-C12]-based gels showing a higher thermal stability than the [p-C10]-based ones. This is reminiscent of the different CGCs and once again hints at the importance of van der Waals interactions in underpinning the formation of the gel network. Comparison of Tgel for a given diimidazolium salt highlights a general trend for the effect of the IL anion. In particular for the [p-C12]-based gels, the Tgel increases following the order [bmim][NTf2] < [bmim][SCN] < [bmim][PF6]. This is consistent with the [p-C10]-based salts, with the ionogels in [bmim][PF6] showing a much higher Tgel than those prepared in [bmim][SCN]. 3.3. Gelation kinetics: RLS and UV–vis measurements To probe gelation kinetics, we performed RLS and UV–vis experiments. These two methodologies can give insights into the process of gelation as a function of time, yielding different information. In particular, RLS measurements provide insight on the evolution of the aggregate size as a function of time, because the scattering intensity is correlated with the aggregate size. More specifically, higher scattering intensity results from the presence of larger aggregates [40]. For this reason, RLS has been successfully used to study a wide range of aggregated systems from ILs to nanoparticles [41], p-conjugated dyes and by us to study the formation of gel-phase yielding aggregates [23–25,27,28]. Moreover, measuring the opacity of a gel as a function of time by means of Uv–vis spectroscopy yields qualitative information into the number and size of polydisperse structures within the gel. This is also related to its degree of crystallinity [42]. Both investigations shed light on the mechanism by which aggregates form over time. We could not perform RLS kinetics for some ionogels because the slow gel formation made the measurement unfeasible. This occurred for all [p-C10]based ionogels and for the [p-C12]-based ionogels in [bmim][PF6]. For the same reasons, we were not able to carry out opacity measurements for the [p-C10]-based ionogels. Plots relevant to the RLS and opacity measurements for the [p-C12]-based ionogels in [bmim][SCN] are reported in Fig. 1 while the others are reported in Fig. S2. These preliminary data outline that in general the length of the alkyl chain in the imidazolium cation affects to a great extent the gelation kinetics and it is much faster for the [p-C12]-based ionogel compared with the relevant [p-C10]-based ones. A closer look at opacity plots allows us to highlight some recurring trends. Unlike what observed for the gelation propensity of each salt expressed by CGC, the anion isomerism plays a different role in the kinetic of gelation depending on the IL used. In particular, when [bmim][SCN] is used as IL, we observed no appreciable difference in gelation rates as evidenced by the plateau being

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Fig. 1. Plots relevant to opacity and RLS measurements for the ionogels (a), (b) [p-C12][Mal]/[SCN] and (c), (d) [p-C12][Fum]/[SCN].

reached at comparable times (300 s and 350 s for [p-C12][Mal] and [p-C12][Fum], respectively). Conversely, we found a markedly different picture for [bmim][PF6] where an induction time of 2.8 h occurs for [p-C12][Fum], while it amounts to 8 min for the isomeric salt. Similar conclusions can be drawn considering the time at which a plateau is reached (Fig. S2a, b). We observed an opposite picture in the case of the ionogels obtained in [bmim][NTf2]. In particular, while a lag of 3.6 h is required for aggregation to start for [p-C12][Mal], no induction time was observed for the ionogel obtained from [p-C12][Fum] in the same solvent (Fig. S2c, d). Accordingly, opacity reaches a plateau after 4 min for the latter and more than 8 h for the former. The trend observed indicates that the nature of the IL anion affects the relative rates of the gelation process and in turn, selectivity. Indeed, differences as a function of the gelator anion were observed only as a consequence of a drop in the rate of the gelation process. The IL anion nature also affects the crystallinity of the gel phases, as accounted for by the decrease in absorbances on going from [bmim][PF6] to [bmim][NTf2]. Some mechanistic considerations can be drawn from RLS measurements. Plots reported in Fig. 1b and d point out that gelation occurs following a two-step pathway in which intermediate aggregates undergo size contraction in a subsequent step to yield the final aggregates. In Table S2 we report the values relevant to the time and scattering intensity at the intermediate formation (tm, Im) together with the corresponding gelation values (te, Ie). Looking at the RLS intensity, we can infer that larger aggregates form in the presence of [p-C12][Fum] compared with the isomeric salt. However, when larger intermediate aggregates form, they

undergo a more pronounced size contraction. Such a trend matches similar findings in organo- [43], hydro- [24] and ionogels [27] formed by diimidazolium salts. 3.4. Rheological properties We investigated rheological properties of our ionogels carrying out strain sweep measurements at an angular frequency of 1 rad/s and frequency sweep experiments using strains from 0.1% to 0.25%. These values fall within the linear viscoelastic region (LVR) of the ionogels. We could not perform the measurements relevant to the [p-C10]-based ionogels because the gels were too soft and were broken down even at the lowest strains used. Plots relevant to rheological measurements for [p-C12][Mal]/ [PF6] are reported in Fig. 2, while those relevant to the other ionogels are reported in Fig. S4. In all strain sweep measurements, the values of the storage modulus G0 were higher than those of the loss modulus G00 at low strains, indicating solid-like behavior. In contrast, at high strains G00 exceeds G0 , reminiscent of liquid-like behavior. Moreover, at the strains (c) employed for the frequency sweeps, G0 was almost independent from the angular frequency and consistently higher than G00 . All these findings strongly support the gel nature of our samples [44]. A measure of the gel strength was extracted from the crossover points of the yield strain, (c) at which G0 = G00 and the loss tangent (tan d = G00 / G0 ). These values express the amount of stress (c) required to observe flow in the material, whereas tan d is a measure of the gel stiffness and is related to the strength of colloidal forces operating within the gel network. In general,

S. Marullo et al. / Journal of Colloid and Interface Science 517 (2018) 182–193

187

Fig. 2. Strain (a) and frequency (b) sweep measurements for the [p-C12][Mal]/[PF6] ionogel at 25 °C.

stronger gels feature high yield strain and low tan d values. The parameters obtained by rheological measurements are reported in Table 3. Examination of data reported in Table 3 shows that in all cases tan d < 1, indicating strong association of the particles within the gel network [27]. Comparing tan d for the same salt in different ILs allow us to single out the effect of the IL anion on the gel strength. Among [p-C12][Mal]-based ionogels, tan d increases following the order [bmim][PF6] < [bmim][SCN] < [bmim][NTf2]. Consequently, mechanically stronger gels are obtained in [bmim][PF6] and the weakest ones in [bmim][NTf2]. The trend found for the strains at crossover points is consistent with this explanation. Further support to these findings comes from the Tgel, which can be considered as another indication of the gel strength. Notably, the trend found for Tgel in [p-C12][Mal]-based gels follows the same order. All these observations suggest a prominent role for solvent viscosity in determining the mechanical properties of these ionogels: indeed the strongest gel is obtained in the presence of the most viscous IL and the weakest gel in the presence of the least viscous (g = 209 cp for [bmim][PF6] [45] and 52 cp for both [bmim] [SCN] [46] and [bmim][NTf2]) [47]. It is however important to underline that the viscoelastic properties of our materials are not dictated solely by those of the ILs employed, since even the most viscous IL employed behaves as a classical Newtonian fluid. Interestingly, the occurrence of stiffer ionogels in the presence of [bmim][PF6] in respect to the ones formed in [bmim][NTf2] is in agreement with the results observed for ionogels of closely related salts, bearing the same [p-C12] cation [27]. We found a similar behavior for ionogels based on the isomeric [p-C12][Fum] salt, although the value of c at crossover point suggests that differences among the rheological properties of these ionogels are less pronounced. Looking at the values of tan d when the IL is the same, reveals the prominence of the effect exerted by the IL anion on the mechanical properties over the gelator anion: in all cases we

observed values falling in a quite narrow range. In the case of the ionogels formed by [p-C12][Fum] a higher value of G00 counteracts the higher values of G0 measured by strain and frequency sweeps. 3.5. POM measurements The morphology of ionogels was investigated by POM measurements (Fig. 3). In particular, analysis was performed on [p-C12][Mal]/[PF6] and [p-C12][Fum]/[PF6], which showed the best performance in dyes adsorption (see later). Furthermore, to have information about the IL anion effect on the ionogels morphology, we took in consideration [p-C12][Mal]-based ionogels and we investigated also [p-C12][Mal]/[SCN] and [p-C12][Mal]/[NTf2]. In the microphotographs under crossed Nicols, the amorphous gel matrix is black, while the crystallites are bright. When using a first order red plate, the matrix is magenta, and the crystallites are either blue or yellow according to their orientation. First of all, the POM images reported in Fig. 3 show anisotropic features that could hypothetically be related to the occurrence of liquid crystals in our materials. To address this issue, we carried out DSC measurements for all [p-C12]-based ionogels. The relevant plots are reported in Fig. S4. Inspection of these plots reveals that in all cases, the gel-sol transition temperatures are consistently higher than those determined with the falling drop method. This can be explained by considering the transition temperatures measured by DSC indicate the complete break down of the gel network. Conversely, the falling drop method detects the temperature at which the gel is not self-supporting and aggregates are still present to a lower extent. Moreover, in all cases, with the only exceptions of [p-C12][Mal]/[PF6] and [p-C12][Mal]/[SCN], the only transitions observed are those relevant to the gel-sol transition. This rules out the occurrence of mesophases. In the cases of [p-C12][Mal]/ [PF6] and [p-C12][Mal]/[SCN], we observed an additional transition. However, further observation of the gels under polarized light microscope at increasing temperature did not show any obvious

Table 3 G0 , G00 , tan d = G00 /G0 at c = 0.1% and values of c at G0 = G00 for ionogels investigated at 6.5 wt% gelator concentrations at 25 °C. Error limits are based on average of three different experiments with different aliquots. Gel

G0 (Pa)

G00 (Pa)

tan d

c at G0 = G00

[p-C12][Mal]/[PF6] [p-C12][Mal]/[SCN] [p-C12][Mal]/[NTf2] [p-C12][Fum]/[PF6] [p-C12][Fum]/[SCN] [p-C12][Fum]/[NTf2]

6300 ± 500 1500 ± 400 4000 ± 1000 11,000 ± 1500 1200 ± 400 17,000 ± 1000

1750 ± 80 700 ± 200 2100 ± 1600 3300 ± 500 600 ± 200 7500 ± 1400

0.27 ± 0.03 0.47 ± 0.01 0.57 ± 0.05 0.32 ± 0.08 0.48 ± 0.03 0.47 ± 0.02

4.0 ± 0.1% 2.5 ± 0.1% 1.52 ± 0.04% 1.55 ± 0.03% 1.55 ± 0.03% 1.62 ± 0.05%

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Fig. 3. POM images corresponding to (a) [p-C12][Mal]/[PF6] (crossed Nicols, XN); (b) [p-C12][Mal]/[PF6] (XN + red plate); (c) [p-C12][Mal]/[SCN] (XN); (d) [p-C12][Mal]/[SCN] (XN + red plate); (e) [p-C12][Fum]/[PF6] (XN); (f) [p-C12][Fum]/[PF6] (XN + red plate); (g) [p-C12][Mal]/[NTf2] (XN); (h) [p-C12][Mal]/[NTf2] (XN + red plate); (i) [p-C12][Mal]/ [NTf2] (XN + red plate).

evidence of mesophase or liquid crystal textures before the gel-sol transition. Anisotropic features have already been reported for supramolecular gels [48,49] and suggest a certain long range order of the fibers [49]. Interestingly enough, other imidazolium-based supramolecular gels reported in the literature display features analogous to our materials when observed with POM [50]. Analysis of the images clearly evidences the occurrence of three different morphological motifs: thick texture for [p-C12][Mal]/[PF6] and [p-C12][Mal]/[SCN] (Fig. 3a–d), spherulitic network of acicular crystals for [p-C12][Fum]/[PF6] (Fig. 3e,f) and prismatic crystals for [p-C12][Mal]/[NTf2] (Fig. 3g,i). In the case of thick texture morphology, very small and abundant crystallites of different sizes (around 1–2 lm and 5–8 lm for [p-C12][Mal]/[PF6] and [p-C12] [Mal]/[SCN], respectively) appear in the gelatinous network. Interestingly, for [p-C12][Mal]/[PF6], the crystallites are loosely clustered together in larger regions of about 50–100 lm, alternating with regions of similar size that are devoid of crystallites. According to data above discussed, this observation evidences the role played by the IL anion in determining properties of ionogels and this evidence is further supported by the significant changes occurring in morphology on going from [p-C12][Mal]/[PF6] to [p-C12] [Mal]/[NTf2]. In the latter case, large and well-formed crystals with blocky prismatic habit, ranging from 20 to 200 lm in length, are observed. Finally, differently from what above discussed, POM investigation clearly shows the effect of the anion isomerism on morphology, as accounted for by the change from thick to spherulitic network observed on going from [p-C12][Mal]/[PF6] to [p-C12] [Fum]/[PF6]. In this case, the sparse crystalline regions are 10–20

lm in size, and they appear as spherulitic aggregates, featuring either spherical or fan morphology. The attempt to correlate Tgel values and rheological properties of ionogels to morphology of soft materials allows drawing a general consideration. Indeed, the thick texture observed for [p-C12] [Mal]/[PF6] and [p-C12][Mal]/[SCN] gives more thermally stable and mechanically stronger gelatinous network than the other ones.

3.6. X-ray diffraction To have a deeper understanding of the structural features of soft material studied, X-ray (XRD) measurements were carried out on both neat gelators and ionogels (Fig. 4 and Fig. S5). For a useful comparison, XRD investigation was performed on the same samples used for POM analysis. The gelators [p-C12][Fum] and [p-C12][Mal] show typical XRD patterns for organic salts, with many intense peaks in the 10–60° region. The XRD patterns of the gels always show different sets of Bragg peaks compared to the corresponding gelators. Some peaks around 7.3–7.8 Å, 4.46–4.53 Å, 4.36–4.38 Å are common features shared by different gels and some of the above peaks (4.46–4.53 Å, 4.36–4.38 Å) may be due to self-assembly through hydrogen bond interactions. The above evidence is a common structural feature in imidazolium salts that have been frequently described as polymeric supramolecular structures [51]. On the other hand, it perfectly agrees with previously reported DFT data on some

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Fig. 4. Diffractograms of neat gelators and of their 6.5 wt% gel in [bmim][PF6] .

diimidazolium salts showing that the anion of the gelators may act as a bridge among diimidazolium cations [43]. In all cases also peaks in the range from 3.43 Å to 3.68 Å were observed. These ones could be ascribed to the occurrence of p-p interactions [52–54]. The XRD patterns of [p-C12][Fum]/[PF6] and [p-C12][Mal]/[PF6] are very similar, with broad peaks. [p-C12][Mal]/[NTf2], on the other hand, displays few very intense and very sharp peaks. [p-C12][Mal]/[SCN] represents an intermediate situation, as also witnessed by the analysis of the peak width (FWHM), which increases from 0.08° ([p-C12][Mal]/[NTf2]) to 0.13° ([p-C12][Mal]/ [SCN]) to 0.2° ([p-C12][Mal]/[PF6] and [p-C12][Fum]/[PF6]). Since the peak width is inversely proportional to the crystallite size, these results are in line with the POM images. 3.7. Dye adsorption Firstly, we checked for the resistance of each gel upon placing 500 lL of water on top of them for 72 h. We found that all [p-C12]-based ionogels remained unaltered after 72 h as evidenced by the tube inversion test. Conversely, [p-C10]-based ionogels, underwent partial breakdown after this treatment and were not used for dye absorption. This finding agrees with the lower rheological resistance observed. Initially, we used the [p-C12][Mal]/[PF6] to assess whether our ionogels show selectivity toward cationic or anionic dyes. To this aim, we used MO as anionic dye and RhB as the cationic one. Preliminarily, we determined a standard calibration curve for each dye, then we calculated the removal efficiency (RE) by means of Eq. (1) [55]:

RE ¼

C0  CðtÞ  100 C0

ð1Þ

where C0 is the initial concentration of dye in the aqueous solution and C(t) is the dye concentration at a given time in the aqueous phase. Plots of the absorption efficiency as a function of time are reported in Fig. 5, together with representative pictures of the gels before and after dye adsorption. Fig. 5 clearly shows that the removal of the cation dye RhB is faster and more efficient of that of the anionic MO. In particular, RhB is almost quantitatively removed from the aqueous phase in 24 h, while for MO the same time is not sufficient to remove half of the amount of dye. Notably, the higher removal efficiency of cationic dyes makes our ionogels complementary to other imidazolium based gels which feature show better removal performance with anionic dyes [50]. For this reason, we compared all ionogels

by measuring the amount of RhB adsorbed from an aqueous solution over time. In Table 4 times corresponding to the maximum removal efficiencies achieved are reported together with some literature data, while plots of RE as a function of time are reported in Fig. S6 and representative UV–vis spectra recorded at different times are reported in Fig. S7. Examination of the results reported in Table 4 reveals some recurring trends. Firstly, comparing the kinetics of removal in the presence of the [p-C12][Fum]/[PF6] and [p-C12][Mal]/[PF6] points out the effect of the isomeric anions. In particular, for [p-C12] [Fum]/[PF6], 95% of removal could be achieved in 6 h, while in the case of [p-C12][Mal]/[PF6], 24 h are required to observe the same level of dye removal. The above differences recall the ones observed analyzing gel morphology, indicating that a spherulitic network works better in dyes removal than a thick texture. Notably, IL being the same, this different behavior is apparent in all cases, although for ionogels prepared in [bmim][SCN] we found lower differences. Such behavior is reminiscent of the lower differences in mechanical properties found by rheological measurements on the ionogels in [bmim][SCN] compared to the other ones and also of the very similar gelation kinetics as shown by RLS measurements. Moreover, comparing the results obtained for ionogels based on the same salt in different ILs allows us to single out the effect of the IL anion on the removal performance. Indeed, for the [p-C12][Mal]based ionogels, the time required to achieve 90% of dye removal is 24 h in [bmim][PF6], 15 h in [bmim][NTf2], while the worst performance was found in [bmim][SCN] in which case the removal efficiency could not exceed 40% even after 72 h. Interestingly, we found a very similar trend for the [p-C12][Fum]-based ionogels with those prepared in [bmim][PF6] and [bmim][NTf2] displaying the fastest dye removal, although in this case the differences are less marked. Once again, they outperform the ionogel prepared in [bmim][SCN]. On the other hand, also in this case, the dye removal efficiency could be related to morphology of used materials, with the best performance detected in the presence of a spherulitic network. It is worth noting that in general ILs can dissolve dyes [64,65]. In the light of this, we carried out the same experiment of RhB removal using the neat ILs instead of the gels. In all cases ILs removed RhB more rapidly than the ionogels, with more than 90% of dye removed in 4 h (RE% = 90%, 99% and 91% for [bmim] [PF6], [bmim][NTf2] and [bmim][SCN] respectively). However, we would like to underline that using the gels is more convenient because of the much easier separation between the two phases, which is one of the general advantages of the adsorption over

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Fig. 5. (a) Plot of removal efficiencies of MO and RhB by [p-C12][Mal]/[PF6] as a function of time, (b) pictures of MO solution on top of [p-C12][Mal]/[PF6] at t = 0 and (c) gel after t = 48 h. (d) Pictures of RhB solution on top of [p-C12][Mal]/[PF6] at t = 0 and (e) gel after t = 24 h.

Table 4 Dye removal efficiencies of our ionogelsa and data from literature.

a

Gel or sorbent

Dye

t (h)

REmax (%)

[p-C12][Mal]/[PF6] [p-C12][Mal]/[PF6] [p-C12][Fum]/[PF6] [p-C12][Mal]/[SCN] [p-C12][Fum]/[SCN] [p-C12][Mal]/[NTf2] [p-C12][Fum]/[NTf2] Imidazolium-based gel in H2O/DMSO [50] Imidazolium-based gel in H2O/DMSO [50] Benzimidazolium-based gel in H2O/DMSO [56] Organogel [57] Organogel [58] Hydrogel [59] Hydrogel [60] Metallogel [61] Graphene Oxide-doped membrane [62] Hollow Carbon microspheres [63]

MO RhB RhB RhB RhB RhB RhB MO Rh6G RhB RhB RhB RhB Rh6G MO RhB RhB

48 24 6 72 48 15 6 24 24 24 24 28 28 0.33 4 4 1

71 92 95 31 36 93 97 100 Partial 85 96 99 >90 >99 99 90 90

T = 25 °C. REs are reproducible within 2%.

extraction methods. In addition, from the standpoint of sustainability, it is desirable to reuse the sorbent, avoiding the disposal of dye-laden exhausted material as waste. For this reason, we assessed the reusability of our gels by performing adsorption for several cycles. To this aim, the [p-C12][Fum]/[PF6] gel was loaded with the RhB solution as previously described, then after 6 h the decolorized aqueous solution was removed and replaced with a fresh batch of 500 lL of a 103 M RhB solution in water. The

Fig. 6. Plot of removal efficiency of RhB from fresh batches placed on dye laden [pC12][Fum]/[PF6].

removal efficiencies determined for each cycle are reported in Fig. 6. Results reported in Fig. 6 clearly show that the ionogel could remove RhB from water up to 20 times without showing any appreciable drop in removal efficiency. Moreover, using the same gel repeatedly did not affect the appearance and characteristics of the material which remained self supporting as assessed by tube inversion test. To further assess the feasibility of our ionogels for dye removal, we analyzed the effect of initial amount of dye on the removal efficiency. To this aim, we placed increasingly concentrated aqueous solutions of RhB onto the best performing ionogel, namely [p-C12][Fum]/[PF6], for a contact time of 6 h. The results obtained are reported in Fig. 7. Examination of the plot reported in Fig. 7 clearly shows that an almost quantitative removal of RhB could be achieved within 6 h even if the initial amount of dye is raised by more than 20 times. Consequently, this ionogel maintains its performance even in very concentrated dye solutions, without problems ensuing from saturation of the adsorbing material. These two experiments clearly show the advantage of using a condensed phase like ionogels as opposed to neat ILs. Finally, it may be useful to assess the performance of our ionogels by comparing it to other related sorbent system previously reported in literature. To the best of our knowledge, to date only one report describes the use of ionogels for dye removal, although no quantitative indication of the removal efficiency was given [66]. The dye adsorbing systems closest to ours are those reported by Yu [50] and Ghosh [56], constituted by supramolecular gels in DMSO/water of carboxylic

Fig. 7. Plot of removal efficiency of RhB by [p-C12][Fum]/[PF6] after 6 h as a function of initial amount of dye.

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191

Chart 1. Diimidazolium salts, ILs gelled and dyes considered.

acid functionalized imidazolium salts and cholesteryl-appended benzimidazolium salts, respectively. Notably, the former system worked better than ours with methyl orange displaying complete removal of this dye within 24 h. In contrast, some of our ionogels performed better with cationic dye, enabling almost total uptake of RhB in 6 h, while the other achieved only partial removal of the structurally related Rhodamine 6G (Rh6G) even after extensive contact time. Moreover, some of our ionogels worked better than the benzimidazoliumbased gels [56] both in terms of RE% and time elapsed. In Table 4, we also report the removal of Rhodamine B by a set of different sorbents reported in the literature. In our choice of the representative materials we did not intend to compile an exhaustive review of the various sorbent that is outside the scope of the work. Rather, we chose works dealing with the same dyes and with experimental conditions as close as possible to ours. However, since a complete correspondence of experimental parameters could not be found, the comparison between RE% and contact times should be taken as indicative. Examination of results reported in Table 4 reveals that the performance of our ionogels is well in line or superior with that of other gel-based systems when removal of cationic dyes is concerned with the only exceptions of a litocholate-based hydrogels and hollow carbon microspheres [63] which achieve faster removal of the dyes Rh6G and RhB respectively. When the anionic dye MO is considered, we observe a reverse picture. In other words, our ionogels appear consistently more selective towards cationic dyes, being thus complementary to other gel-based sorbents.

4. Conclusions On the basis of the ability of diimidazolium salts of gelling ILs [26–28] and high solubility of dyes in ILs [64,65] we sought to combine these properties obtaining a series of ionogels that exhibit a remarkable ability to adsorb organic dyes from wastewaters. To our knowledge, this is one of the very few works addressing the

use of ionogels in the context of environmental remediation [66]. We found that our soft materials, differently from similar systems reported in literature [50,56], have a special affinity for cationic dyes, like Rhodamine B. Indeed, one of ionogels tested, [p-C12] [Fum]/[PF6], is able to perform a quantitative dye removal within 6 h. This gel maintains its adsorbing performance even upon raising the dye initial concentration more than 20 times without showing saturation events. Moreover, the ionogel can be reused for more than 20 times without reduction of removal efficiency. Overall, our ionogels proved suitable materials for dye removal and particularly advantageous over neat ILs which would require larger volumes and a more difficult phase separation. Comparison with the performance of other sorbent materials reported in the literature shows that our materials outperform most of the gel-based systems so far reported [50,56–59]. Another significant advantage brought about by our ionogels over the common gels based on conventional solvents is that the properties of ionogels can be varied in a straightforward way simply by changing the IL anion. The attempt to correlate performance of gel phases to their structural features highlights that morphology of materials plays a significant role in determining the adsorption ability, with the best performance detected in the presence of spherulitic network. Full structural characterization of ionogels shows that the gelling propensity, as expressed by the CGC, is mainly influenced by the hydrogen bonding ability of the IL anion and the length of the alkyl chains in the cationic heads, whereas the effect of the dicarboxylate anion isomerism appears marginal. Finally, rheological measurements point out a favorable effect of a longer alkyl chain length and a more viscous IL on the mechanical properties of the ionogels. Future work will be devoted to the valorisation of post-sorption material as fluorescent sensors.

Declaration of interest None.

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