Supramolecular Eutecto Gels: Fully Natural Soft Materials - ACS

Letter pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Supramolecular Eutecto Gels: Fully Natural Soft Materials Salvatore Marullo,† Alessandro Meli,† Francesco Giannici,§ and Francesca D’Anna*,† †

Università degli Studi di Palermo, Dipartimento di Scienze Biologiche, Chimiche e Farmaceutiche, Viale delle Scienze, Ed. 17, 90128 Palermo, Italia § Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, Viale delle Scienze, Ed. 17, 90128 Palermo, Italia

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S Supporting Information *

ABSTRACT: The obtainment of materials featured by high environmental compatibility is one of the main goals of modern research. On this subject, we herein report the first example of supramolecular gel in deep eutectic solvents. In particular, we prepared gels of the L-amino acids isoleucine and tryptophan in choline chloride/ phenylacetic acid 1:2. All gel components are readily available and nontoxic. Gels have been fully characterized by standard gelation tests, rheology, X-ray diffraction, morphology and gelation kinetics. Data collected show that gels properties depend on the gelator nature. In particular, gel phases exhibit strong colloidal forces and, this high mechanical resistance, together with their environmental friendly nature, make them promising candidates for applications in all those fields requiring good performance and low environmental impact. KEYWORDS: Supramolecular gels, Deep eutectic solvents, Amino acids

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INTRODUCTION Supramolecular gels prominently feature research in materials science due to their intriguing properties and application in vast range of fields.1 Held together solely by reversible noncovalent interactions, they often display stimuli responsiveness or self-healing ability. The most used way of classifying supramolecular gels considers the nature of the solvent which is hardened. The most studied gels are hydro- and organogels, originated from aqueous and organic solvent solutions, respectively. The latest development in the field is represented by ionogels which form in solution of ionic liquids.2 In this work, we present the first example of supramolecular gels in another class of solvents, namely deep eutectic solvents (DES).3 DES are mixtures of two or more compounds characterized by a lower melting point than any of the individual components. These latter comprise general hydrogen bond acceptors (HBA) like quaternary ammonium salts and hydrogen bond donors (HBD) like amides, carboxylic acids or carbohydrates. DES are often composed of readily available, nontoxic components deriving from renewable resources.4 Moreover, their synthesis is straightforward, involving only mixing of components without any reaction, any purification steps and no generation of waste, in full compliance with the principles of Green Chemistry.5 Similarly to ionic liquids, DES have a high solubilizing ability. This explains their wide use in extraction processes, frequently aimed at the removal of environmental contaminants, like dyes, heavy metals and sulfur compounds from fuels.6−8 We herein report supramolecular gels of the L-amino acids isoleucine (L-Ile) and tryptophan (L-Trp) in the DES choline chloride/phenylacetic acid (ChCl/PhAA) 1:2. (Figure 1). © XXXX American Chemical Society

Despite examples of polymeric gels in DES are present in the literature,9−12 to the best of our knowledge, no instance of supramolecular gels in DES has been reported to date. It is worth noting that our gels are entirely composed of cheap, readily available and nontoxic components fully respecting the “biorefinery concept”.13 L-amino acids are indeed simple building blocks for obtaining self-assembled nanostructures underpinned by H-bonding and π−π interactions.14 These include supramolecular hydro-15 and organogels.16 Besides the L-amino acids used as gelators, PhAA is a compound naturally occurring in honey,17,18 plants19 and is involved in the catabolysm of phenylalanine.20 Furthermore, ChCl is a nontoxic compound used as additive in chicken food.21 The resulting DES fall within the realm natural deep eutectic solvents (NADES)4 which are widely recognized as solvents with low or nonexistent toxicity.22 Our gels were fully characterized by standard gelation tests, thermal behavior and rheological properties, whereas structure and morphology were investigated by X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM) measurements. We also studied gelation kinetics by UV−vis spectroscopy.

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RESULTS AND DISCUSSION First, we tested the gelling ability of these L-amino acids in a range of DES. Results of gelation tests are reported in Table S1. Using amino acid concentration as low as 3 wt %, we Received: August 27, 2018 Revised: September 13, 2018 Published: September 17, 2018 A

DOI: 10.1021/acssuschemeng.8b04278 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Gelators and DES used and (b) pictures of the eutecto gels formed by L-Ile (left), L-Trp (center) and L-Trp under a UV-lamp (right), in ChCl/PhAA 1:2.

Figure 2. Strain and frequency sweeps measurements for the L-Ile eutecto gel at 3 wt % in ChCl/PhAA 1:2.

Trp-based gels respectively).27 Moreover, rheological measurements point out that the L-Ile gel is stronger than the L-Trpbased one as shown by comparing storage modulus G′ values and the strain at the crossover point at which G′ = G″ (γ). This parameter represents the amount of strain able to destroy the gel and is an estimation of the mechanical strength of the material (Table 1).

obtained opaque gels in the DES ChCl/PhAA 1:2 which were stable to tube inversion test for at least three months at room temperature. The observation of the L-Trp-gel under a UV-lamp reveals that this gel is fluorescent, as the amino acid keeps its emissive property also in the self-assembled network. Unfortunately, because of the opaque nature of materials obtained, assessing if chirality of gelators was also retained in the gel phases required solid state circular dichroism,23 which is not under our current availability. To characterize our materials, we first determined their thermal stability by measuring the gel−sol transition temperature (Tgel) by means of the falling drop method.24 In particular, for 3 wt % gels, transition temperatures of 34 and 38 °C for the L-Ile- and L-Trp-based gels, were detected, respectively. It is worth noting that these temperatures are much higher than the melting temperature of the DES, 25 °C.25 This shows that the transition is truly due to the breakdown of the gel network and not to the mere melting of the DES. To further confirm the gel nature of our materials, we carried out rheological measurements such as strain and frequency sweeps experiments. Plots relevant to rheological measurements are reported in Figures 2 and S1. In both cases, G′ is higher than G″ at low strains, whereas G′′ overcomes G′ at high strains. Moreover, both parameters are almost constant over a wide range of angular frequencies. These rheological behavior are well established hallmarks of gels and lend further support for the gel nature of our materials.26 The values of tan δ (= G″/G′) are lower than 1 in both cases, evidencing the occurrence of strong colloidal forces within the gel networks (tan δ = 0.68 and 0.60 for L-Ile- and L-

Table 1. G′, G″, tan δ = G′′/G′ at γ = 0.025% and ω = 1 rad/s Together with Values of γ at G′= G′′ for Eutecto Gels Investigated at 3% wt and 25 °Ca Gelator L-Ile L-Trp

G′ (Pa)

G″ (Pa)

tan δ

γ at G′ = G″ (%)

11000 ± 3000 1160 ± 40

8000 ± 1000 840 ± 90

0.68 ± 0.02 0.60 ± 0.04

1.57 ± 0.05 0.55 ± 0.02

a

Error limits are based on average of three different experiments with different aliquots.

Such values point out good mechanical properties of our gels which are in line with those of gels of organic salts in ionic liquids.26,28,29 We also investigated whether the gels show reversible response to external stimuli such as mechanical stress and ultrasonic irradiation. In both cases, gels were unaffected by these stresses and remained stable (Table S2). Subsequently, we investigated the gelation kinetics by means of opacity measurements carried out by UV−vis spectroscopy (Figure 3). This investigation provides information on the rate of gelation. Measuring the opacity of a gel as a function of time yields also qualitative information into the number and size of B


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Figure 3. Plots of opacity measurements for the eutecto gel of (a) L-Ile and (b) L-Trp 3 wt % in ChCl/PhAA 1:2 at 25 °C.

Figure 4. XRD patterns for gels at 3 wt % in ChCl/PhAA 1:2 (red) and neat gelators (black) for (a) L-Trp, (b) L-Ile and (c) DES ChCl/PhAA 1:2.

Figure 5. POM images relevant to eutecto gels at 3 wt % in ChCl/PhAA 1:2 (a) L-Trp (Crossed Nicols, XN), (b) L-Trp (XN+red plate), (c) L-Ile (XN), (d) L-Ile (XN+red plate). In image panels a and b, the total width is 2200 μm; in image panels c and d, total width is 700 μm. SEM images of xerogels of at 3 wt % in ChCl/PhAA 1:2 of (e) L-Trp and (f) L-lle.

polydisperse structures within the gel. This is also related to its degree of crystallinity.30

In both cases, gelation follows a single-step mechanism. Comparing the times required to reach a plateau (tg), we can C



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CONCLUSIONS In conclusion, we have reported and studied supramolecular gels in DES, namely eutecto gels. All gel features are heavily affected by the side chain structure of the gelator. Gelation is driven by the ability of L-amino acids to interact with the hydrogen bond network of the solvent. However, the possibility of establishing interactions different in nature, as a function of the side chain structure of amino acid, determines different packing and properties. Interestingly, these supramolecular gels were obtained in solvents that, like ionic liquids, show ultralow volatility, designability and high solubilizing ability. However, they are superior in cost effectiveness, synthesis and biodegradability.33 Gelators are also cheap, readily available and nontoxic. Consequently, the supramolecular gels obtained in this work perfectly fit the continous needs of Green Chemistry to develop technology and design materials that are inherently nontoxic to living things and the environment. Regarding the possible application, the use of solid-like materials such as eutecto gels has the advantage of entrapping the solvent in a 3D-network. This should warrant a decrease in solvent loss, sometimes detected in liquid−liquid extraction. Consequently, they could have great potential in all those applications for which a minimal environmental impact is mandatory. Currently, we are studying dye removal tests from water with eutecto gels (Figure S2), on the grounds of the good results observed with ionogels.34 However, these materials will also be tested in the near future as a sorbent system for removal of sulfur compound from fuels or the adsorption of atmospheric pollutants, like ammonia, toward which DES have already proven very efficient.6,35

state that gelation of the L-Ile-based gel occurs faster than that of the L-Trp-based one (tg = 8 and 20 h for the L-Ile-based gel and for the L-Trp-based one, respectively). Furthermore, it is also characterized by a higher degree of crystallinity as accounted for by the absorbances detected at the plateau. To gain insights into the structure and morphology of our gels, we carried out XRD, POM and SEM measurements. For a useful comparison, in Figure 4, we report the X-ray patterns of each gel, along with those obtained for the relevant neat gelators. The diffractograms of the gels show the occurrence of few sharp peaks, suggestive of a significant degree of order. For LTrp, the gel pattern does not display any peaks of gelator. Moreover, the mismatch of the XRD patterns between gel and gelator suggests the occurrence in the gel of a different crystalline state than the gelator, which in the DES forms a distinct crystalline entity. In both cases, the diffraction patterns of the gels are dominated by peaks ascribable to the solvent, which although visually in the liquid state, displays few well-defined Bragg peaks (Figure 4c), suggestive of a high-symmetry lattice. It is worth noting that such peaks are not attributable to either component of the DES. Therefore, the DES appears as a fluid endowed with an unusually high degree of order and organization. These unusual features can be tentatively hypothesized also to stem from the highly organized nature of DES which possess a complex and persistent structuring underpinned by thick and complex network of hydrogen bonds.31 POM images of the gels are reported in Figure 5. In the microphotographs under crossed Nicols, the amorphous matrix is black, whereas the aggregates are bright. When a first-order red plate is used, the matrix is magenta, and the aggregates are either blue or yellow according to their orientation. Analysis of the images reported in Figure 5a−d reveals slightly different appearance for the two gels. For the L-Trpbased gel, abundant aggregates with different shapes (mostly matching highly symmetric shapes, such as dipyramids, octahedra, dodecahedra, etc.), and sparse elongated structures. Conversely, the gel formed by L-Ile in the same DES features more densely packed aggregates with different size and shape, interlocked in a complex texture. In agreement with the results obtained by POM measurements, the SEM images obtained from the xerogels, reported in Figure 5e,f, evidence very different morphologies. In particular, the L-Trp-based gels shows a flake-like structure, (Figure 5e) whereas the L -Ile-based one shows a windflower-like morphology (Figure 5f). The differences detected in morphology could be ascribed to the different interactions operating in the gel network. As recently reported, differently from ionic liquids, DES are supramolecular complexes of individual components.32 They are fragile liquids, which can be preserved from decomposition only by the occurrence of strong interactions with a third party, in our case the gelator. Obviously, the obtainment of strong gelatinous networks demonstrates in our case the strength of interactions occurring between solvent and gelator. Amino acids are able to insert in the hydrogen bond network of the solvent. However, crossed van der Waals interactions, in the case of L-Ile and π−π interactions, in the case of L-Trp could be considered key factors determining the different packing of the gel phase.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04278. Materials and methods; strain and frequency sweeps measurements; tables of gelation and self-repairing ability tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*F. D’Anna. E-mail: [email protected]. ORCID

Salvatore Marullo: 0000-0001-9932-9823 Francesco Giannici: 0000-0003-3086-956X Francesca D’Anna: 0000-0001-6171-8620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Carla Rizzo for useful discussion. We thank MIUR for financial support (FIRB 2010RBFR10BF5 V and FFABR PJ_RIC_FFABR_2017_161917).

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Supramolecular Eutecto Gels: Fully Natural Soft Materials - ACS

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