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Funding Information: The authors thank Fundação para a Ciência e Tecnologia (FC&T) for project PTDC/BII‐BIO/30884/2017 and EXPL/BII‐BIO/0436/2021 and also for the researcher contract 2021.03255.CEECIND (M.C.C) and 2020.01614.CEECIND/CP1596/CT0007 (A.F.P.). Authors also thank the support by the Associate Laboratory for Green Chemistry – LAQV, which is financed by national funds from FCT/MCTES (UIDB/50006/2020 and UIDP/50006/2020) and i3 N (LA/P/0037/2020, UIDP/50025/2020 and UIDB/50025/2020 . The National NMR Facility is supported by FC&T (ROTEIRO/0031/2013 – PINFRA/22161/2016, co‐financed by FEDER through COMPETE 2020, POCI, and PORL and FC&T through PIDDAC) and CERMAX through project 022162. We thank Prof. Dr. Carlos A. M. Afonso from the Faculty of Pharmacy, ULisboa for the scientific discussions on the experiments carried out with furfuryl alcohol derivatives. Publisher Copyright: © 2023 The Authors. ChemCatChem published by Wiley-VCH GmbH. Sulfonic acid carbon-(bio)based and natural clays-based catalysts were prepared and investigated for the first time as heterogeneous catalysts for amide bond synthesis by a Ritter reaction. The different SO3H-catalysts were screened using benzyl alcohol and acetonitrile as model substrates, and MWCNT-CSP revealed to be an efficient catalyst, affording the amide in 75 % yield. The practical utility of the catalysts was demonstrated by a diverse range of amides, obtained from alcohols and nitriles, in moderate to good yields. Biomass derived platform alcohols, such as 5-HMF and furfuryl alcohol, were also tested as potential building blocks for the synthesis of biopolymers. The SO3H-catalysts revealed to be a highly efficient and environmentally friendly alternative to the conventional acid catalysts commonly used in the Ritter reaction. publishersversion published

The purpose of this chapter is to demonstrate alternative porous IL-grafted materials for CO2 capture and reuse. ILs are known for their affinity and selectivity towards CO2, however, their liquid nature restrains the scope of applications. The effort to gather IL features within porous materials has therefore widened their functionalities. Since the choice of support is essential to achieve the adequate porosity, the chapter is organized according to the nature of support, namely organic, inorganic and hybrid or crystalline frameworks materials. The discussion is mainly based on organic supports since the desired functionality with modulated properties can be introduced into the porous materials by using PIL-based materials. In addition, in all sections, the methodologies and support trends are highlighted to facilitate the development of future tailored materials.

Extracts from plants have considerable significance as bioactive compounds with several pharmacological applications. Polyphenols have attracted the attention as anti-inflammatory and anti-oxidative materials. Nonetheless, the amount of these compounds in the extracts is typically very low. Consequently, green extraction techniques with higher efficiency for phenolic compounds are of paramount importance. Ionic liquids (ILs), which are also known as designer solvents can be used to extract polyphenols, however the search for ideal solvents is mostly done by trial and error. In this work, nuclear magnetic resonance (NMR) is used to study the profile of ILs molecular interactions with model compounds that mimic polyphenols. The ILs that exhibit the strongest molecular interactions were proven to have the highest efficiency when extracting polyphenols from matcha Japanese green tea, known to be extremely rich in these compounds. Both the IL cation and anion have an influence on the solvent behaviour. The best IL solvents for matcha polyphenols were imidazolium derivatives with shorter alkyl side chains and weakly basic anions such as tricyanomethanide, dicyanamide and triflate. Thus, the NMR approach avoids an exhaustive testing and allows the rational selection of the best ILs for the extraction.

The demand for better cellulose solvents has driven the search for new and improved materials to enable the processing of this polysaccharide. Ionic liquids have been debated for a long time as interesting alternatives, but the molecular details on the solubilization mechanism have been a matter of controversy. Herein, for the first time, the structure and dynamics of hydroxypropylcellulose (HPC) liquid crystal solutions were probed in the presence of imidazolium ionic liquids (ILs), conjugating rheological measurements with magnetic resonance spectroscopy. This study provides a characterization of the solutions macroscopic behaviour, where the liquid crystalline (LC) properties were maintained. Using ILs with different side chain lengths, the influence of the hydrophobic IL domain in the solvation abilities of ILs towards a cellulose derived polymer was accessed, providing experimental evidence on these interactions.

Understanding the behavior of a chemical compound at a molecular level is fundamental, not only to explain its macroscopic properties, but also to enable the control and optimization of these properties. The present work aims to characterize a set of systems based on the ionic liquids [Aliquat][Cl] and [Aliquat][FeCl4] and on mixtures of these with different concentrations of DMSO by means of 1H NMR relaxometry, diffusometry and X-ray diffractometry. Without DMSO, the compounds reveal locally ordered domains, which are large enough to induce order fluctuation as a significant relaxation pathway, and present paramagnetic relaxation enhancement for the [Aliquat][Cl] and [Aliquat][FeCl4] mixture. The addition of DMSO provides a way of tuning both the local order of these systems and the relaxation enhancement produced by the tetrachloroferrate anion. Very small DMSO volume concentrations (at least up to 1%) lead to enhanced paramagnetic relaxation without compromising the locally ordered domains. Larger DMSO concentrations gradually destroy these domains and reduce the effect of paramagnetic relaxation, while solvating the ions present in the mixtures. The paramagnetic relaxation was explained as a correlated combination of inner and outer-sphere mechanisms, in line with the size and structure differences between cation and anion. This study presents a robust method of characterizing paramagnetic ionic systems and obtaining a consistent analysis for a large set of samples having different co-solvent concentrations.

Aiming to rationalize the release profile of an incorporated pharmaceutical drug in terms of its mobility, driven by guest-host interactions, the poorly water-soluble Ibuprofen drug was loaded in a mesoporous inorganic silica matrix with unmodified (MCM-41) and modified surface (MCM-41sil) by post-synthesis silylation, both having pore sizes ∼ 3 nm. The single calorimetric detection of a broad glass transition step for both ibuprofen composites indicates full drug amorphization, confirmed by the only appearance of an amorphous halo in the powder XRD patterns. Moreover, a gradient profile is disclosed by the heat flux derivative plot in the glass transition, in coherence with the thermogravimetric profile that shows a multi-step decomposition trace for confined ibuprofen in these matrixes. While identical guest dynamics, as probed by dielectric relaxation spectroscopy, were found in both dehydrated composites, a significant molecular population with faster relaxation exists in the hydrated state for the drug inside the unmodified matrix. This was rationalized as the concurrence of true confinement effects, which manifest under nanometer dimensions, and greater water affinity of the unmodified matrix, forcing the drug molecules to be placed mostly in the pore core. Finite size effects are also felt in both dehydrated composites, however guest-host interactions give origin to a dominant population with slowed down mobility that governs the overall guest dynamics. In spite of an inferior number of active sites for drug adsorption in the silylated matrix, a faster Ibuprofen delivery in phosphate buffer (pH = 6.8) was observed when the drug is released from unmodified MCM-41 in the hydrated state. Therefore, our results suggest that a relevant role is played by water molecules, which impair a strong guest adsorption in the host surface more efficiently than the limited surface modification, influence the higher ratio of a faster population in the pore core and facilitate the diffusion of the aqueous releasing media inside pores.

Understanding the behavior of a chemical compound at a molecular level is fundamental, not only to explain its macroscopic properties, but also to enable the control and optimization of these properties. The present work aims to characterize a set of systems based on the ionic liquids [Aliquat][Cl] and [Aliquat][FeCl4] and on mixtures of these with different concentrations of DMSO by means of 1H NMR relaxometry, diffusometry and X-ray diffractometry. Without DMSO, the compounds reveal locally ordered domains, which are large enough to induce order fluctuation as a significant relaxation pathway, and present paramagnetic relaxation enhancement for the [Aliquat][Cl] and [Aliquat][FeCl4] mixture. The addition of DMSO provides a way of tuning both the local order of these systems and the relaxation enhancement produced by the tetrachloroferrate anion. Very small DMSO volume concentrations (at least up to 1%) lead to enhanced paramagnetic relaxation without compromising the locally ordered domains. Larger DMSO concentrations gradually destroy these domains and reduce the effect of paramagnetic relaxation, while solvating the ions present in the mixtures. The paramagnetic relaxation was explained as a correlated combination of inner and outer-sphere mechanisms, in line with the size and structure differences between cation and anion. This study presents a robust method of characterizing paramagnetic ionic systems and obtaining a consistent analysis for a large set of samples having different co-solvent concentrations.

CO2 separation from natural gas is considered to be a crucial strategy to mitigate global warming problems, meet product specification, pipeline specs and other application specific requirements. Silica xerogels (SX) are considered to be potential materials for CO2 capture due to their high specific surface area. Thus, a series of silica xerogels functionalized with imidazolium, phosphonium, ammonium and pyridinium-based room-temperature ionic liquids (RTILs) were synthesized. The synthesized silica xerogels were characterized by NMR, helium pycnometry, DTA-TG, BET, SEM and TEM. CO2 sorption, reusability and CO2/CH4 selectivity were assessed by the pressure-decay technique. Silica xerogels containing IL demonstrated advantages compared to RTILs used as separation solvents in CO2 capture processes including higher CO2 sorption capacity and faster sorption/desorption. Using fluorinated anion for functionalization of silica xerogels leads to a higher affinity for CO2 over CH4. The best performance was obtained by SX- [bmim] [TF2N] (223.4 mg CO2/g mg/g at 298.15 K and 20 bar). Moreover, SX- [bmim] [TF2N] showed higher CO2 sorption capacity as compared to other reported sorbents. CO2 sorption and CO2/CH4 selectivity results were submitted to an analysis of variance and the means compared using Tukey's test (5%).

Imidazolium-based ionic liquids (ILs) with different cation alkyl chain ([i-C5mim] or [C4mim]) and inorganic anions ([Cl−], [Tf2N−], [PF6−] and [DCA−]) were synthesized and immobilized in commercial mesoporous silica. The synthesized supported ILs (SILs) were characterized using NMR, FTIR, TGA, BET, SEM and TEM. CO2 sorption capacity, reusability and CO2/N2 selectivity were assessed by the pressure-decay technique. The effects of IL concentration, cation and anion chemical structure in CO2 sorption capacity and CO2/N2 separation performance were evaluated. Tests evidenced that the presence of branching on the cation alkyl side chain increases CO2/N2 selectivity. The immobilization of the IL [i-C5TPIm][Cl] on mesoporous silica in different concentrations (50, 20, 10 and 5 %) revealed that lower IL concentration results in higher CO2 sorption capacity. Immobilization of ILs containing fluorinated anions at low concentrations in the mesoporous silica support may promote the improvement of the CO2/N2 selectivity without interfering on CO2 sorption capacity of the original support. CO2 sorption capacity value shown by sample SIL-5 % - [i-C5TPIm][Tf2N] (79.50 ± 0.70 mg CO2 g-1) was close to the value obtained for the pristine mesoporous silica (81.70 ± 2.20 mg CO2 g-1) and the selectivity (4.30 ± 0.70) was more than twice of the one obtained for the support alone (2.32 ± 0.4). Recycle tests demonstrated that the ILs immobilized in mesoporous silica samples are stable, providing a new option to be used in CO2 capture processes.

The mechanism of cellulose dissolution in ionic liquid (IL)/dimethyl sulfoxide (DMSO) solvent systems has attracted much attention due to the possible replacement of synthetic materials. However, the solvent behaviour is not completely understood. This work has found an explanation for the solvent behaviour in cellulose dissolution, considering the almost unavoidable presence of the water. Ternary [C4mim]Cl/DMSO/H2O mixtures were studied with Nuclear Magnetic Resonance experiments and molecular dynamics simulations to explore IL/molecular solvents interactions and disclose the water interactions in these complex media. Titration of binary and ternary solvent systems with water and DMSO disclosed a relation between water’s proton chemical shift and the molar fraction of the mixture components, creating an unprecedent theory to predict the cellulose solvation ability. A “working range” for IL/DMSO/H2O ratio was observed, tested in cellulose dissolution, and rationalized using cellobiose interaction. Within this solvent ratio, the interactions between components are maximized, being switched on, while out of the range, the interactions are no longer detected.

The development of efficient CO2 capture routes to tackle the unprecedently high concentrations of this gas in the atmosphere is one of the world’s top-priority technological targets. Here, we describe, for the first time, the combined use of the enzyme carbonic anhydrase (CA) and an aqueous solution of choline hydroxide, to effectively absorb CO2. Variables such as choline hydroxide and enzyme concentrations and temperature were investigated. Overall sorption coefficients of up to 105 gCO2 kg−1solution were found (up to 3.8 M absorbent), whereas reaction rate improvements were demonstrated within a low enzyme concentration range (0.1–0.5 mg g−1). FT-IR and in situ 1H and 13C NMR experiments supported the elucidation of reaction mechanisms. It was found that in the absence of CA, the choline carbonate species is rapidly shifted to the bicarbonate form, whereas in the presence of the enzyme, the carbonate form is stable, conferring a buffered-like environment. Finally, the solvents containing the enzyme were used for the preparation of supported liquid membranes, which showed CO2 selectivity to N2 and CH4 of up to 90.5 and 28.9 times, respectively. The results here discussed add relevant knowledge towards the implementation of efficient CO2 capture.

In this work 2H NMR spectroscopy and 1H NMR relaxometry and diffusometry were used to characterise water order and dynamics in cellulose acetate/silica asymmetric membranes. Two hydrated membranes were characterised allowing the identification of extra ordering of the water molecules and the presence in each membrane of up to two spectral components with different degrees of order and different T1 values. The mechanism behind this order increase was ascribed to the rapid exchange of the water molecules between the pore walls and its interior. T1 relaxometry dispersions allowed for the identification of the relevant mechanisms of pore-confined water motion, with rotations mediated by translational displacements (RMTD) as the dominant mechanism in the low frequency region. Using the RMTD low cut off frequency along with the in situ directly measured diffusion constant it was possible do determine characteristic lengths of correlated water motion in both membranes studied, which fall in ranges compat...

Unexpected and unusual reactivity of 2-methylimidazolium salts toward aryl-N-sulfonylimines and aryl aldehydes is here reported. Upon reaction with aryl-N-sulfonylimines, the addition product, arylethyl-2-imidazolium-1-tosylamide (3), is formed with moderate to good yields, while upon reaction with aldehydes, the initial addition product (6) observed in NMR and HPLC-MS experimental analysis is postulated by us as an intermediate to the final conversion to carboxylic acids. Studies in the presence and absence of molecular oxygen allow us to conclude that the imidazolium salts is crucial for the oxidation. A detailed mechanistic study was carried out to provide insights regarding this unexpected reactivity.

This work was partly funded through the Associate Laboratory for Green Chemistry- LAQV which is financed by national funds from FCT/MCTES (POCI-01-0145-FEDER-007265, UID/QUI/50006/2019) and CENIMAT, I3N (POCI-01-0145-FEDER-007688, UID/CTM/50025/2013), also financed by national funds through the FCT/MCTES, and when appropriate co-financed by the Fundo Europeu de Desenvolvimento Regional (FEDER) under the PT2020 Partnership Agreement. This work is funded by national funds (OE), through FCT – Fundação para a Ciência e a Tecnologia. European Social Fund through the program POPH of QREN. The authors also acknowledge the Portuguese Nuclear Magnetic Resonance Network (PTNMR). This work compares the stability and the catalytic efficiency of different ionic liquid phosphomolybdates ([BPy]3[PMo12O40] and [BMIM]3[PMo12O40]) with a cationic (propylpyridinium) functionalized mesoporous silica nanoparticle composite (PMo12O40@PPy-MSN). These were used as solid catalysts for the oxidative desulfurization of a multicomponent model diesel using hydrogen peroxide as oxidant and a polar immiscible extraction solvent. Ionic liquid ([BMIM][PF6] was successfully used as solvent to extract sulfur compounds from model diesel. The ionic liquid phosphomolybdates showed partial solubility in the ionic liquid phase, occurring some decomposition of their Keggin structure in the soluble reaction media, probably caused by their interaction with oxidant. On the other hand, the phosphomolybdate composite PMo12O40@PPy-MSN presented high structural stability and only negligible leaching occurrence after various consecutive reaction cycles. The model diesel was near complete desulfurized after 3 h and consecutive desulfurization cycles were performed without loss of activity. Therefore, the immobilization of Keggin phosphomolybdate structure [PMo12O40]3− using cationic propylpyridinium silica nanoparticle is an assertive strategy to produce stable and active heterogeneous catalysts. authorsversion published

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[EN] The monolacunary Keggin-type [PWO] (PW) heteropolyanion was immobilized on porous framework of mesoporous silicas, namely SBA-15 and an ethylene-bridged periodic mesoporous organosilica (PMOE). The supports were functionalized with a cationic group (N-trimethoxysilypropyl-N, N, N-trimethylammonium, TMA) for the successful anchoring of the anionic polyoxometalate. The PW@TMA-SBA-15 and PW@TMA-PMOE composites were evaluated as heterogeneous catalysts in the oxidative desulfurization of a model diesel. The PW@TMA-SBA-15 catalyst showed a remarkable desulfurization performance by reaching ultra-low sulfur levels (, This work was partly funded through the projects REQUIMTE-LAQV [FCT (Fundação para a Ciência e a Tecnologia) Ref. LAQV, REQUIMTE (POCI-01-0145-FEDER-007265, UID/QUI/50006/2019)], and CENIMAT/I3N (POCI-01-0145-FEDER-007688, UID/CTM/50025/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by the Fundo Europeu de Desenvolvimento Regional (FEDER) under the PT2020 Partnership Agreement. This work was funded by national funds (OE), through FCT—Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. The FCT and the European Union are acknowledged for the post-graduation grant SFRH/BD/95571/2013 (to SR) co-funded by Ministério da Ciência, Tecnologia e Ensino Superior (MCTES), and the European Social Fund through the program POPH of QREN. The authors also acknowledge the Portuguese Nuclear Magnetic Resonance Network (PTNMR—ROTEIRO/0031/2013, PINFRA/22161/2016) and the research group of Prof. Isabel Gonçalves from CICECO Laboratory, University of Aveiro, Portugal.

The preorganization and cooperation mechanism of imide-based ionic liquids reported in a recent Communication was evocated to rationalize the extremely high gravimetric CO2 capture displayed by these fluids. An analysis of the reported spectroscopic evidences together with additional experiments led to the proposition of an alternative, simpler, and feasible mechanism involving the formation of bicarbonate.

Over the past few years porous materials have become a topic of intense research. Porous poly(ionic liquid)s combine the porous architecture with intrinsic ionic liquids properties. In all research areas, the quest for new and improved materials has targeted functional materials with enhanced specificity and efficiency towards the final application. The application of porous materials ranges from sensing, protein separation, solid-phase extraction, catalysis, to CO2 capture and reuse. Recently, the design, synthesis, and porosity control of poly(ionic liquid)s have been attempted through strategies that include classic polymerization techniques as well as molecular imprinting and aerogels production. This review aims at providing the recent advances on porous poly(ionic liquid)s, giving a critical perspective about the works in which key requirements for porosity induction are discussed. Several applications that rely on molecular interactions between the porous material and target compounds are presented, focusing mainly on CO2 capture and reuse, along with some challenges that the scientific community in this field need to be aware of.

The present study focus on the preparation of chromatography supports for affinity-based chromatography of supercoiled plasmid purification. Three l-tryptophan based supports are prepared through immobilization on epoxy-activated Sepharose and characterized by HR-MAS NMR. The SPR is employed for a fast screening of l-tryptophan derivatives, as potential ligands for the biorecognition of supercoiled isoform, as well as, to establish the suitable experimental conditions for the chromatography. The results reveal that the overall affinity is high (KD=10(-9) and 10(-8)M) and the conditions tested show that the use of HEPES 100mM enables the separation and purification of supercoiled at T=10°C. The STD-NMR is performed to accomplish the epitope mapping of the 5'-mononucleotides bound to l-tryptophan derivatives supports. The data shows that the interactions between the three supports and the 5'-mononucleotides are mainly hydrophobic and π-π stacking. The chromatography experiments are performed with l-tryptophan support and plasmids pVAX-LacZ and pPH600. The supercoiled isoform separation is achieved at T=10°C by decreasing the concentration of (NH4)2SO4 from 2.7 to 0M in HEPES for pVAX-LacZ and 2.65M to 0M in HEPES for pPH600. Overall, l-tryptophan derivatives can be a promising strategy to purify supercoiled for pharmaceutical applications.

The developments in the use of plasmid DNA (pDNA) in gene therapy and vaccines have motivated the search and improvement of optimized purification processes. In this context, dipeptides l-tyrosine-l-tyrosine and l-tyrosine-l-arginine are synthetized to explore their application as affinity ligands for supercoiled (sc) plasmid DNA (pDNA) purification. The synthesis is based on the protection of N-Boc-l-tyrosine, followed by condensation with l-tyrosine or l-arginine methyl esters in the presence of dicyclohexylcarbodiimide (DCC), which after hydrolysis and acidification give the afforded dipeptides. The supports are then obtained by coupling l-tyrosine, l-tyrosine-l-tyrosine and l-tyrosine-l-arginine to epoxy-activated Sepharose and are characterized by high resolution magic angle spinning (HR-MAS) NMR and Fourier transform infrared spectroscopy (FTIR). Surface plasmon resonance (SPR) biosensor is used to establish the promising ligand to be used in the chromatographic experiments and ascertain experimental conditions. Sc isoform showed the highest affinity to the dipeptides, followed by linear (ln) pDNA, being the open circular (oc) the one that promoted the lowest affinity to l-tyrosine-l-arginine. Saturation transfer difference (STD)-NMR experiments show that the interaction is mainly hydrophobic with the majority of the 5'-mononucleotides, except for 5'-GMP with l-tyrosine-l-arginine Sepharose that is mainly electrostatic. The support l-tyrosine Sepharose used in chromatographic experiments promotes the separation of native pVAX1-LacZ and pcDNA3-FLAG-p53 samples (oc+sc) by decreasing the salt concentration. The results suggest that it is possible to purify different plasmids with the l-tyrosine Sepharose, with slight adjustments in the gradient conditions.

In the last few years, ionic liquids (ILs) have been the focus of extensive studies concerning the relationship between structure and properties and how this impacts their application. Despite a large number of studies, several topics remain controversial or not fully answered, such as: the existence of ion pairs, the concept of free volume and the effect of water and its implications in the modulation of ILs physicochemical properties. In this paper, we present a critical review of state-of-the-art literature regarding structure–property relationship of ILs, we re-examine analytical theories on the structure–property correlations and present new perspectives based on the existing data. The interrelation between transport properties (viscosity, diffusion, conductivity) of IL structure and free volume are analysed and discussed at a molecular level. In addition, we demonstrate how the analysis of microscopic features (particularly using NMR-derived data) can be used to explain and predict macroscopic properties, reaching new perspectives on the properties and application of ILs.

The mitigation of climate change effects requires the use of alternative materials and technologies to control CO2 atmospheric levels through its capture, storage and use. In this field, the current work presents the evaluation of two poly(ionic liquid)s (PILs) (poly-1-vinyl-3-ethylimidazolium acetate and hydroxide) combined with free ionic liquid (IL) 1-butyl-3-methylimidolium acetate (BMI·OAc) for CO2 capture. The sorption capacity of PIL@IL composites was evaluated under 20 bar of CO2 at 298 K. Nuclear Magnetic Resonance (NMR) spectroscopy allowed quantification of CO2 sorption (physisorption and/or chemisorption) and in situ study of the PIL−CO2 interaction mechanism. NMR in combination with Molecular Dynamics (MD) simulations suggested a 3D organization of PIL composites, maintaining a similar organization to ILs. Also, the use of aqueous solutions of PIL@IL composites was tested, identifying the optimum conditions for water activation (intrinsic water trapped inside IL structure) for chemisorption. As our main contribution, we demonstrate the possibility to control the sorption pathway towards CO2 physisorption, or CO2 conversion (chemisorption) through carbonation (HCO3−/CO32-) according to the PIL/IL ratio, ions structure and water amount. The use of PIL/IL composites is a promising advance for further CO2 reuse approaching a biomimetic carbonation process.

Fungal stains affect documents and artworks on paper all over the world, diminishing their chemical stability and compromising their readability. The present paper studies the suitability of agarose and gellan gum hydrogels to remove fungal stains from paper, using paper impregnated with alizarin as a model system to simulate the most common colorant molecules produced by fungi - polyketide quinones. The effect of pH variation on the efficacy of the gels was evaluated by UV spectrometry. The results show that the cleaning efficacy of the gels greatly depends on the gel matrix, the colorant molecules, and the pH balance of the process.