The New Approach to the Preparation of Polyacrylamide-Based Hydrogels: Initiation of Polymerization of Acrylamide with 1,3-Dimet

Communication The New Approach to the Preparation of Polyacrylamide-Based Hydrogels: Initiation of Polymerization of Acrylamide with 1,3-Dimethylimidazolium (Phosphonooxy-)Oligosulphanide under Drying Aqueous Solutions

Natalia Tarasova 1,2, Alexey Zanin 1,* , Efrem Krivoborodov 1, Ilya Toropygin 3, Ekaterina Pascal 1 and Yaroslav Mezhuev 1

1 Institute of Chemistry and Problems of Sustainable Development, Dmitry Mendeleev University of Chemical Technology of Russia, 12047 Moscow, Russia; [email protected] (N.T.); [email protected] (E.K.); [email protected] (E.P.); [email protected] (Y.M.) 2 Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, 119017 Moscow, Russia 3 V.N. Orekhovich Research Institute of Biomedical Chemistry, Russian Academy of Medical Sciences, 119121 Moscow, Russia; [email protected] * Correspondence: [email protected]

Citation: Tarasova, N.; Zanin, A.; Abstract: The new initiator of the polymerization of acrylamide, leading to the formation Krivoborodov, E.; Toropygin, I.; of crosslinked polyacrylamide, was discovered. The structure of the synthesized polyacrylamide Pascal, E.; Mezhuev, Y. The New was characterized by XRD, 1H NMR, and 13C NMR spectroscopy. It was shown that 1,3- Approach to the Preparation of dimethylimidazolium (phosphonooxy-)oligosulphanide is able to initiate radical polymerization Polyacrylamide-Based Hydrogels: under drying aqueous solutions of acrylamide, even at room temperature. According to XRF data, Initiation of Polymerization of the synthesized polyacrylamide gel contains 0.28 wt% of sulphur. The formed polymer network Acrylamide with 1,3- has a low crosslinking density and a high equilibrium degree of swelling. The swelling rate of Dimethylimidazolium polyacrylamide gel in water corresponds to the ﬁrst order kinetic equation with the rate constant (Phosphonooxy-)Oligosulphanide 6.2 × 10−2 min−1. The initiator is promising for combining acrylamide polymerization with the under Drying Aqueous Solutions. processes of gel molding and drying. Polymers 2021, 13, 1806. https://doi.org/10.3390/ polym13111806 Keywords: polyacrylamide; polymerization of acrylamide; polymerization initiator; ionic liquids; sulphur Academic Editor: Adriana Kovalcik

Received: 9 May 2021 Accepted: 27 May 2021 1. Introduction Published: 30 May 2021 Linear and crosslinked polyacrylamide, as well as acrylamide copolymers, are of significant applied interest and are used in water treatment [1–3], enhanced oil recovery (EOR) Publisher’s Note: MDPI stays neutral technologies [3–6], stabilization of nanoparticles [7–9], electrophoresis-induced separation with regard to jurisdictional claims in and purification of substances [10], and adsorption removal of Pb2+ ions from aqueous published maps and institutional affil- solutions [11]. The biocompatibility and hydrophilicity of polyacrylamide have ensured its iations. wide application in biomedical fields [12], including in systems of pharmacologically active substance delivery [13] and the binding of toxins [14]. Several acrylamide copolymers have proven to be stimulus-sensitive polymers. For example, nanoparticles of a copolymer of acrylamide and acrylonitrile are described that are capable of transitioning from worm-like Copyright: © 2021 by the authors. morphology to spherical when heated in water [15]. Licensee MDPI, Basel, Switzerland. For biomedical applications, polymer networks based on crosslinked polyacrylamide, This article is an open access article capable of large, limited swelling in water, which are obtained by radical copolymerization distributed under the terms and of acrylamide and methylene-bis-acrylamide, are of particular importance. Although conditions of the Creative Commons polyacrylamide gels are widely used, significant difficulties arise in the need for poly- Attribution (CC BY) license (https:// merization molding of articles. Therefore, the development of a method that provides creativecommons.org/licenses/by/ the initiation of acrylamide polymerization under the conditions of drying its aqueous 4.0/).

Polymers 2021, 13, 1806. https://doi.org/10.3390/polym13111806 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1806 2 of 8

Polymers 2021, 13, 1806 2 of 8 for polymerization molding of articles. Therefore, the development of a method that provides the initiation of acrylamide polymerization under the conditions of drying its aqueoussolutions solutions is an important is an appliedimportant problem. applied Although problem. the Although kinetics andthe thekinetics mechanism and the of mechanismpolyacrylamide of polyacrylamide synthesis have synthesis been thoroughly have been studied thoroughly [16,17 ],studied the search [16,17], for newthe search meth- forods new of initiating methods theof initiating polymerization the polymerization of acrylamide of in acrylamide mild conditions in mild continues conditions [18 con-–20]. tinuesEarlier, [18–20]. we discovered Earlier, we the discovered possibility ofthe ring possibility opening of of ring cyclooctasulphur opening of cyclooctasulphur in the presence inof the 1,3-dimethylimidazolium presence of 1,3-dimethylimidazolium dimethyl phosphate dimethyl with phosphate the formation with of the phosphorylated formation of phosphorylatedlinear sulphur oligomers linear sulphu (Schemer oligomers1)[21]. (Scheme 1) [21].

SchemeScheme 1. 1. SynthesisSynthesis of of 1,3-dimethylimidazolium 1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide. (phosphonooxy-)oligosulphanide.

ThisThis work work shows shows for for the the first first time the possibility of using 1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide(phosphonooxy-)oligosulphanide toto initiate initiate polymerization polymerization of of acrylamide acrylamide under under conditions condi- tionsof prolonged of prolonged drying drying of its aqueous of its aqueous solution solu at roomtion at temperature. room temperature. As a result, As the a result, formation the formationof a crosslinked of a polyacrylamidecrosslinked polyacrylamide was observed inwas the observed absence of in methylene-bis-acrylamide. the absence of methylene-bis-acrylamide. 2. Materials and Methods 2.2.1. Materials Synthesis and of Polyacrylamide Methods in the Presence of 1,3-Dimethylimidazolium (Phosphonooxy-)Oligosulphanide 2.1. Synthesis of Polyacrylamide in the Presence of 1,3-Dimethylimidazolium 0.2 mL of a solution of 1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide, (Phosphonooxy-)Oligosulphanide obtained according to the previously described method [21], was added to a solution of 0.5 g0.2 of mL acrylamide of a solution (“Sigma-Aldrich”, of 1,3-dimethylimidazolium purity over 99%, (phosphonooxy-)oligosulphanide, St. Louis, MI, USA) in 2 mL of obtainedwater (bidistillate), according followedto the previously by stirring describe for 20 min.d method Then [21], the reaction was added mixture to a wassolution left at of a 0.5temperature g of acrylamide of 298 K(“Sigma-Aldrich”, (25 ◦C) and dried. purity The structure over 99%, of the St. obtainedLouis, MI, polyacrylamide USA) in 2 mL was of watercharacterized (bidistillate), by XRD followed (Benchtop by stirring X-ray Diffractometer,for 20 minutes. MalvernThen the Panalytical reaction mixture Aeris XRD) was leftand at X-ray a temperature fluorescent of analysis 298 K (Bruker (25 °C) Kappa and dried. APEX The DUO). structure of the obtained poly- acrylamideThe dynamics was characterized of acrylamide by XRD polymerization (Benchtop X-ray was investigatedDiffractometer, in situMalvern in an Pana- open lyticalampoule, Aeris daily XRD) recording and X-ray the fluorescen1H NMR spectrumt analysis (Bruker CXPKappa 200) APEX of the DUO). reaction system. For theThe sample dynamics isolated of acrylamide four days after polymerization the start of the was reaction investigated and washing in situ with in methanolan open 1 ampoule,(in the calculation daily recording of 10 mL the of H CH NMR3OH spectrum (“Sigma-Aldrich”, (Bruker CXP purity 200) over of the 99%) reaction per 1 gsystem. of gel), Forthe the13C sample NMR spectrum isolated wasfour recorded.days after the start of the reaction and washing with methanol (in the calculation of 10 mL of CH3OH (“Sigma-Aldrich”, purity over 99%) per 1 g of gel),2.2. Swellingthe 13C NMR Kinetics spectrum of Polyacrylamide was recorded. in Water 0.115 g of synthesized polyacrylamide, thoroughly washed with methanol and dried, 2.2.was Swelling placed inKinetics 100 mL of Polyacrylamide of water. The in system Water was thermostated at 298 K (25 ◦C), and the mass0.115 of the g sampleof synthesized was recorded polyacrylamide, at specified time thoroughly intervals. washed with methanol and dried, was placed in 100 mL of water. The system was thermostated at 298 K (25 °C), and 3. Results and Discussion the mass of the sample was recorded at specified time intervals. The 1H NMR spectrum contains signals of 5.69 ppm and 6.10 ppm related to the 3.vinyl Results moiety and of Discussion the residual acrylamide (Figure1A), as well as signals of 2.07 ppm and 1.52 ppm, belonging to the CH and CH groups of the polyacrylamide chain (Figure1D). The 1H NMR spectrum contains signals2 of 5.69 ppm and 6.10 ppm related to the The assignment of signals on the 13C NMR and 1H NMR spectra of polyacrylamide gels vinyl moiety of the residual acrylamide (Figure 1A), as well as signals of 2.07 ppm and are consistent with the previously described outcomes [22,23]. The 1H NMR spectra of 1.52 ppm, belonging to the CH and CH2 groups of the polyacrylamide chain (Figure 1D). the reaction system, obtained during the drying of an aqueous solution of acrylamide The assignment of signals on the 13C NMR and 1H NMR spectra of polyacrylamide gels in the presence of 1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide, indicate are consistent with the previously described outcomes [22,23]. The 1H NMR spectra of the the gradual formation of polyacrylamide (Figure1). The reaction proceeds almost until reaction system, obtained during the drying of an aqueous solution of acrylamide in the the complete conversion of the monomer, so that after 6 days no vinyl proton signals presenceof the initial of acrylamide1,3-dimethylimidazolium were observed in(pho thesphonooxy-)oligosulphanide,1H NMR spectrum of the reaction indicate system the (Figure1D).

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gradual formation of polyacrylamide (Figure 1). The reaction proceeds almost until the Polymers 2021, 13, 1806 complete conversion of the monomer, so that after 6 days no vinyl proton signals of3 ofthe 8 initial acrylamide were observed in the 1H NMR spectrum of the reaction system (Figure 1D).

Figure 1. The 1HH NMR NMR spectra spectra of of the the reaction reaction system system obtain obtaineded in in the the process process of drying an aqueous solution of of acrylamide acrylamide at at a atemperature temperature of of 298 298 K K(25 (25 °C)◦ C)after: after: (A) (1A day;) 1 day; (B) 4 ( Bdays;) 4 days; (C) 5 (days;C) 5 days;(D) 6 (daysD) 6 after days the after start the of start the of reaction. the reaction.

It is noteworthynoteworthy that polymerizationpolymerization begins only when most of thethe waterwater hashas beenbeen removed and does not develop inin a sealed ampoule.ampoule. The polymerization ofof acrylamide inin the presence ofof 1,3-dimethylimidazolium1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide(phosphonooxy-)oligosulphanide is evidencedevidenced 1313 by the CC NMR spectrum recorded in D2O for gel washed with methanol (Figure2 2).). AsAs observed, thethe13 13CC NMRNMR spectrum spectrum contains contains signals signals of 171of 171 ppm, ppm, 129.99 129.99 ppm ppm and 128.99and 128.99 ppm relatedppm related to the to carbonthe carbon atoms atoms of residualof residual acrylamide, acrylamide, as as well well as as signals signals ofof 180.00180.00 ppm, 42.25 ppm and 35.77 ppm, relatedrelated toto thethe carboncarbon atomsatoms ofof thethe C=O,C=O, CHCH andand CHCH22 moietiesmoieties Polymers 2021, 13, 1806 4 of 8 of the resulting polyacrylamide. The signal with a chemical shift of 50 ppm refers to thethe methanol carbon used for rinsing.

Figure 2.13 13С NMR spectrum of polyacrylamide gel after rinsing with methanol at the rate of 10 mL Figure 2. C NMR spectrum of polyacrylamide gel after rinsing with methanol at the rate of 10 mL of CH3OH per 1 g of crosslinked polymer. of CH3OH per 1 g of crosslinked polymer. XRD data indicate the formation of an amorphous substance, which is characteristic of polyacrylamide (Figure 3) [24]. According to X-ray fluorescent analysis, the mass proportion of sulphur within the polyacrylamide gel sample repeatedly purified by rinsing with methanol and distilled water is 0.28 wt%, while the 31P NMR spectrum recorded by using methanol for rinsing shows a singlet with a chemical shift of 3.28 ppm (relative to 85% aqueous phosphoric acid), which is characteristic for the phosphorus atom of the dimethyl phosphate anion.

Figure 3. XRD of dried polyacrylamide gel.

The synthesized polyacrylamide was only capable of limited swelling in water, which indicates the formation of a three-dimensional polymer network (Figure 4). The linearity of the kinetic curve of the swelling of polyacrylamide in water in coordinates ( ) ln[ ] vs. 𝑡 indicates the subordination of the process to the quasi-first order Equa- tion (1) and its integral form (2): =𝑘(𝛼 −𝛼) (1)

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Polymers 2021, 13, 1806 4 of 8 Figure 2. 13С NMR spectrum of polyacrylamide gel after rinsing with methanol at the rate of 10 mL of CH3OH per 1 g of crosslinked polymer.

XRDXRD data data indicate indicate the the formation formation of of an an amorphous substance, which is characteristic ofof polyacrylamide (Figure (Figure 33))[ [24].24]. Accordin Accordingg to to X-ray X-ray fluorescent ﬂuorescent analysis, the mass proportionproportion ofof sulphursulphur within within the the polyacrylamide polyacrylamide gel samplegel sample repeatedly repeatedly puriﬁed purified by rinsing by rinsingwith methanol with methanol and distilled and distilled water iswater 0.28 is wt%, 0.28 whilewt%, while the 31 Pthe NMR 31P NMR spectrum spectrum recorded rec- ordedby using by methanolusing methanol for rinsing for rinsing shows ashows singlet a withsinglet a chemicalwith a chemical shift of 3.28shift ppm of 3.28 (relative ppm (relativeto 85% aqueous to 85% phosphoricaqueous phosphoric acid), which acid), is characteristic which is characteristic for the phosphorus for the phosphorus atom of the atomdimethyl of the phosphate dimethyl anion.phosphate anion.

Figure 3. XRDFigure of dried 3. XRD polyacrylamide of dried polyacrylamide gel. gel.

TheThe synthesizedsynthesized polyacrylamidepolyacrylamide was was only only capable capable of limitedof limited swelling swelling in water, in water, which whichindicates indicates the formation the formation of a three-dimensional of a three-dimensional polymer polymer network network (Figure4 ).(Figure The linearity 4). The (α − α) linearity of the kinetic curve of the swelling of polyacrylamide in water in coordinates[ ∞ ] of( the kinetic) curve of the swelling of polyacrylamide in water in coordinates ln ln[ ] vs. 𝑡 indicates the subordination of the process to the quasi-first orderα Equa-∞ vs. tindicates the subordination of the process to the quasi-ﬁrst order Equation (1) and its tionintegral (1) and form its (2): integral form (2): dα ==𝑘(𝛼k(α −−𝛼)α) (1)(1) dt ∞ (α − α) ln [ ∞ ] = −kt (2) α∞

where: α = (m − m0)/m0—the degree of swelling and α∞—the equilibrium degree of

swelling of the polyacrylamide gel in water; m0, m—initial and current value of gel weight; t—time; k—swelling rate constant. The swelling rate constant of polyacrylamide in water was 6.2 × 10−2 min−1, and the equilibrium degree of swelling is 7.09. The obtained kinetic data make it possible to quantitatively estimate the average molecular weight of the chain segments between the network nodes according to the Flory and Rehner Equation (3) [25]:

1 dPVmS ϕ 3 − 0.5ϕ Mc = − (3) ln(1 − ϕ) + ϕ + χϕ2

where: Mc—average molecular weight of chain segments between network nodes; ϕ— volume fraction of the polymer that has reached equilibrium swelling; dP—density of the polymer; VmS—molar volume of the solvent; χ—Huggins interaction parameter. Polymers 2021, 13, 1806 5 of 8

( ) ln[ ]=−𝑘𝑡 (2)

Polymers 2021, 13, 1806 where: 𝛼=(𝑚−𝑚)/𝑚—the degree of swelling and 𝛼– the equilibrium degree5 ofof 8 swelling of the polyacrylamide gel in water; 𝑚,𝑚—initial and current value of gel weight; 𝑡—time; 𝑘—swelling rate constant.

FigureFigure 4. KineticKinetic curvescurves of of the the swelling swelling of of polyacrylamide polyacrylamide in waterin water at a temperatureat a temperature of 298 of K 298 (25 ◦KC) (25 in coordinates:°C) in coordinates: (A)—m ( − )() A)—vs. t;𝑚 (B vs.)—ln t; B)—[ α∞ ln[α ] vs. t], wherevs. 𝑡, wherem—current 𝑚—current value of value gel weight. of gel weight. α∞

−3 TheTaking swelling into account rate constant the density of polyacrylamide of polyacrylamide in water 1.3 was g × 6.2cm × 10, the−2 min volume−1, and frac- the equilibriumtion of the polymer degree of (ϕ )swelling upon reaching is 7.09. the The equilibrium obtained kinetic degree ofdata swelling make isit 0.098.possible Since to quantitativelythe Huggins interactionestimate the parameter average molecula for ther polyacrylamide—water weight of the chain segments system between is close the to 4 network0.493 at anodes temperature according of to 298 the K Flory (25 °C and)[26 Rehner], Mc isEquation about 10 (3). [25]: Therefore, the use of 1,3- dimethylimidazolium (phosphonooxy-)oligosulphanide as an initiator of acrylamide polymerization under conditions of slow drying of its aqueous solutions allows the formation 𝑑𝑉(𝜑 −0.5𝜑) of a lightly crosslinked polymer𝑀 network= − with high water absorption. (3) ln(1−𝜑) + 𝜑+ 𝜒𝜑 The listed circumstances can be explained if we admit the mechanism shown in Scheme2. In accordance with this mechanism, the initiation of polymerization is possible where: 𝑀 —average molecular weight of chain segments between network nodes; only under the condition of a high concentration of acrylamide in the reaction system. 𝜑—volume fraction of the polymer that has reached equilibrium swelling; 𝑑—density In addition, according to the HSAB theory, only a low rate of processes leading to chain of the polymer; 𝑉—molar volume of the solvent; 𝜒—Huggins interaction parameter. crosslinkingTaking into can beaccount expected, the whichdensity is of consistent polyacrylamide with the 1.3 formation g × cm− of3, the polymer volume gels fraction with a ofhigh the equilibrium polymer (𝜑 degree) upon ofreaching swelling the and equilibrium low crosslinking degreedensity. of swelling is 0.098. Since the HugginsThe possibilityinteraction of parameter initiating polymerizationfor the polyacry oflamide—water acrylamide with system 1,3-dimethylimidazolium is close to 0.493 at (phosphonooxy-)oligosulphanide is consistent with the increased4 ability for the formation a temperature of 298 K (25 ℃) [26], 𝑀 is about 10 . Therefore, the use of of S–S bonds to homolytic decomposition, compared to S [27–31] and the low stability of 1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide8 as an initiator of acrylamide hydrogen polysulphides. Although the details of the mechanism of the reaction described polymerization under conditions of slow drying of its aqueous solutions allows the for- above have yet to be established, we report on the fundamental possibility of initiating mation of a lightly crosslinked polymer network with high water absorption. radical polymerization of acrylamide with 1,3-dimethylimidazolium (phosphonooxy-) The listed circumstances can be explained if we admit the mechanism shown in oligosulphanide. Scheme 2. In accordance with this mechanism, the initiation of polymerization is possible only under the condition of a high concentration of acrylamide in the reaction system. In addition, according to the HSAB theory, only a low rate of processes leading to chain crosslinking can be expected, which is consistent with the formation of polymer gels with a high equilibrium degree of swelling and low crosslinking density.

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SchemeScheme 2.2. The proposedproposed mechanismmechanism forfor thethe formationformation of of crosslinked crosslinked polyacrylamide polyacrylamide in thein the presence presence of 1,3-of dimethylimidazolium1,3-dimethylimidazolium (phosphonooxy-)oligosulphanide. (phosphonooxy-)oligosulphanide.

4. ConclusionsThe possibility of initiating polymerization of acrylamide with 1,3-dimethylimidazoliumThe product of the interaction (phosphonooxy-)oligosulphanide of elemental sulphur with is 1,3-dimethylimidazolium consistent with the in- dimethylphosphatecreased ability for the is formation capable of of initiating S–S bonds acrylamide to homolytic polymerization decomposition, upon compared the slow to evaporationS8 [27–31] and of the water low at stability room temperature. of hydrogen Thepolysulphides. resulting polyacrylamide Although the details is capable of the of limitedmechanism swelling of the in reaction water, anddescribed the equilibrium above have mass yet degreeto be established, of swelling we is 7.09,report and on thethe swellingfundamental rate constantpossibility at aof temperature initiating radical of 298 Kpolymerization (25 ◦C) is 6.2 ×of10 acrylamide−2 min−1. Thus,with simultaneously1,3-dimethylimidazolium with the formation (phosphonooxy-) of polyacrylamide oligosulphanide. chains, the process responsible for their crosslinking develops, which opens the possibility of obtaining polymer networks by drying4. Conclusions aqueous solutions of acrylamide. This reaction opens new perspectives and modes of formingThe product articles of from the crosslinkedinteraction polyacrylamide.of elemental sulphur with 1,3-dimethylimidazolium dimethylphosphateApparently, both is thecapable initiation of initiating of polymerization acrylamide of polymerization acrylamide and upon the crosslink-the slow ingevaporation of the chains of water of the at resultingroom temperature. polyacrylamide The resulting are a consequence polyacrylamide of the is low capable stability of oflimited the S–S swelling bonds in of water, linear sulphurand the chainsequilibrium of 1,3-dimethylimidazolium mass degree of swelling (phosphonooxy-) is 7.09, and the oligosulphanide.swelling rate constant at a temperature of 298 K (25 °C) is 6.2 × 10−2 min−1. Thus, simultaneously with the formation of polyacrylamide chains, the process responsible for their Authorcrosslinking Contributions: develops,Conceptualization, which opens the N.T.; possibility methodology, of obtaining E.K. and I.T.;polymer validation, networks E.K., I.T.by anddrying E.P.; aqueous formal analysis, solutions E.K. of and acrylamide. I.T.; investigation, This I.T.;reaction resources, opens E.K. new and perspectives E.P.; data curation, and E.K.; writing—original draft preparation, E.K. and Y.M.; writing—review and editing, N.T., A.Z. modes of forming articles from crosslinked polyacrylamide. and Y.M.; visualization, A.Z., E.K. and Y.M.; supervision, N.T. and A.Z.; project administration, N.T. and A.Z.;Apparently, funding acquisition,both the initiation A.Z. All of authors polymeri havezation read and of acrylamide agreed to the and published the crosslinking version of theof the manuscript. chains of the resulting polyacrylamide are a consequence of the low stability of the S–S bonds of linear sulphur chains of 1,3-dimethylimidazolium (phos- Funding:phonooxy-)oligosulphanide.This research was funded by D. Mendeleev University of Chemical Technology of Russia, grant number 2020-040. InstitutionalAuthor Contributions: Review Board Conceptualization, Statement: Not N.T.; applicable. methodology, E.K. and I.T.; validation, E.K., I.T. and E.P.; formal analysis, E.K. and I.T.; investigation, I.T.; resources, E.K. and E.P.; data curation, Informed Consent Statement: E.K.; writing—original draft prepNotaration, applicable. E.K. and Y.M.; writing—review and editing, N.T., A.Z. Dataand Y.M.; Availability visualization, Statement: A.Z.,Not E.K. applicable. and Y.M.; supervision, N.T. and A.Z.; project administration,

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Acknowledgments: This research was funded by D. Mendeleev University of Chemical Technology of Russia, grant number 2020-040. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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