Enhancing Lignin Dissolution and Extraction: the Effect of Surfactants

Article Enhancing Lignin Dissolution and Extraction: The Effect of Surfactants

Elodie Melro 1,* , Artur J. M. Valente 1 , Filipe E. Antunes 1, Anabela Romano 2 and Bruno Medronho 2,3

1 Department of Chemistry, University of Coimbra, CQC, Rua Larga, 3004-535 Coimbra, Portugal; [email protected] (A.J.M.V.); [email protected] (F.E.A.) 2 MED—Mediterranean Institute for Agriculture, Environment and Development, Faculdade de Ciências e Tecnologia, Campus de Gambelas, Universidade do Algarve, Ed. 8, 8005-139 Faro, Portugal; [email protected] (A.R.); [email protected] (B.M.) 3 FSCN, Surface and Colloid Engineering, Mid Sweden University, SE-851 70 Sundsvall, Sweden * Correspondence: [email protected]

Abstract: The dissolution and extraction of lignin from biomass represents a great challenge due to the complex structure of this natural phenolic biopolymer. In this work, several surfactants (i.e., non-ionic, anionic, and cationic) were used as additives to enhance the dissolution efﬁciency of model lignin (kraft) and to boost lignin extraction from pine sawdust residues. To the best of our knowledge, cationic surfactants have never been systematically used for lignin dissolution. It was found that ca. 20 wt.% of kraft lignin is completely solubilized using 1 mol L−1 octyltrimethylammonium bromide aqueous solution. A remarkable dissolution efﬁciency was also obtained using 0.5 mol L−1 polysorbate 20. Furthermore, all surfactants used increased the lignin extraction with formic acid, even at low concentrations, such as 0.01 and 0.1 mol L−1. Higher concentrations of cationic surfactants improve the extraction yield but the purity of extracted lignin decreases.

Citation: Melro, E.; Valente, A.J.M.; Keywords: dissolution; extraction; lignin; lignocellulosic waste; maritime pine; surfactants Antunes, F.E.; Romano, A.; Medronho, B. Enhancing Lignin Dissolution and Extraction: The Effect of Surfactants. Polymers 2021, 13, 714. 1. Introduction https://doi.org/10.3390/ polym13050714 The interaction of surfactants with macromolecules, such as proteins, dates from the early 1950s [1]. This research focusing on these interactions was mainly triggered Academic Editor: Debora Puglia by their biological relevance. Later, synthetic polymers with well-defined characteristics appeared, and research was further extended to other polymer–surfactant systems. Due to Received: 8 February 2021 their uses in different applications, such as cosmetics, paints, foods, detergents, pesticides, Accepted: 22 February 2021 and pharmaceutics, polymer–surfactant systems have received a lot of attention in the last Published: 26 February 2021 several decades [2,3]. The combination of polymers and surfactants can offer myriad effects, such as controlling interfacial properties, network formation, and rich phase behavior. The Publisher’s Note: MDPI stays neutral latter effect is particularly relevant in the context of the solubilization of water-insoluble with regard to jurisdictional claims in polymers [4]. Lignin is an example of a polymer that is insoluble in water and many other published maps and institutional affil- traditional solvents. This complex polyphenolic biopolymer is considered the second most iations. available macromolecule on Earth and works as a “natural glue,” holding together cellulose and hemicellulose in the fibers of plant cell walls [5]. Lignin possesses great potential to obtain value-added compounds, but its uses are still scarce; lignin is mainly burnt for energy generation purposes [6]. As mentioned, lignin dissolution is complicated but frequently Copyright: © 2021 by the authors. required as a first step for several applications. Despite the dissolution limitations, several Licensee MDPI, Basel, Switzerland. solvent systems have been successfully developed [7]. In some cases, different additives This article is an open access article have proven to enhance lignin dissolution [8]. In this respect, surfactants are typically good distributed under the terms and candidates to improve polymer solubilization, since their association with macromolecules conditions of the Creative Commons can turn them more hydrophilic, decrease hydrophobic interactions, introduce charges, Attribution (CC BY) license (https:// etc. Surfactant (and polymer) properties, such as hydrophobicity, chain flexibility, charge creativecommons.org/licenses/by/ density, and ionic strength, are expected to affect the binding and aggregation phenomena, 4.0/).

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Polymers 2021, 13, 714 2 of 13 interactions, introduce charges, etc. Surfactant (and polymer) properties, such as hydrophobicity, chain flexibility, charge density, and ionic strength, are expected to affect the binding and aggregation phenomena, and even control the dissolution process [9]. De- andspite even being control appealing, the dissolution surfactant process application [9]. Despite in lignin being dissolution appealing, has surfactant been surprisingly application inscarce. lignin Among dissolution the few has reports, been surprisingly Xu et al. ha scarce.ve demonstrated Among the fewthe enhancement reports, Xu et of al. lignin have demonstrateddissolution in aqueous-based the enhancement systems of lignin containing dissolution the in nonionic aqueous-based surfactant systems tween containing 80 [10]. In theanother nonionic work, surfactant Norgren tweenet al. have 80 [10 used]. In bile another acid work,salts (a Norgren special class et al. of have biological used bile surfac- acid saltstants) (a to special successfully classof increase biological the solubility surfactants) and to colloidal successfully stability increase of kraft the lignin solubility [11]. andThe colloidalsearch for stability efficient of lignin kraft lignin extraction [11]. Themethods search is for a topic efficient of great lignin scientific extraction and methods industrial is a topicinterest. of great Lignin scientific is an effective and industrial source interest.of biomass, Lignin with is anenzymatic effective hydrolysis source of biomass, being key with to enzymaticits use [12,13]. hydrolysis In lignocellulosic being key to biomass its use [12 pr,13ocessing,]. In lignocellulosic non-ionic biomasssurfactants processing, are typically non- ionicused surfactantsto enhance arelignin typically extraction used [14,15]. to enhance These lignin surfactants extraction play [14 the,15 ].role These of additives surfactants in playthe soda the rolepulping of additives of bagasse in the [16] soda and pulping increase of the bagasse yield [of16 kraft] and lignin increase removal the yield in the of kraft pre- lignincipitation removal step [17]. in the Non-charged precipitation surfactants, step [17]. Non-charged mainly the tween-type, surfactants, are mainly important the tween- in en- type,zymatic are hydrolysis, important inand enzymatic thus are widely hydrolysis, used andfor the thus conversion are widely of used lignocellulosic for the conversion biomass ofinto lignocellulosic bioethanol. Other biomass surfactant into bioethanol. systems have Other been surfactant reported systems to assist have the beenenzymatic reported hy- todrolysis assist in the different enzymatic lignocellulosic hydrolysis inbiomass different matrixes lignocellulosic [18–26]. During biomass these matrixes processes, [18– 26lig-]. Duringnin acts these as a physical processes, barrier lignin to acts the as enzymes a physical and barrier can even to the absorb enzymes them and via can strong even interac- absorb them via strong interactions (hydrophobic, electrostatic, and hydrogen-bonding) [27,28]. tions (hydrophobic, electrostatic, and hydrogen-bonding) [27,28]. Surfactants facilitate the Surfactants facilitate the access of the enzyme to cellulose by removing the lignin [18]. access of the enzyme to cellulose by removing the lignin [18]. The presence of surfactants The presence of surfactants not only may increase the reaction yield but also allows a not only may increase the reaction yield but also allows a substantial reduction in the substantial reduction in the amount of needed enzyme (up to ca. 50%), while maintaining a amount of needed enzyme (up to ca. 50%), while maintaining a superior yield [23]. The superior yield [23]. The surfactant efficiency in lignin removal can be influenced by several surfactant efficiency in lignin removal can be influenced by several factors, such as its factors, such as its critical micelle concentration (CMC) [22], the chemical and molar mass critical micelle concentration (CMC) [22], the chemical and molar mass properties of lignin properties of lignin [8], hydrolysis conditions [28], and substrate type [29]. [8], hydrolysis conditions [28], and substrate type [29]. To our knowledge, charged surfactants have never been used to dissolve lignin. To our knowledge, charged surfactants have never been used to dissolve lignin. Thorough studies comparing several classes of surfactants are lacking. Therefore, in the Thorough studies comparing several classes of surfactants are lacking. Therefore, in the first part of this study, different cationic and anionic surfactants (Figure1) were used in an first part of this study, different cationic and anionic surfactants (Figure 1) were used in aqueous solution to dissolve model kraft lignin. Non-ionic surfactants were also tested for comparison.an aqueous solution In this first to dissolve part, the model interaction kraft lig betweennin. Non-ionic lignin and surfactants a cationic were surfactant also tested was alsofor comparison. followed by In electrical this first conductance, part, the interaction which has between not been lignin reported and before.a cationic Inthe surfactant second partwas ofalso this followed study, the by mostelectrical promising conductance, surfactants which used has for not model been ligninreported dissolution before. In were the testedsecond as part potential of this additives study, the to most improve promising the yield surfactants of lignin extractionused for model from lignocellulosiclignin dissolu- biomasstion were (maritime tested as pinepotential sawdust). additives to improve the yield of lignin extraction from lignocellulosic biomass (maritime pine sawdust).

O Na+ n = 8 Sodium octyl sulfate (SOS) Anionic H OOS - n = 10 Sodium decyl sulfate (SDeS) n n = 12 Sodium docecyl sulfate (SDS) O Br n = 8 Octyltrimethylammonium bromide (OTAB) n = 10 Decyltrimethylammonium bromide (DeTAB) Cationic H N n = 14 Tetradecyltrimethylammonium bromide (TTAB) n = 16 Hexadecyltrimethylammonium bromide (CTAB) n

O OH n Non-ionic

n = 9–10 Triton X-100 (TX-100)

Figure 1. Cont.

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O O w O OH O x

HO OH O O z y w + x + y + z = 20 Polysorbate 20 (PS20)

Figure 1. Molecular structures of the surfactantssurfactants usedused inin thisthis work.work.

2. Materials and Methods 2.1. Materials Kraft lignin,lignin, sodiumsodium dodecyl dodecyl sulfate sulfate (SDS), (SDS), decyltrimethylammonium decyltrimethylammonium bromide bromide (DeTAB), (De- hexadecyltrimethylammoniumTAB), hexadecyltrimethylammonium bromide bromide (CTAB), (CTAB), and Triton and X-100 Triton (TX-100) X-100 were (TX-100) purchased were frompurchased Merck from (Alg Merckés, Portugal). (Algés, Portugal). Sodium octyl Sodium sulfate octyl (SOS), sulfate sodium (SOS), decylsodium sulfate decyl (SDeS), sulfate and(SDeS), polysorbate and polysorbate 20 (PS20) 20 were (PS20) acquired were fromacquired Fluka from (Bunchs, Fluka Switzerland). (Bunchs, Switzerland). Octyltrimethy- Oc- lammoniumtyltrimethylammonium bromide (OTAB) bromide was (OTAB) obtained was from obtained Tokyo from Kasei Tokyo Kogyo Kasei Co., Kogyo Ltd. Co., (Tokyo, Ltd. Japan)(Tokyo, and Japan) tetradecyltrimethylammonium and tetradecyltrimethylammo bromidenium (TTAB)bromide was (TTAB) purchased was purchased from LabKemi from (Stockholm,LabKemi (Stockholm, Sweden). Sodium Sweden). hydroxide Sodium (NaOH) hydroxide was purchased(NaOH) was from purchased José Manuel from Gomes José dosManuel Santos, Gomes Lda., dos (Porto, Santos, Portugal) Lda., (Porto, and formic Portugal) acid fromand Merckformic (Algacidé froms, Portugal). Merck All(Algés, the chemicalsPortugal). wereAll the used chemicals as received. were Maritime used as pine received. (Pinus Maritime pinaster Ait.) pine sawdust (Pinus waspinaster received Ait.) fromsawdust Valco was - Madeirasreceived from e Derivados, Valco - Madeiras S.A. (Leiria, e Derivados, Portugal) S.A. as (Leiria, a kind giftPortugal) and was as a dried kind beforegift and further was dried use. before further use. 2.2. Conductivity Measurements 2.2. Conductivity Measurements The surfactant–lignin interactions (DeTAB surfactant was selected) were probed by electricalThe surfactant–lignin conductometry of interactions different solutions (DeTAB using surfactant a Wayne–Kerr was selected) model were 4265 Automaticprobed by LCRelectrical meter conductometry (Wayne Kerr Electronicsof different Ltd, solu Bognortions using Regis, a UnitedWayne–Kerr Kingdom), model at 4265 1 kHz Auto- and 25.00matic ◦LCRC. A meter dip-type (Wayne conductivity Kerr Electronics cell, with Ltd, a cell Bognor constant Regis, of United 0.1002 cmKingdom),−1 and an at uncer-1 kHz −1 taintyand 25.00 of 0.02%, °C. A dip-type was used conductivity [30]. The temperature cell, with a wascell keptconstant constant of 0.1002 by using cm and a HAAKE an un- Phoenixcertainty IIof P2 0.02%, thermostat was used bath [30]. (Thermo The temperature Fisher Scientific, was kept Waltham, constant MA, by US).using In a aHAAKE typical experiment,Phoenix II P2 30 thermostat mL of water bath or lignin(Thermo solution Fisher was Scientific, placed inWaltham, the conductivity MA, US). cell In and, a typical after theexperiment, thermostabilizing, 30 mL of aliquotswater or of lignin the DeTAB solution solution was placed (1.5 mol in Lthe−1 )conductivity were added stepwisecell and, after the thermostabilizing, aliquots of the DeTAB solution (1.5 mol L−1) were added step- with a micropipette (50 µL). The experimental electrical specific conductance, κexp, was measuredwise with aftera micropipette each addition (50 μ ofL). surfactant The experimental and computed electrical using specific an in-house-made conductance, soft-κexp, ware.was measured The specific after electrical each addition conductance, of surfactantκ, was and obtained computed after using subtracting an in-house-made the specific conductancesoftware. The of specific the solvent. electrical conductance, κ, was obtained after subtracting the specific conductance of the solvent. 2.3. Dissolution Efficiency 2.3. DissolutionThe dissolution Efficiency tests were performed with 20 wt.% model kraft lignin using different concentrationsThe dissolution of surfactant. tests were The performed solutions with were 20 wt.% stirred model at 1000 kraft rpm lignin using using a magnetic different stirrerconcentrations for 24 h atof 20–25surfactant.◦C. The The dispersions solutions were were stirred centrifuged at 1000 at rpm 14,000 using rpm a for magnetic 1 h and stir- the supernatantrer for 24 h at was 20–25 removed °C. The and dispersions diluted with were 1 wt.% centrifuged NaOH aqueous at 14,000 solution. rpm for The 1 h dissolved and the ligninsupernatant content was was removed obtained byand measuring diluted with the absorbance 1 wt.% NaOH of the aqueous diluted solution,solution. at The 288 nm,dis- usingsolved a lignin UV–VIS content spectrophotometer, was obtained by Shimadzu measuring UV-2450 the absorbance (Shimadzu of Corporation,the diluted solution, Tokyo, Japan).at 288 nm, The using results a UV–VIS presented spectrophotometer, are the average of threeShimadzu samples. UV-2450 The dissolution(Shimadzu efficiencyCorpora- wastion, further Tokyo, evaluated Japan). The by opticalresults microscopy,presented are Linkam the average LTS 120 of microscope three samples. (Linkam The Scientific dissolu- Instrumentstion efficiency Ltd., was Tardworth, further evaluated United Kingdom), by optical andmicroscopy, a QImaging Linkam station,-QICAM LTS 120 microscope Fast 1394 Digital(Linkam Camera Scientific (Teledyne Instruments Photometrics, Ltd., Tardworth, Tucson, AZ,United US) Kingdom), was used for and image a QImaging acquisition. station,-QICAM Fast 1394 Digital Camera (Teledyne Photometrics, Tucson, AZ, US) was used for image acquisition.

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2.4.2.4. Lignin Lignin Extraction Extraction and and Purity Purity Determination Determination LigninLignin was was extracted extracted from from pine sawdust (1 g) using 10 mL formic acid and different concentrationsconcentrations of of surfactant. surfactant. The The mixtures mixtures were were kept kept at at 160 160 °C◦C for for 2 2 h h in in a a paraffin paraffin bath. bath. Afterward, thethe liquidliquid fractionfraction was was separated separated by by vacuum vacuum filtration, filtration, and and 500 500 mL mL of deionizedof deion- izedwater water was addedwas added to trigger to trigger lignin lignin precipitation. precipitation. After After centrifugation, centrifugation, the collected the collected solid solidwas driedwas dried with with a freeze a freeze dryer. dryer. An experimentalAn experimental scheme scheme of the of the lignin lignin extraction extraction can can be beobserved observed in in Figure Figure2. The2. The purity purity of of the the recovered recovered lignin lignin was was determined determined by by the the sumsum ofof thethe acid-insoluble acid-insoluble and and acid-soluble acid-soluble lignin, lignin, using using the the procedures procedures LAP-003 LAP-003 and and LAP-004, LAP-004, respectively,respectively, by the National Renewable Energy Laboratory [31,32]. [31,32]. Briefly, Briefly, ca. 0.1–0.3 g ◦ ofof the extracted material was hydrolyzed with 3 mL of 72% H2SO44 forfor 1 1 h h at at 30 30 °C.C. Then, Then, thethe mixture mixture was was diluted diluted with with deionized deionized water water to to4% 4% H2 HSO2SO4 and4 and placed placed in the in theoven oven at 121 at °C121 for◦C 1 for h. 1 The h. The solution solution was was cooled cooled and and vacuum-filtered vacuum-filtered with with previously previously dried dried and weighted filters.filters. TheThe totaltotal solidsolid content content was was used used to to determine determine the the insoluble insoluble lignin lignin fraction. fraction.The filtrateThe filtrate obtained obtained was was used used to estimate to estima thete the acid-soluble acid-soluble lignin lignin content content by by measuring measur- ingthe the absorbance absorbance at 205 at nm205 innm a UV–VISin a UV–VIS spectrophotometer, spectrophotometer, Shimadzu Shimadzu UV-2450 UV-2450 (Shimadzu (Shi- madzuCorporation, Corporation, Tokyo, Japan). Tokyo, All Japan). analyses All (extraction analyses (extraction and purity determination)and purity determination) were carried wereout in carried duplicates. out in duplicates.

FigureFigure 2. 2. ExperimentalExperimental scheme scheme for for lignin lignin extraction. extraction.

3.3. Results Results and and Discussion Discussion 3.1.3.1. Lignin Lignin Dissolution Dissolution InIn the the first first part part of of this study, different surfactantssurfactants were tested regarding their effect onon the the dissolution dissolution of lignin (kraft lignin selected as a “model” lignin). As can be observed inin Figure Figure 33,, cationiccationic surfactantssurfactants stronglystrongly interactinteract withwith lignin,lignin, thusthus impactingimpacting thethe dissolu-dissolu- tiontion efficiency. efficiency. Two strikingstriking observationsobservations cancan bebe made:made: (1)1) the dissolution performance remarkablyremarkably increases increases above the the critical critical aggreg aggregationation concentration concentration (CAC) (CAC) of of the the surfactant, surfactant, andand 2) (2) longer longer aliphatic aliphatic chains induceinduce aa superiorsuperior interaction interaction between between lignin lignin and and the the surfac- sur- factants,tants, resulting resulting in ain lower a lower CAC CAC value value [33 ,[33,34].34]. Tetradecyltrimethylammonium Tetradecyltrimethylammonium bromide bromide was wasobserved observed to have to have a stronger a stronger affinity affinity with with lignin lignin and itsand effect its effect on the on dissolution the dissolution efficiency effi- −1 ciencystarts atstarts lower at concentrations.lower concentrations. Note that Note the that CMC the of CMC TTAB of (0.00386TTAB (0.00386 mol L mol[35 L])−1 is [35]) lower is −1 lowerthan DeTAB, than DeTAB, while while OTAB OTAB shows shows the highest the highest CMC CMC (0.22 mol(0.22 L mol[ 36L−])1 [36]) because because of its of less its lesspronounced pronounced hydrophobicity. hydrophobicity. This This effect effect demonstrates demonstrates the important the important role of role hydrophobic of hydro- phobicinteractions interactions in the surfactant–lignin in the surfactant–lignin complex complex formation. formation. The pronounced The pronounced beneficial benefi- effect cialof cationic effect of surfactants cationic surfactants in lignin dissolution in lignin dissolution is mainly driven is mainly by the driven favorable by the electrostatic favorable electrostaticinteractions interactions between the between positively the charged positively surfactant charged headgroups surfactant headgroups and the ionized and car-the

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ionized carbonyl groups of lignin [17]. The high electrostatic attraction has also been foundbonyl to be groups beneficial of lignin in other [17 ].applications, The high electrostatic such as in dye attraction adsorption has also[37]. been found to be beneﬁcial in other applications, such as in dye adsorption [37].

100

20 Dissolution efficiency (%) efficiency Dissolution 0 0.0 0.2 0.4 0.6 0.8 1.0 Concentration of surfactant (mol L1)

FigureFigure 3. Dissolution 3. Dissolution efficiency efﬁciency of kraft of lignin kraft in lignin water inas watera function as aof function cationic surfactants: of cationic surfactants:oc- tyltrimethylammoniumoctyltrimethylammonium bromide bromide (OTAB) (OTAB) (squares), (squares), decyltrimethylammonium decyltrimethylammonium bromide bromide (DeTAB) (DeTAB) (circles),(circles), and andtetradecyltrimethylammo tetradecyltrimethylammoniumnium bromide bromide (TTAB) (TTAB) (triangles). (triangles).

In contrastIn contrast to cationic to cationic surfactants, surfactants, anioni anionicc surfactants surfactants have havea modest a modest effect effecton disso- on dis- lution.solution. Regardless Regardless of the anionic of the anionic surfactant surfactant used (i.e., used SOS, (i.e., SDeS, SOS, and SDeS, SDS), and the SDS), lignin the dis- lignin solutiondissolution is weakly is weakly affected affected and rather and similar; rather similar;at a surfactant at a surfactant concentration concentration range between range be- 0.1 tweenand 0.5 0.1 mol and L 0.5−1 (well mol Labove−1 (well the above CMCs), the the CMCs), dissolution the dissolution efficiency efﬁciency is observed is observed to in- to creaseincrease to 24%, to 24%,while while above above this concentration this concentration (i.e., from (i.e., from0.5 to 0.5 1.0 to mol 1.0 L mol−1) their L−1) effect their effectis veryis subtle—only very subtle—only a 4% improvement a 4% improvement is observed is observed (Figure (Figure 4). It is4 ).worth It is worthmention mention that the that obtainedthe obtained solutions solutions have an have average an averagepH slightly pH slightlyhigher (5.30–5.50) higher (5.30–5.50) than the than pKa theof car- pKa of bonylcarbonyl groups groups (pKa 4–5 (pKa [38]). 4–5 [Consequently,38]). Consequently, electrostatic electrostatic repulsio repulsionn between between the theionized ionized carbonylcarbonyl groups groups of lignin of lignin and and the theionized ionized sulfate sulfate groups groups of the of theanionic anionic surfactants surfactants may may limitlimit their their interaction interaction and and eventual eventual favorable favorable effects effects on lignin on lignin dissolution. dissolution. On Onthe theother other hand,hand, such such deprotonation deprotonation of lignin of lignin carbonyl carbonyl groups groups may give may rise give to risethe stretching to the stretching of the of ligninthe structure, lignin structure, inducing inducing the increase the in increase its solubility in its solubility[39]. Another [39]. possibility Another possibilityfor ration- for alizingrationalizing the moderate the increase moderate in increase the lignin in dissolution the lignin dissolutionis related to isthe related increase to thein counter- increase in ion counter-ioncondensation, condensation, both on the bothsurfactant on the and surfactant lignin, as and suggested lignin, as elsewhere suggested [40]. elsewhere [40]. To better understand the role of ionic interactions on the lignin dissolution mech-

anism, nonionic surfactants were also tested. Two different non-ionic surfactants with different molecular weights and CMCs, PS20 (CMC = 0.0489 mmol L−1 [41]) and TX-100 (CMC = 0.240 mmol L−1 [41]), were used at concentrations well above their CMCs. As it can be observed in Figure5, both non-ionic surfactants show a notable beneficial effect on the lignin dissolution. Nevertheless, whilst the TX-100 reaches a dissolution efficiency of ca. 60% and becomes concentration-independent, the PS20 shows a sharp increase, between 0.1 and 0.5 mol L−1, reaching 100% dissolution efficiency for the higher concentrations. The reason for the observed behavior can be related to the relative number of ethylene and hydroxyl groups in TX-100 and PS20, and the occurrence of hydrogen bonds between the ethylene oxide and hydroxyl units from the surfactants and the hydroxyl groups of the lignin. Those interactions give rise to the formation of lignin–surfactant complexes, similar to those described for guar gum [42]. Lignin contains both hydrogen bond donors and acceptor groups, and it is thus capable of forming intra- and intermolecular hydrogen bonds. The presence of the nonionic surfactants may compete and/or break some of the intra- and intermolecular hydrogen bonds among lignin molecules, driving its dissolution [43].

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100

20 Dissolution efficiency (%) efficiency Dissolution 0 0 0.1 0.3 0.5 0.6 0.7 0.8 0.9 1 Concentration of surfactant (mol L1)

Polymers 2021, 13, x FOR PEER REVIEW 7 of 14 FigureFigure 4. Dissolution 4. Dissolution efficiency efﬁciency of kraft of kraftlignin lignin in water in wateras a function as a function of the anionic of the anionicsurfactants surfactants so- sodium diumoctyl octyl sulfate sulfate (SOS) (SOS) (black), (black), sodiumsodium decyl decyl sulfate sulfate (SDeS) (SDeS) (gray), (gray), and andsodium sodium dodecyl dodecyl sulfate sulfate (SDS) (SDS) (light gray). (light gray).

To100 better understand the role of ionic interactions on the lignin dissolution mechanism, nonionic surfactants were also tested. Two different non-ionic surfactants with different molecular weights and CMCs, PS20 (CMC = 0.0489 mmol L−1 [41]) and TX-100 (CMC = 0.240 80mmol L−1 [41]), were used at concentrations well above their CMCs. As it can be observed in Figure 5, both non-ionic surfactants show a notable beneficial effect on the lignin dissolution. Nevertheless, whilst the TX-100 reaches a dissolution efficiency of ca. 60% and60 becomes concentration-independent, the PS20 shows a sharp increase, between 0.1 and 0.5 mol L−1, reaching 100 % dissolution efficiency for the higher concentrations. The reason for the observed behavior can be related to the relative number of ethylene 40 and hydroxyl groups in TX-100 and PS20, and the occurrence of hydrogen bonds between the ethylene oxide and hydroxyl units from the surfactants and the hydroxyl groups of the lignin.20 Those interactions give rise to the formation of lignin–surfactant complexes, similar to those described for guar gum [42]. Lignin contains both hydrogen bond donors and (%) efficiency Dissolution acceptor groups, and it is thus capable of forming intra- and intermolecular hydrogen bonds. The0 presence of the nonionic surfactants may compete and/or break some of the intra- and intermolecular0.0 0.2 hydrogen 0.4 bonds amon 0.6g lignin 0.8 molecules, 1.0 driving its dissolution [43]. Concentration of surfactant (mol L1)

FigureFigure 5. Dissolution 5. Dissolution efficiency efﬁciency of kraft oflignin kraft as lignin a function as a functionof polysorbate of polysorbate 20 (PS20) (squares), 20 (PS20) and (squares), and TritonTriton X-100 X-100 (TX-100) (TX-100) (circles). (circles).

SeveralSeveral techniques techniques have havebeen beenused usedto study to study the interaction the interaction between between different different poly- polymers mersand and surfactants, surfactants, suchsuch asas small-anglesmall-angle ne neutronutron scattering scattering [44], [44 ],fluorescence fluorescence [45], [45 spe-], specific con- cificductivity conductivity [46], [46], dynamic dynamic light light scattering scattering [47 [47],], surface surface tension tension [48 [48],], NMR NMRspectroscopy spectros- [49,50], copyand [49,50], rheometry and rheometry [42,51]. To[42,51]. elucidate To elucidate the dissolution the dissolution mechanism, mechanism, the surfactant–lignin the surfac- inter- tant–lignin interactions were further evaluated by conductimetry. It is important to em- actions were further evaluated by conductimetry. It is important to emphasize that electrical phasize that electrical conductivity is an important tool to unravel polymer–surfactant in- conductivity is an important tool to unravel polymer–surfactant interactions [52]. Among teractions [52]. Among the tested surfactants, the cationic DeTAB was selected due to its the tested surfactants, the cationic DeTAB was selected due to its striking beneficial effect striking beneficial effect (see Figure 3) and availability. In Figure 6, the electrical conductance(see of DeTAB Figure 3in) andwater, availability. in the presence In Figure and absence6, the electrical of lignin, conductance is represented. of The DeTAB varia- in water, in tionthe in κ presence with the andconcentration absence of of lignin, DeTAB is shows represented. an expected The variation profile with in κ twowith linear the concentrationre- gions,of DeTABat pre- and shows post-CMC an expected regions, profile with an with estimated two linear CMC regions, of 0.065 at (± pre- 0.005) and mol post-CMC L−1, in regions, −1 agreementwith an with estimated the literature CMC of[53,54]. 0.065 In (± the0.005) presence mol Lof 10, inwt.% agreement kraft lignin, with the the conduc- literature [53,54]. tometricIn the behavior presence of DeTAB of 10 wt.% changes kraft signific lignin,antly, the and conductometric two remarks are behavior worth noting: of DeTAB 1) changes the electricalsignificantly, conductivity and two is remarkslower than are in worththe absence noting: of lignin, (1) the which electrical may be conductivity related to is lower an increasethan in in the the absence viscosity of or lignin, due to which the su mayrfactant–polymer be related to attractive an increase interactions in the viscosity induc- or due to ing somethe surfactant–polymer charge neutralization attractive (this is cons interactionsistent with inducingthe decrease some in the charge slope neutralization of κ=f([De- (this is TAB])consistent [55], and with 2) the the dependence decrease in of the κ slopewith[DeTAB] of κ=f ([DeTAB]) shows three [55 ],inflection and (2) thepoints dependence ra- of κ tionalizedwith [DeTAB] as follows. shows The first three one, inflection occurring points at a DeTAB rationalized concentration as follows. around The 0.015 first one,mol occurring L−1, can be assigned to the CAC [56] relative to polymer-free micelle formation, which marks the onset of the association between the polymer and the surfactant; the second inflection point corresponds to the polymer saturation point (PSP), attributed to the surfactant concentration needed to saturate the polymer chains [34,57–59]. Between CAC and PSP, a dynamic equilibrium between surfactant-saturated polymer and micelles exists [52]. Above the PSP, the surfactant in excess forms regular free micelles, in dynamic equilibrium with the polymer–surfactant complexes [57]. This is in line with a third inflection point, which is justified by the micelle formation and corresponds to the so-called “apparent critical micelle concentration” (CMC*) of the DeTAB. The effect of lignin concentration on the micellization properties of DeTAB is shown in Table 1. The CAC values decrease with lignin concentration since the association between lignin and the surfactant becomes more favorable [59]. On the other hand, the PSP values increase with lignin concentration because of the higher polymer chains/surfactant micelles ratio. This phenomenon has been observed in other systems [34,58,59]. It can also be seen that the formation of surfactant micelles, at concentrations above the CMC, cannot be justified on thermodynamic

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at a DeTAB concentration around 0.015 mol L−1, can be assigned to the CAC [56] relative to polymer-free micelle formation, which marks the onset of the association between the polymer and the surfactant; the second inflection point corresponds to the polymer saturation point (PSP), attributed to the surfactant concentration needed to saturate the polymer chains [34,57–59]. Between CAC and PSP, a dynamic equilibrium between surfactant- saturated polymer and micelles exists [52]. Above the PSP, the surfactant in excess forms regular free micelles, in dynamic equilibrium with the polymer–surfactant complexes [57]. This is in line with a third inflection point, which is justified by the micelle formation and corresponds to the so-called “apparent critical micelle concentration” (CMC*) of the DeTAB. The effect of lignin concentration on the micellization properties of DeTAB is shown in Table1. The CAC values decrease with lignin concentration since the association between lignin and the surfactant becomes more favorable [59]. On the other hand, the PSP Polymers 2021, 13, x FOR PEER REVIEW 8 of 14 values increase with lignin concentration because of the higher polymer chains/surfactant micelles ratio. This phenomenon has been observed in other systems [34,58,59]. It can also be seen that the formation of surfactant micelles, at concentrations above the CMC, cannot grounds.be justified However, on thermodynamic once the polymer grounds. favors However,the interaction once with the the polymer surfactant, favors it should the interaction bewith expected the surfactant, that surfactants it should consumed be expected in the thatinteraction surfactants are subtracted consumed from in thethe interactionCMC*; are consequently,subtracted from the effective the CMC*; CMC consequently, of DeTAB in th thee presence effective of lignin CMC should of DeTAB be calculated in the presence asof CMC’ lignin = CMC*- should PSP be calculated(see Table 1). as ACMC’ deeper = analysis CMC*- PSPof the(see effect Table of the1). DeTAB A deeper micelli- analysis of zationthe effect can be of done the DeTAB based on micellization the standard canGibbs be energy done basedof micellization, on the standard ΔG0m Gibbs energy of G0 micellization, ∆ m Δ𝐺 =(2−𝛼)𝑅𝑇𝑙𝑛𝑋 (1) ∆G0 = (2 − α)RTlnX (1) where R is the gas constant and X is the mmole fraction of the surfactant at the CMC. The variablewhere Rα representsis the gas the constant degree and of dissociationX is the mole of counterions fraction of which, the surfactant in the present at the case, CMC. The isvariable computedα representsas the slope the of the degree electrical of dissociation conductance of data counterions as a function which, of surfactant in the present con- case, is centration,computed at as the the post-micelle slope of the region, electrical and conductancepre-CAC region, data following as a function the recommenda- of surfactant concen- tionstration, of Zanette at the et post-micelle al. [60]. Based region, on the and values pre-CAC reported region, in Table following 1, and although the recommendations the data areof somewhat Zanette et scattered, al. [60]. Basedit is possible on the to values conclude reported that lignin, in Table at the1 ,lowest and although concentration, the data are 0 leadssomewhat to the formation scattered, of it ismore possible stable to DeTAB conclude micelles, that lignin,with the at  theG m lowest ca. 14% concentration, lower than leads that obtained for the lignin-free micellization of DeTAB; this is also driven0 by a very low to the formation of more stable DeTAB micelles, with the ∆G m ca. 14% lower than that α, suggesting the occurrence of highly dense micelles in the presence of lignin. This result obtained for the lignin-free micellization of DeTAB; this is also driven by a very low α, agrees with the highest solubility found for this lignin concentration. suggesting the occurrence of highly dense micelles in the presence of lignin. This result agrees with the highest solubility found for this lignin concentration.

0.004

CMC

0.003

0.002 CMC* PSP 0.001 CAC

Specific conductivity (S/cm) 0.000 0.00 0.05 0.10 0.15 Concentration of DeTAB (mol L1)

FigureFigure 6. 6.SpecificSpecific conductivity conductivity of DeTAB of DeTAB at different at different concentrations concentrations in water in (empty water squares) (empty and squares) and inin an an aqueousaqueous solution solution of 10 of wt.% 10 wt.% kraft kraftlignin lignin (full squares). (full squares). The different The different transition transition regions high- regions high- lighted (i.e., critical aggregation concentration (CAC), polymer saturation point (PSP), critical mi- lighted (i.e., critical aggregation concentration (CAC), polymer saturation point (PSP), critical micelle celle concentration (CMC), and apparent critical micelle concentration (CMC*)) are described in theconcentration text. (CMC), and apparent critical micelle concentration (CMC*)) are described in the text.

Table 1. The CMC, CAC, and PSP of DeTAB for different initial concentrations of lignin obtained from the conductivity data at 25 °C.

Kraft Lignin (wt.%) CAC (mol L−1) PSP (mol L−1) CMC’ (mmol L−1) G0m (kJ mol−1) 0.0 0.065 ± 0.005* - 64.6 −29.8 0.1 0.020 ± 0.002 0.036 ± 0.004 48.0 −33.9 1.0 0.020 ± 0.002 0.040 ± 0.006 70.0 −31.2 2.5 0.018 ± 0.002 0.039 ± 0.006 52.0 −32.5 5.0 0.018 ± 0.002 0.041 ± 0.007 72.0 −30.9 10.0 0.015 ± 0.002 0.049 ± 0.002 53.0 −28.9 * This value corresponds to the CMC of DeTAB in the absence of lignin.

3.2. Lignin Extraction

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Table 1. The CMC, CAC, and PSP of DeTAB for different initial concentrations of lignin obtained from the conductivity data at 25 ◦C.

−1 −1 −1 0 −1 Kraft Lignin (wt.%) CAC (mol L ) PSP (mol L ) CMC’ (mmol L ) ∆G m (kJ mol ) 0.0 0.065 ± 0.005 * - 64.6 −29.8 0.1 0.020 ± 0.002 0.036 ± 0.004 48.0 −33.9 1.0 0.020 ± 0.002 0.040 ± 0.006 70.0 −31.2 2.5 0.018 ± 0.002 0.039 ± 0.006 52.0 −32.5 5.0 0.018 ± 0.002 0.041 ± 0.007 72.0 −30.9 10.0 0.015 ± 0.002 0.049 ± 0.002 53.0 −28.9 * This value corresponds to the CMC of DeTAB in the absence of lignin.

3.2. Lignin Extraction In the first part of this study, several surfactants were tested regarding their effect on lignin dissolution. Among them, the non-ionic PS20, cationic CTAB, and the anionic SDS were selected and used as additives to improve the lignin extraction from pine sawdust. The solvent used was formic acid (98–100%) [61], to guarantee that the phenolic hydroxyl and carbonyl groups are protonated. Note that CTAB was selected for the extraction trials due to the higher availability and solubility of this surfactant in the acidic solvent. As can be observed in Figure7, the SDS and PS20 have a positive effect on the extraction yield at lower concentrations (i.e., at concentrations lower than 0.1 mol L−1). Higher amounts of these surfactants have a negative effect on the extraction yield, due to the presence of micelles. On the other hand, the addition of CTAB in a concentration range from 0.01 to 1 mol L−1 leads to an enhanced lignin extraction; the extraction yield has a threefold increase when the concentration increases from 0 to 0.6 mmol L−1. Nevertheless, the increase in surfactant concentration favors the extraction of impurities, such as sugars [62,63], proteins, and minerals [64]. This is verified by the gradual decrease in lignin purity, from 87 (± 1) to 61 (± 2) %, using 0.01 and 1 mol L−1, respectively. The same trend was observed using SDS and PS20, with the lignin purity decreasing from 80.1 (± 0.9) to 72 (± 1) % for SDS and from 79 (± 1) and 73.4 (± 0.6) % for PS20, using 0.01 and 0.1 mol L−1 of each type of surfactant, respectively. In previous studies, using a related acid solvent (H2SO4 (aq.)), the addition of sodium dodecylbenzene sulfonate (SDBS), tween 80, and dodecylbenzene sulfonic acid (DDBSA) were also observed to increase the lignin extraction from biomass [14,15]. Hamzeh et al. used non-ionic surfactants in soda pulping and verified that, in most cases, increasing amounts of surfactants decreases pulping yields and delignification rate [16]. It is suggested that the prevalent forces are hydrophobic and π –π interactions driving the lignin molecular backbone to contract and clamp adsorbed the surfactant [39]. The addition of surfactants improves penetration efficiency through wetting and emulsifying-like effects [65,66]. The lignins recovered using 0.01 mol L−1 of surfactant were analyzed by FTIR and TG, and the results compared with lignin recovered without surfactant. Charac- teristic bands of lignin were found in all extracted lignins (Figure8): O-H vibrations (3200–3500 cm−1)[67]; CH stretching in aromatic methoxyl or methylene groups of side −1 −1 chain (2925–2940 cm )[68]; CH vibration of -OCH3 groups (2840–2870 cm )[68]; C=O stretching in unconjugated ketone, carbonyl, and in ester groups (1716 cm−1)[69]; aromatic skeletal vibrations plus the C=O stretch (1600 cm−1)[69]; aromatic skeleton vibration (1506 cm−1)[69]; C-H bending from methyl or methylene groups (1455 cm−1)[68]; aromatic skeletal vibration combined with CH asymmetric deformation of methyl groups (1423 cm−1)[69]; guaiacyl ring breathing with CO stretch in lignin; CO linkage in guaiacyl aromatic methoxyl groups (1266 cm−1)[68]; aromatic C-H in plane deformation plus secondary alcohols plus C=O stretch (1142–1152 cm−1); C-O deformation in secondary alcohol and aliphatic ethers (1082–1088 cm−1)[69]; aromatic C-H in-plane deformation plus C-O deformation in primary alcohols; and C=O stretch (unconjugated) (1029 cm−1)[69]. Polymers 2021, 13, x FOR PEER REVIEW 9 of 14

In the first part of this study, several surfactants were tested regarding their effect on lignin dissolution. Among them, the non-ionic PS20, cationic CTAB, and the anionic SDS were selected and used as additives to improve the lignin extraction from pine sawdust. The solvent used was formic acid (98–100%) [61], to guarantee that the phenolic hydroxyl and carbonyl groups are protonated. Note that CTAB was selected for the extraction trials due to the higher availability and solubility of this surfactant in the acidic solvent. As can be observed in Figure 7, the SDS and PS20 have a positive effect on the extraction yield at lower concentrations (i.e., at concentrations lower than 0.1 mol L−1). Higher amounts of these surfactants have a negative effect on the extraction yield, due to the presence of micelles. On the other hand, the addition of CTAB in a concentration range from 0.01 to 1 mol L−1 leads to an enhanced lignin extraction; the extraction yield has a threefold increase when the concentration increases from 0 to 0.6 mmol L−1. Nevertheless, the increase in surfactant concentration favors the extraction of impurities, such as sugars [62,63], proteins, and minerals [64]. This is verified by the gradual decrease in lignin purity, from 87 (± 1) to 61 (± 2) %, using 0.01 and 1 mol L−1, respectively. The same trend was observed using SDS and PS20, with the lignin purity decreasing from 80.1 (± 0.9) to 72 (± 1) % for SDS and from 79 (± 1) and 73.4 (± 0.6) % for PS20, using 0.01 and 0.1 mol L−1 of each type of surfactant, respectively. In previous studies, using a related acid solvent (H2SO4 (aq.)), the addition of sodium dodecylbenzene sulfonate (SDBS), tween 80, and dodecylbenzene sulfonic acid (DDBSA) were also observed to increase the lignin extraction from biomass [14,15]. Hamzeh et al. used non-ionic surfactants in soda pulping and verified that, in most cases, increasing amounts of surfactants decreases pulping yields and delignification rate [16]. It is suggested that the prevalent forces are hydrophobic and π –π interactions driving the lignin molecular backbone to contract and clamp adsorbed the surfactant [39]. The addition of surfactants improves penetration efficiency through wetting and emulsifying- Polymers 2021, 13, 714 like effects [65,66]. 9 of 13

Polymers 2021, 13, x FOR PEER REVIEW 10 of 14 25

unconjugated20 ketone, carbonyl, and in ester groups (1716 cm−1) [69]; aromatic skeletal vibrations plus the C=O stretch (1600 cm−1) [69]; aromatic skeleton vibration (1506 cm−1) [69]; −1 C-H bending15 from methyl or methylene groups (1455 cm ) [68]; aromatic skeletal vibration combined with CH asymmetric deformation of methyl groups (1423 cm−1) [69]; guaiacyl ring breathing with CO stretch in lignin; CO linkage in guaiacyl aromatic methoxyl groups10 (1266 cm−1) [68]; aromatic C-H in plane deformation plus secondary alcohols plus C=O stretch (1142–1152 cm−1); C-O deformation in secondary alcohol and aliphatic ethers −1 (1082–10885 cm ) [69]; aromatic C-H in-plane deformation plus C-O deformation in pri- −1 mary (%) yield Extraction alcohols; and C=O stretch (unconjugated) (1029 cm ) [69]. Depending on the solvent and operational conditions used in the extraction, the lignin structure0 may differ. In this case, the spectrum of extracted lignin using SDS as an additive is very0 similar 0.01 to the one 0.1 of lignin 0.3 extracted 0.6 with only 1 formic acid (surfactant- free). Using CTABConcentration and TX-100, it is of possible surfactant to observe (mol someL1) changes in the relative absorbance in the characteristic bands of lignin. FigureFigure 7. Yield 7. Yield of the of recovered the recovered lignin using lignin different using different concentrations concentrations of SDS (black), of SDS hexadecyltri- (black), hexade- methylammoniumcyltrimethylammonium bromide bromide (CTAB) (gray), (CTAB) and (gray), PS20 and(light PS20 gray). (light gray).

The lignins recovered using 0.01 mol L−1 of surfactant were analyzed by FTIR and TG, and the results compared with lignin recovered without surfactant. Characteristic bands of lignin were found in all extracted lignins (Figure 8): O-H vibrations (3200–3500 cm−1) [67]; CH stretching in aromatic methoxyl or methylene groups of side chain (2925– 2940 cmd−1) [68]; CH vibration of -OCH3 groups (2840-2870 cm−1) [68]; C=O stretching in

b Absorbance (a.u) Absorbance a

3600 3200 2800 1600 1200 800 Wavenumber (cm1)

FigureFigure 8. 8. NormalizedNormalized FTIR FTIR spectra spectra of lignin of lignin obtained obtained after extraction after extraction with (a) withformic (a) acid formic and 0.01 acid and − mol0.01 L mol−1 of L(b)1 SDS,of (b) (c) SDS, CTAB, (c) and CTAB, (d) andPS20. (d) PS20.

TheDepending thermal ondecomposition the solvent andprofiles operational of the recovered conditions lignins used are in the shown extraction, in Figure the 9. lignin Usingstructure SDS, may the differ.extracted In thislignin case, has the a peak spectrum at 137 of°C, extracted due to the lignin volatilization using SDS of aslow-mo- an additive lecular-weightis very similar compounds to the one ofin ligninaddition extracted to removal with of only water formic [67]. acidThis (surfactant-free). value shifts to 163 Using °CCTAB for the and lignin TX-100, obtained it is possiblewith the other to observe surfactants some and changes the intensity in the relative is much absorbancelower. Using in the CTAB,characteristic the lignin bands obtained of lignin. contains hemicellulose impurities, due to the peak at 237 °C [70]. AllThe lignins thermal show decomposition a temperature proﬁlesat the ma ofximum the recovered rate of weight lignins loss are between shown 373 in andFigure 9 . 382Using °C, due SDS, to thethe fragmentation extracted lignin of inter-unit has a peak linkages at 137 [71].◦C, These due tovalues the volatilizationare in accordance of low- withmolecular-weight those found for compounds other lignins in (from addition 300 totoremoval 450 °C) of[70]. water The [larger67]. This proportion value shifts of to weight163 ◦C loss for thebetween lignin 200 obtained and 600 with°C, observed the other in surfactants lignin recovered and the without intensity surfactant is much and lower. withUsing SDS, CTAB, can be the attributed lignin obtained to the presence contains of carbohydrates hemicellulose [72]. impurities, due to the peak at 237 ◦C[70]. All lignins show a temperature at the maximum rate of weight loss between 373 and 382 ◦C, due to the fragmentation of inter-unit linkages [71]. These values are in accordance with those found for other lignins (from 300 to 450 ◦C) [70]. The larger proportion of weight loss between 200 and 600 ◦C, observed in lignin recovered without surfactant and with SDS, can be attributed to the presence of carbohydrates [72].

Polymers 2021, 13, x FOR PEER REVIEW 11 of 14

Polymers 2021, 13, 714 10 of 13 ) 1 100  0.0

80 -0.2

-0.4 60 -0.6 Weight (%) Weight 40 -0.8 20 Derivative Weight (% min -1.0 200 400 600 200 400 600 º º Temperature ( C) Temperature ( C)

(a) (b)

FigureFigure 9. (a) 9. Thermograms (a) Thermograms and and (b )(b) the the corresponding corresponding differential thermograms thermograms (DTGs) (DTGs) of recovered of recovered lignin lignin from extraction from extraction with formic acid (gray line), and 0.01 mol L−1 of SDS (red line), CTAB (blue line), and PS20 (black line). with formic acid (gray line), and 0.01 mol L−1 of SDS (red line), CTAB (blue line), and PS20 (black line). 4. Conclusions 4. Conclusions In this study, a systematic analysis on the effect of different surfactants on the disso- lutionIn this of model study, kraft a systematic lignin and analysis their influence on the effecton lignin of different extraction surfactants from lignocellulose on the dissolu- tionwaste of model (maritime kraft pine lignin sawdust) and their was influenceconducted. on Overall, lignin surfactants extraction proved from lignocellulose to be remark- waste (maritimeable additives pine sawdust)to enhance waskraft conducted. lignin dissolution Overall, and surfactants boost lignin provedextraction to from be remarkable bio- additivesmass. Using to enhance cationic amphiphiles, kraft lignin the dissolution dissolution and efficiency boost is lignin proportional extraction to the from concen- biomass. Usingtration cationic of surfactant, amphiphiles, being possible the dissolution to fully dissolve efficiency the isinitial proportional amount of to lignin the concentration (i.e., 20 ofwt.%). surfactant, A much being less possiblesignificant to effect fully was dissolve observed the when initial using amount anionic of ligninsurfactants (i.e., with 20 wt.%). a A muchmaximum less significant dissolution effect efficien wascy observed of ca. 30% when (0.9 mol using L−1 SDS). anionic With surfactants non-ionic withsurfactants, a maximum −1 dissolutionit was possible efficiency to completely of ca. dissolve 30% (0.9 lignin mol using L−1 0.5SDS). molWith L PS20. non-ionic The data surfactants, suggests that it was the hydrophobic interactions play an important role in the surfactant–lignin complex for- possible to completely dissolve lignin using 0.5 mol L−1 PS20. The data suggests that the mation. This effect is boosted by the favorable electrostatic interactions between the cati- hydrophobic interactions play an important role in the surfactant–lignin complex forma- onic surfactants and ionized lignin, while it is decreased when anionic surfactants are used tion.due This to the effect repulsion is boosted between by like thewise favorable charges. electrostatic At low surfactant interactions concentrations between (i.e., the 0.01 cationic surfactantsand 0.1 mol and L− ionized1), all surfactants lignin, while studied it is are decreased observed whento have anionic a positive surfactants effect on are lignin used due to theextraction repulsion from between biomass. likewiseThe increase charges. in surf Atactant low concentration surfactant concentrations only proved to (i.e.,be ben- 0.01 and − 0.1eficial mol Lin 1the), allextracted surfactants lignin studied yield when are using observed CTAB. to Nevertheless, have a positive the effecthigher onextraction lignin extrac- tionyield from is also biomass. followed The by increase a growing in number surfactant of impurities. concentration Therefore, only provedthis implies to bea fine- beneficial intune the extracted control to strictly lignin yieldbalance when the yield using and CTAB. purity Nevertheless,regarding the hypothetical the higher final extraction appli- yield is alsocations. followed This work by not a growing only introduces number the of us impurities.efulness of surfactants Therefore, as this valuable implies additives a fine-tune controlin lignin to strictly dissolution balance and theextraction yield and butpurity also sheds regarding light on the the hypothetical mechanisms final of dissolu- applications. Thistion/extraction work not only and introduces the concomitant the usefulness role of electrostatic of surfactants and ashydrophobic valuable additivesinteractions. in lignin This knowledge is of critical importance for the future development of more efficient dis- dissolution and extraction but also sheds light on the mechanisms of dissolution/extraction solution/extraction strategies, thus paving the way for lignin valorization. and the concomitant role of electrostatic and hydrophobic interactions. This knowledge is ofAuthor critical Contributions: importance forconceptualization, the future development E.M, A.J.M.V., ofand more B.M.; efficientmethodology, dissolution/extraction E.M., A.J.M.V., strategies,and B.M.; thusinvestigation, paving E.M.; the wayresources, for lignin A.J.M.V. valorization. and F.E.A.; writing—original draft preparation, E.M.; writing—review and editing, A.J.M.V., F.E.A., A.R., and B.M.; visualization, E.M.; supervision, AuthorA.J.M.V., Contributions: F.E.A., and B.M.;Conceptualization, project administration, E.M, B.M.; A.J.M.V., funding and acquisition, B.M.; methodology, E.M. and B.M. E.M., All au- A.J.M.V., andthors B.M.; have investigation, read and agreed E.M.; to the resources, published A.J.M.V. version of and the F.E.A.; manuscript. writing—original draft preparation, E.M.; writing—review and editing, A.J.M.V., F.E.A., A.R., and B.M.; visualization, E.M.; supervision, A.J.M.V., F.E.A., and B.M.; project administration, B.M.; funding acquisition, E.M. and B.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Portuguese Foundation for Science and Technology (FCT), through the projects PTDC/AGR-TEC/4814/2014, PTDC/ASP-SIL/30619/2017, UIDB/05183/2020, researcher grant CEECIND/01014/2018 (B.M.), and PhD grant SFRH/BD/132835/2017 (E.M.). The CQC is supported by FCT through the projects UID/QUI/00313/2020 and COMPETE. Polymers 2021, 13, 714 11 of 13

Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in this article. Acknowledgments: The authors acknowledge FCT for the funded projects. Elodie Melro is very grateful to FCT for the PhD grant and B.M. for the researcher grant. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Goddard, E.D.; Ananthapadmanabhan, K.P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, USA, 2018; ISBN 9781351082235. 2. Kwak, J.C.T. Polymer-Surfactant Systems; CRC Press: Boca Raton, FL, USA, 2020; ISBN 1000110206. 3. Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons Ltd.: Hoboken, NJ, USA, 2003. 4. Kronberg, B.; Holmberg, K.; Lindman, B. Surface Chemistry of Surfactants and Polymers; John Wiley & Sons Ltd.: Hoboken, NJ, USA, 2014; ISBN 9781118695968. 5. Kai, D.; Tan, M.J.; Chee, P.L.; Chua, Y.K.; Yap, Y.L.; Loh, X.J. Towards lignin-based functional materials in a sustainable world. Green Chem. 2016, 18, 1175–1200. [CrossRef] 6. Zhang, L.; Gellerstedt, G. NMR observation of a new lignin structure, a spiro-dienone. Chem. Commun. 2001, 24, 2744–2745. [CrossRef] 7. Melro, E.; Alves, L.; Antunes, F.E.; Medronho, B. A brief overview on lignin dissolution. J. Mol. Liq. 2018, 265, 578–584. [CrossRef] 8. Jiang, F.; Qian, C.; Esker, A.R.; Roman, M. Effect of Nonionic Surfactants on Dispersion and Polar Interactions in the Adsorption of Cellulases onto Lignin. J. Phys. Chem. B 2017, 121, 9607–9620. [CrossRef][PubMed] 9. Ren, B.; Gao, Y.; Lu, L.; Liu, X.; Tong, Z. Aggregates of alginates binding with surfactants of single and twin alkyl chains in aqueous solutions: Fluorescence and dynamic light scattering studies. Carbohydr. Polym. 2006, 66, 266–273. [CrossRef] 10. Xu, A.; Li, W.; Zhang, Y.; Xu, H. Eco-friendly polysorbate aqueous solvents for efficient dissolution of lignin. RSC Adv. 2016, 6, 8377–8379. [CrossRef] 11. Norgren, M.; Edlund, H. Stabilisation of kraft lignin solutions by surfactant additions. Colloids Surf. Physicochem. Eng. Asp. 2001, 194, 239–248. [CrossRef] 12. Singh, S.K. Biological treatment of plant biomass and factors affecting bioactivity. J. Clean. Prod. 2020, 279, 123546. [CrossRef] 13. Zheng, Y.; Guo, M.; Zhou, Q.; Liu, H. Effect of lignin degradation product sinapyl alcohol on laccase catalysis during lignin degradation. Ind. Crops Prod. 2019, 139, 111544. [CrossRef] 14. Wang, B.; Yang, G.; Wang, Q.; Liu, S.; Chen, J.; Fang, G. A new surfactant assisted acid prehydrolysis process for enhancing biomass pretreatment. Cellulose 2020, 27, 2149–2160. [CrossRef] 15. Qing, Q.; Yang, B.; Wyman, C.E. Impact of surfactants on pretreatment of corn stover. Bioresour. Technol. 2010, 101, 5941–5951. [CrossRef][PubMed] 16. Hamzeh, Y.; Abyaz, A.; Niaraki, M.O.S.M.; Abdulkhani, A. Application of surfactants as pulping additives in soda pulping of bagasse. BioResources 2009, 4, 1267–1275. [CrossRef] 17. Norgren, M.; Mackin, S. Sulfate and surfactants as boosters of kraft lignin precipitation. Ind. Eng. Chem. Res. 2009, 48, 5098–5104. [CrossRef] 18. Wang, W.; Zhuang, X.; Tan, X.; Wang, Q.; Chen, X.; Yu, Q.; Qi, W.; Wang, Z.; Yuan, Z. Dual Effect of Nonionic Surfactants on Improving the Enzymatic Hydrolysis of Lignocellulose. Energy Fuels 2018, 32, 5951–5959. [CrossRef] 19. Mesquita, J.F.; Ferraz, A.; Aguiar, A. Alkaline-sulfite pretreatment and use of surfactants during enzymatic hydrolysis to enhance ethanol production from sugarcane bagasse. Bioprocess. Biosyst. Eng. 2016, 39, 441–448. [CrossRef] 20. Zhang, H.; Lyu, G.; Zhang, A.; Li, X.; Xie, J. Effects of ferric chloride pretreatment and surfactants on the sugar production from sugarcane bagasse. Bioresour. Technol. 2018, 265, 93–101. [CrossRef][PubMed] 21. Qi, B.; Chen, X.; Wan, Y. Pretreatment of wheat straw by nonionic surfactant-assisted dilute acid for enhancing enzymatic hydrolysis and ethanol production. Bioresour. Technol. 2010, 101, 4875–4883. [CrossRef] 22. Parnthong, J.; Kungsanant, S.; Chavadej, S. The Influence of Nonionic Surfactant Adsorption on Enzymatic Hydrolysis of Oil Palm Fruit Bunch. Appl. Biochem. Biotechnol. 2018, 186, 895–908. [CrossRef] 23. Alkasrawi, M.; Eriksson, T.; Börjesson, J.; Wingren, A.; Galbe, M.; Tjerneld, F.; Zacchi, G. The effect of Tween-20 on simultaneous saccharification and fermentation of softwood to ethanol. Enzyme Microb. Technol. 2003, 33, 71–78. [CrossRef] 24. Li, K.; Wan, J.; Wang, X.; Wang, J.; Zhang, J. Comparison of dilute acid and alkali pretreatments in production of fermentable sugars from bamboo: Effect of Tween 80. Ind. Crops Prod. 2016, 83, 414–422. [CrossRef] 25. Chen, C.; Jiang, L.; Ma, G.; Jin, D.; Zhao, L.; Ouyang, X. Lignin Removal from Tobacco Stem with Laccase Improved by Synergistic Action of Weak Alkali and Tween 80. Waste Biomass Valorization 2019, 10, 3343–3350. [CrossRef] 26. Zheng, Y.; Pan, Z.; Zhang, R.; Wang, D.; Jenkins, B. Non-Ionic Surfactants and Non-Catalytic Protein Treatment on Enzymatic Hydrolysis of Pretreated Creeping Wild Ryegrass; Humana Press: Totowa, NJ, USA, 2007. Polymers 2021, 13, 714 12 of 13

27. Méndez Arias, J.; de Oliveira Moraes, A.; Modesto, L.F.A.; de Castro, A.M.; Pereira, N. Addition of Surfactants and Non- Hydrolytic Proteins and Their Influence on Enzymatic Hydrolysis of Pretreated Sugarcane Bagasse. Appl. Biochem. Biotechnol. 2017, 181, 593–603. [CrossRef] 28. Chen, Y.A.; Zhou, Y.; Liu, D.; Zhao, X.; Qin, Y. Evaluation of the action of Tween 20 non-ionic surfactant during enzymatic hydrolysis of lignocellulose: Pretreatment, hydrolysis conditions and lignin structure. Bioresour. Technol. 2018, 269, 329–338. [CrossRef] 29. Kim, H.J.; Kim, S.B.; Kim, C.J. The effects of nonionic surfactants on the pretreatment and enzymatic hydrolysis of recycled newspaper. Biotechnol. Bioprocess. Eng. 2007, 12, 147–151. [CrossRef] 30. Barthel, J.; Feuerlein, F.; Neueder, R.; Wachter, R. Calibration of conductance cells at various temperatures. J. Solut. Chem. 1980, 9, 209–219. [CrossRef] 31. Templeton, D.; Ehrman, T. Chemical Analysis and Testing Task: LAP-003 (Determination of Acid-Insoluble Lignin in Biomass); National Renewable Energy Laboratory: Golden, CO, USA, 1995. 32. Ehrman, T. Determination of Acid Soluble Lignin in Biomass, Chemical Analysis and Testing Task, Laboratory Analytical Procedure (LAP-004); Technical Report; National Renewable Energy Laboratory: Golden, CO, USA, 1996. 33. Ansari, A.A.; Kamil, M.; Kabir-ud-Din. Polymer-Surfactant Interactions and the Effect of Tail Size Variation on Micellization Process of Cationic ATAB Surfactants in Aqueous Medium. J. Dispers. Sci. Technol. 2013, 34, 722–730. [CrossRef] 34. Banipal, T.S.; Kaur, H.; Banipal, P.K.; Sood, A.K. Effect of head groups, temperature, and polymer concentration on surfactant— Polymer interactions. J. Surfactants Deterg. 2014, 17, 1181–1191. [CrossRef] 35. Ruiz, C.C. Thermodynamics of micellization of tetradecyltrimethylammonium bromide in ethylene glycol-water binary mixtures. Colloid Polym. Sci. 1999, 277, 701–707. [CrossRef] 36. Jang, J.; Bae, J.; Park, E. Selective fabrication of poly(3,4-ethylenedioxythiophene) nanocapsules and mesocellular foams using surfactant-mediated interfacial polymerization. Adv. Mater. 2006, 18, 354–358. [CrossRef] 37. Albadarin, A.B.; Collins, M.N.; Naushad, M.; Shirazian, S.; Walker, G.; Mangwandi, C. Activated lignin-chitosan extruded blends for efficient adsorption of methylene blue. Chem. Eng. J. 2017, 307, 264–272. [CrossRef] 38. Notley, S.M.; Norgren, M. Adsorption of a strong polyelectrolyte to model lignin surfaces. Biomacromolecules 2008, 9, 2081–2086. [CrossRef] 39. Pang, Y.; Wang, S.; Qiu, X.; Luo, Y.; Lou, H.; Huang, J. Preparation of Lignin/Sodium Dodecyl Sulfate Composite Nanoparticles and Their Application in Pickering Emulsion Template-Based Microencapsulation. J. Agric. Food Chem. 2017, 65, 11011–11019. [CrossRef] 40. Notley, S.M.; Norgren, M. Measurement of interaction forces between lignin and cellulose as a function of aqueous electrolyte solution conditions. Langmuir 2006, 22, 11199–11204. [CrossRef] 41. Helenius, A.; Simons, K. Solubilization of membranes by detergents. BBA Rev. Biomembr. 1975, 415, 29–79. [CrossRef] 42. Fijan, R.; Šostar-Turk, S.; Lapasin, R. Rheological study of interactions between non-ionic surfactants and polysaccharide thickeners used in textile printing. Carbohydr. Polym. 2007, 68, 708–717. [CrossRef] 43. Wang, J.; Li, Y.; Qiu, X.; Liu, D.; Yang, D.; Liu, W.; Qian, Y. Dissolution of lignin in green urea aqueous solution. Appl. Surf. Sci. 2017, 425, 736–741. [CrossRef] 44. Appell, J.; Porte, G.; Rawiso, M. Interactions between Nonionic Surfactant Micelles Introduced by a Telechelic Polymer. A Small Angle Neutron Scattering Study. Langmuir 1998, 14, 4409–4414. [CrossRef] 45. Winnik, F.M.; Regismond, S.T.A. Fluorescence methods in the study of the interactions of surfactants with polymers. Colloids Surf. Physicochem. Eng. Asp. 1996, 118, 1–39. [CrossRef] 46. Bakshi, M.S.; Sachar, S. Surfactant polymer interactions between strongly interacting cationic surfactants and anionic polyelectrolytes from conductivity and turbidity measurements. Colloid Polym. Sci. 2004, 282, 993–999. [CrossRef] 47. Ali, M.S.; Suhail, M.; Ghosh, G.; Kamil, M.; Kabir-ud-Din. Interactions between cationic gemini/conventional surfactants with polyvinylpyrrolidone: Specific conductivity and dynamic light scattering studies. Colloids Surf. Physicochem. Eng. Asp. 2009, 350, 51–56. [CrossRef] 48. Kaur, R.; Kumar, S.; Aswal, V.K.; Mahajan, R.K. Interactional and aggregation behavior of twin tail cationic surfactants with pluronic L64 in aqueous solution. Colloid Polym. Sci. 2012, 290, 127–139. [CrossRef] 49. Cabane, B. Structure of some polymer-detergent aggregates in water. J. Phys. Chem. 1977, 81, 1639–1645. [CrossRef] 50. Pettersson, E.; Topgaard, D.; Stilbs, P.; Söderman, O. Surfactant/Nonionic Polymer Interaction. A NMR Diffusometry and NMR Electrophoretic Investigation. Langmuir 2004, 20, 1138–1143. [CrossRef] 51. Rivero, D.; Gouveia, L.M.; Müller, A.J.; Sáez, A.E. Shear-thickening behavior of high molecular weight poly(ethylene oxide) solutions. Rheol. Acta 2012, 51, 13–20. [CrossRef] 52. Khan, M.Y.; Samanta, A.; Ojha, K.; Mandal, A. Interaction between aqueous solutions of polymer and surfactant and its effect on physicochemical properties. Asia-Pac. J. Chem. Eng. 2008, 3, 579–585. [CrossRef] 53. Ribeiro, A.C.; Lobo, V.M.M.; Valente, A.J.M.; Azevedo, E.F.; Miguel, M.D.G.; Burrows, H.D. Transport properties of alkyltrimethylammonium bromide surfactants in aqueous solutions. Colloid Polym. Sci. 2004, 283, 277–283. [CrossRef] 54. Evans, D.F.; Allen, M.; Ninham, B.W.; Fouda, A. Critical micelle concentrations for alkyltrimethylammonium bromides in water from 25 to 160 ◦C. J. Solut. Chem. 1984, 13, 87–101. [CrossRef] Polymers 2021, 13, 714 13 of 13

55. Burrows, H.D.; Valente, A.J.M.; Costa, T.; Stewart, B.; Tapia, M.J.; Scherf, U. What conjugated polyelectrolytes tell us about aggregation in polyelectrolyte/surfactant systems. J. Mol. Liq. 2015, 210, 82–99. [CrossRef] 56. Diamant, H.; Andelman, D. Onset of self-assembly in polymer-surfactant systems. Europhys. Lett. 1999, 48, 170. [CrossRef] 57. Dal-Bó, A.G.; Laus, R.; Felippe, A.C.; Zanette, D.; Minatti, E. Association of anionic surfactant mixed micelles with hydrophobically modified ethyl(hydroxyethyl)cellulose. Colloids Surf. Physicochem. Eng. Asp. 2011, 380, 100–106. [CrossRef] 58. Sood, A.K.; Singh, K.; Banipal, T.S. Study of micellization behavior of some alkyldimethylbenzyl ammonium chloride surfactants in the presence of polymers. J. Dispers. Sci. Technol. 2009, 31, 62–71. [CrossRef] 59. Silva, S.M.C.; Antunes, F.E.; Sousa, J.J.S.; Valente, A.J.M.; Pais, A.A.C.C. New insights on the interaction between hydroxypropyl- methyl cellulose and sodium dodecyl sulfate. Carbohydr. Polym. 2011, 86, 35–44. [CrossRef] 60. Zanette, D.; Ruzza, Â.A.; Froehner, S.J.; Minatti, E. Polymer-surfactant interactions demonstrated by a kinetic probe: Degree of ionization. Colloids Surf. Physicochem. Eng. Asp. 1996, 108, 91–100. [CrossRef] 61. Zhang, M.; Qi, W.; Liu, R.; Su, R.; Wu, S.; He, Z. Fractionating lignocellulose by formic acid: Characterization of major components. Biomass Bioenergy 2010, 34, 525–532. [CrossRef] 62. Inkrod, C.; Raita, M.; Champreda, V.; Laosiripojana, N. Characteristics of Lignin Extracted from Different Lignocellulosic Materials via Organosolv Fractionation. Bioenergy Res. 2018, 11, 277–290. [CrossRef] 63. Erdocia, X.; Prado, R.; Corcuera, M.Á.; Labidi, J. Effect of different organosolv treatments on the structure and properties of olive tree pruning lignin. J. Ind. Eng. Chem. 2014, 20, 1103–1108. [CrossRef] 64. Snelders, J.; Dornez, E.; Benjelloun-Mlayah, B.; Huijgen, W.J.J.; de Wild, P.J.; Gosselink, R.J.A.; Gerritsma, J.; Courtin, C.M. Biorefining of wheat straw using an acetic and formic acid based organosolv fractionation process. Bioresour. Technol. 2014, 156, 275–282. [CrossRef] 65. Malkov, S.; Tikka, P.; Gullichsen, J. Towards complete impregnation of wood chips with aqueous solutions. Part I. A retrospective and critical evaluation of the penetration process. Pap. Puu/Pap. Timber 2001, 83, 468–473. 66. Parthasarathy, V.R.; Grygotis, R.C.; Wahoske, K.W.; Bryer, D.M. A sulfur-free, chlorine-free alternative to kraft pulping. Tappi J. 1996, 79, 189–198. 67. Rashid, T.; Kait, C.F.; Regupathi, I.; Murugesan, T. Dissolution of kraft lignin using Protic Ionic Liquids and characterization. Ind. Crops Prod. 2016, 84, 284–293. [CrossRef] 68. Sathawong, S.; Sridach, W.; Techato, K.A. Lignin: Isolation and preparing the lignin based hydrogel. J. Environ. Chem. Eng. 2018, 6, 5879–5888. [CrossRef] 69. Faix, O. Classification of Lignins from Different Botanical Origins by FT-IR Spectroscopy. Holzforschung 1991, 45, 21–28. [CrossRef] 70. Sun, R.C.; Tomkinson, J.; Lloyd Jones, G. Fractional characterization of ash-AQ lignin by successive extraction with organic solvents from oil palm EFB fibre. Polym. Degrad. Stab. 2000, 68, 111–119. [CrossRef] 71. Tejado, A.; Peña, C.; Labidi, J.; Echeverria, J.M.; Mondragon, I. Physico-chemical characterization of lignins from different sources for use in phenol-formaldehyde resin synthesis. Bioresour. Technol. 2007, 98, 1655–1663. [CrossRef] 72. Sameni, J.; Krigstin, S.; dos Santos Rosa, D.; Leao, A.; Sain, M. Thermal characteristics of lignin residue from industrial processes. BioResources 2014, 9, 725–737. [CrossRef]