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Kraft lignin-tannic acid as a green stabilizer for oil/water emulsion Samira Gharehkhani, Nasim Ghavidel, and Pedram Fatehi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05193 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Sustainable Chemistry & Engineering

Kraft lignin-tannic acid as a green stabilizer for oil/water emulsion Samira Gharehkhani, Nasim Ghavidel, Pedram Fatehi* Green Processes Research Centre and Chemical Engineering Department, Lakehead University, 955 Oliver road, Thunder Bay, ON, Canada, P7B 5E1 *Corresponding author: email: [email protected]; tel: 807-343-8697; fax: 807-346-7943 Abstract To overcome hydrophobicity and low charge density of kraft lignin (KL), softwood KL was reacted with tannic acid (TA), a green reagent, under alkaline conditions. The mechanism of this reaction was identified to be proceeded through three steps: i) oxidation of TA ii) transesterification of ester groups of TA (in hydroquinone form) by lignin and iii) phenolic ring opening of TA. The enhancement in carboxylate group was observed; resulting KL-TA with a high charge density and water solubility in a wide pH range. The modified lignin was utilized as an emulsifier to stabilize oil (hexadecane) in water (O/W) emulations. Vertical scan analysis revealed the greater stability of the system against phase separation at low pH values. The stability of the emulsion containing KL-TA could be tuned by pH, while the emulsion containing lignosulfonate (LS) was not pH responsive. The LS containing emulsion had lower stability than

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KL-TA containing one. The rheological studies of KL-TA emulsions showed that all samples displayed shear-thinning characteristics. Keywords: Kraft lignin; Tannic acid; emulsion; Oil water emulsion; Green materials; Interface analysis INTRODUCTION Lignin, a three-dimensional polymer, is generally obtained as a by-product of the pulping industry. It is one of the plant components and consists of three subunits of syringyl (S), guaiacyl (G), and hydroxyphenyl (H) linked by ether and C-C bonds. It has abundant functional groups, mostly aliphatic hydroxyl and phenolic hydroxyl groups. Annually, more than 50 million tons of lignin are produced in the pulping industry.1 Majority of lignin is combusted to produce energy, but its utilization as a filler or binder accounts for a share of 2-5% only. Recently, there has been a surge of interest to extend the utilization of this renewable resource. The economic studies have reported that lignin valorization can provide a higher potential market value as compared to burning it for energy production.2 Various methods, such as grafting with the functional groups or polymers, have been introduced for chemical reactions of lignin.3-5 However, many of them suffer from the usage of environmentally unfriendly reactants. For example, many of reactions were performed in solvents, e.g., dimethylformamide (DMF)6 and dioxane7, which were not favorable from an environmental standpoint. Realizing such disadvantages can lead to the development of simple and eco-friendly methods for lignin modification. Tannin comes after lignin as one of the largest classes of plant components.8 Among tannin substances, tannic acid (TA), a natural water soluble polyphenol, is commercially produced from


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gallnuts.9 It has a macromolecular structure consisting of a glucose core esterified by small molecular weight phenolic compounds (gallic acid, GA).10,11 TA has gained intensive interests in diverse areas, such as functionalization of carbonaceous materials,8 films and hydrogel preparation,12-14 biomaterials,15,16 polymer chemistry17,18 and food industry.19 Lin et al.20 reported the adsorption of TA on carbon nanotube and its role in stabilizing the suspension. Chitosan was modified by TA and the product was used for treating wastewater effluents containing aluminum and lead.21,22 Oxygen uptake was reported as one of the most striking properties of TA.21 In this context, GA and TA can be used for atmospheric oxygen activation.21,22 Recently, engineering the characteristics of lignin has attracted intensive interests, shedding lights on extending lignin applications. Of particular interest is emulsion. Lignin-based emulsions can be implemented, for examples, as a fuel emulsion,23 organic carrier24 or a template for synthesizing the polymers with porous structures.25,26 Lignosulfonate (LS) and kraft lignin (KL) with low and high molecular weights were used in emulsion studies.27,28 However, the high sulfur content of LS and poor solubility of KL limit their utilizations in emulsions. KL originally has a low surface activity and is water soluble only at a high pH. Gupta et al.29 studied the behavior of emulsions stabilized by polymer-grafted lignin particles in which lignin particles were grafted by two different polymers of polyacrylamide and poly (acrylic acid) through the reversible addition–fragmentation chain transfer (RAFT) route. In another study, lignin particles produced by an aerosol flow reactor were effective to form the stable oil in water (O/W) emulsions.30 Very recently, the efficiency of water soluble carboxymethylated lignin to produce stable kerosene in water emulsions was investigated, in which the lignin particles with a high degree of substitution (30%) were able to maintain emulsions fairly stable in a wide pH range.31



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The present study discloses a simple and green method for KL modification using TA in an aqueous medium. For the first time, this study explores the reaction of KL and TA to produce highly charged anionic lignin-based products. The water solubility and anionicity of TA can facilitate the development of a green process for mimicking anionic lignin. Also, the aqueous alkaline modification process is another advantage of KL-TA production process. In this work, atmospheric oxygen is consumed by TA under alkaline conditions in the first stage. The transesterification and phenolic ring degradation are proposed as the second and third steps in the reaction. The results confirmed that KL-TA had superiority in anionic charge density to other carboxymethylated lignin products (with the charge density of 2.8 meq/g) as carboxymethylated lignin had a charge density of 1.5 meq/g.4 In addition to examine the possible utilization of KL-TA for emulsion stabilization, the behavior of the emulsions at different pH values was systematically investigated by means of interfacial tension and rheology analysis. Hereby, it is demonstrated that KL-TA has potential to be used as an O/W emulsion stabilizer. However, we believe that its usage is not limited to this application owing to KL-TA’s water solubility and environmentally friendly production process. EXPERIMENTAL SECTION Materials. Softwood kraft lignin (KL) produced by the LignoForce™ technology was received from FPInnovations. Tannic acid (C76H52O46, ACS reagent), sodium hydroxide (NaOH, 97%), potassium hydroxide (KOH), HCl, 4-hydroxybenzoic acid, sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), deuterated chloroform (CDCl3), pyridine (C5H5N), cyclohexanol (C6H12O), chromium (III) acetylacetonate, lignosulfonic acid sodium 97.0%, cyclohexane (C6H12), dimethyl sulfoxide-d6 (DMSO-d6) and deuterium oxide (D2O) were purchased from Sigma-Aldrich company. All the chemical reagents were used without further purification.


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Dialysis membrane of cellulose acetate (molecular weight cut-off of 1000 g/mol) was obtained from Spectrum Labs. Synthesis of kraft lignin-tannic acid. Kraft lignin-tannic acid (KL-TA) samples were prepared according to the following procedure: KL powder (0.5 g) was ground into fine powder followed by dispersing in deionized water (50 mL) by means of ultrasonication (Omni-Ruptor 4000, Omni International Int.) for 15 min. The obtained suspension was then transferred to a three-neck flask equipped with a stir bar. The reaction was initiated by adding TA and NaOH while stirring. The final volume and pH of mixture were adjusted to 80 mL and 11, respectively. The flask was transferred to a water bath preheated at 80 °C. Upon completion of the reaction after 7 h, the reaction was cooled down to room temperature. The pH of the solution was set to 7 using H2SO4 (1M) and the product was dialyzed against deionized water for 4 days. The sample was subsequently dried at 60 °C. Nuclear magnetic resonance (NMR) spectroscopy. The procedure was followed as stated in a previous study.32 KL and KL-TA were dissolved in 500 L of DMSO-d6 and D2O, respectively, to make 40–50 g/L concentrations. The solutions were stirred overnight at 100 rpm and room temperature prior to analysis. Spin-lattice relaxation (T1) measurements were conducted to find out the accurate relaxation time prior to 1H-NMR. The minimum and the maximum expected T1 were adjusted as 0.5 and 5 s with 90° pulse width. The maximum T1 was 2.581s, which was used in 1H-NMR analysis. 1H-NMR was performed using an INOVA-500 MHz instrument (Varian, USA), and 64 number of scans was obtained in a 90° pulse width. To conduct the 13C-NMR analysis, the samples were prepared by dissolving 120 mg of KL in 500 L of DMSO-d6 or 120 mg of KL-TA in a mixture of 400 L of DMSO-d6 and 100 L of D2O. Chromium (III) acetylacetonate (5 mg) was added to the mixture to enable the complete 5 ACS Paragon Plus Environment


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relaxation of all nuclei. Spectra were recorded with a 90 pulse angle, a pulse delay of 0.5 s, and the transient number of 20000. 31P-NMR test was also conducted for KL sample to determine its functional groups as explained in the supporting information. Fourier transform infrared (FT-IR). A Fourier transform infrared spectrophotometer (Bruker Tensor 37, Germany, ATR accessory) was used to conduct the FT-IR analysis of the samples. A transmittance mode was used to record the spectra in the range of 600 cm−1 and 4000 cm−1 with a 4 cm−1 resolution, and 32 scans per sample. H2O2 concentration measurement. The concentration of H2O2 in the reaction media was measured using a peroxide Vacu-Vials kit (K-5543, CHEMetrics), which employed the ferric thiocyanate chemistry.33 The amount of H2O2 in the sample (i.e., 300 L of the reaction mixture) was measured by UV-Vis spectrophotometer (Thermo Scientific, Madison, USA) at 470 nm wavelength. The test results were obtained within 1 min of experimentation. Zeta potential analysis. Zeta potential measurements were carried out using a ZetaPALS analyzer (Brookhaven Instruments Corp, USA). In this study, 1.5 mL of KL-TA aqueous solution (1.5 wt%) was prepared at different pH via adding it to 20 mL of a 1.0 mM KCl aqueous solution. Then, 2 mL of each solution was collected for conducting the measurements. Each sample was measured three times and the average value was reported. Emulsion preparation. To study the performance of KL-TA as an emulsifier, cyclohexane was used as a water-immiscible phase. In this set of experiments, 0.15 g of KL-TA was added to deionized water (10 g) and stirred at 300 rpm for 1 h. The pH of samples was then adjusted using H2SO4 or NaOH. Afterward, 10 g of cyclohexane was added to the solution followed by ultrasonication for 1 min. The ultrasound machine was set to 240 W power and 30 s interval. The


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samples were stored at room temperature to equilibrate. A similar procedure was used to prepare the emulsions containing lignosulfonate, LS (1.5 wt%). Surface and interfacial tension. A tensiometer equipped with a Du Nouy ring (Sigma 701, Biolin Scientific) and OneAttension software was used to determine the surface and interfacial tension based on force measurements. The measurement specimens (ring and petri dish) were washed with water and ethanol prior to the analysis. The samples with a pre-determined concentration were then prepared at different pH values (3, 5, 7 and 10) to monitor the surface tension variations. For interfacial tension measurements, the aqueous KL-TA solutions (1.5 wt%) with different pHs were poured into petri dish. Then, the cyclohexane was added to the solution. The mixture was allowed to rest for 30 min to reach equilibrium prior to the measurements at room temperature. Then, the ring of the instrument was submerged in the light phase. A minimum of five measurements were recorded for each sample. Droplet size. The particle size distribution of emulsion droplets was determined using a laser diffraction particle size analyzer (Malvern Mastersizer 3000, Worcestershire, UK) at room temperature. The emulsion (300 μL) was introduced to a chamber containing 700 mL of water under stirring. The reflective indices of cyclohexane and water were 1.42 and 1.33, respectively.34 In order to break down the droplets present in the emulsion, the emulsion was stirred at 3000 rpm and their droplet size distributions were identified after 30 s of stirring. The conditions were kept constant for all measurements. D [4,3] and Dx (50) were used to express the mean and median diameters, representing an average diameter of droplets and a value of droplet size, which divides the population into two halves, respectively. D[4,3] as a sensitive index for detection of the large particles is defined as ∑𝐷4𝑖 𝑛𝑖/∑𝐷3𝑖 𝑛𝑖 where 𝑛𝑖 is the number of emulsion droplets with diameter 𝐷𝑖.35



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Stability of emulsions. The stability of emulsions was monitored by a vertical scan analyzer (Turbiscan Lab Expert, Formulaction, France) at room temperature for 18 h. The emulsion sample was transferred into the glass container of the instrument immediately after preparation and subjected to analysis. The entire height of the sample was scanned with a pulsed near infrared light (λ = 880 nm)36 and the transmitted and backscattered lights were recorded by the detectors in which a microscopic fingerprint of the samples could be presented at a given time.37 The stability of samples was presented quantitively by a stability index (TSI), where both coalescence and settling phenomena were considered in the TSI evaluation. The TSI is determined as shown in equation (1). 𝑛

𝑇𝑆𝐼 =

∑𝑖 ― 1(𝑥𝑖 ― 𝑥𝑏𝑠)2

(1)

𝑛―1

Where n, xi, xbs refer to the number of scanning, average of the backscattered light intensity at the scanning time, and average of xi, respectively. Generally, the higher the TSI, the lower the stability is.38 In addition, an optical microscope (Olympus IX 51) was used to observe the emulsion droplets. Rheological properties. A hybrid rheometer (TA instrument) with a parallel plate geometry (8 mm, gap 300 μm) was used to perform the rheological studies of emulsion samples at room temperature. About 1 mL of each sample was carefully taken by pipette and placed on the lower plate of the instrument. A 3 min pre-shear at 100 s-1 was applied on each sample prior to starting the measurements. Viscosity variations were measured from 0.1 to 1000 s-1 at 23°C. To determine the linear viscoelastic region, an amplitude sweep was performed in the range of 0.01 and 100 s-1 at frequency of 10 rad/s. Frequency sweep measurements were then carried out in the range of 0.01 and 500 rad/s with a strain of 0.1% chosen from the linear viscoelastic region.


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RESULTS AND DISCUSSION To pursuit the reaction mechanism of lignin and TA, a series of experiments was performed under various conditions. The goal was to produce water soluble lignin based products. Table S1 presents the initial parameters implemented for lignin modification using TA. Attempts to produce water soluble lignin under acidic conditions or room temperature failed. The results showed that water soluble lignin can be produced under an alkaline environment and elevated temperature. Reaction mechanism between lignin and TA. Scheme 1 presents the proposed mechanism for KL and TA reaction following three steps. KL used in this study was derived from softwood species, which was known to be rich in coniferyl alcohol units.3 The presence of this structure was also confirmed by the 31P-NMR experiments (Figure S1). Therefore, a coniferyl alcohol unit was used as a lignin representative in scheme 1. Several studies have shown that oxygen is consumed by TA in an alkaline medium, which generates hydrogen peroxide.21,22, 39 In the proposed reaction, TA is transformed to hydroquinone upon oxidation to generate oxidized tannic acid (OTA), which ultimately results in aromatic conjugated structure breakage (scheme 1, step 1). In the second stage of reaction, the phenoxide ions generated from the deprotonation of phenol groups in an alkaline condition participate in the transesterification of TA’s ester groups.40 Existence of two distinct ester bonds in the TA structure makes two diverse possibilities for the final products. In this case, either one or two-GA groups substitute with phenol groups of lignin as is shown in scheme 1, step 2. We anticipated that the transesterification reaction would occur after TA oxidation due to the higher rate of TA oxidation reaction compared to the transesterification reaction. Moreover, the oxidation of TA’s hydroxyl groups makes the substituent groups attached to the ester bond more electron



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withdrawing, promoting the affinity of lignin’s phenoxide groups to attack the carbonyl group of the ester bond with lower electron density.41 In the third stage, it is shown that ring opening in quinone structures is favored with further oxidation of carbonyl groups, affording an increase in carboxylic acid content and resulting in muconic acid structure. The oxidation of the quinone structure to muconic acid in the presence of H2O2 was studied previously.42 This reaction probably proceeds through either radical-based frameworks or acid and base-catalyzed oxidative cleavages of diketones, and the latter is more probable under alkaline conditions.42 It is worth mentioning that the presence of hydrogen peroxide is also contributed to the drastic oxidation of lignin. The presence of carboxylate groups leads to an increase in the oxygen content of the product. The elemental analysis confirmed the oxygen content increase from 29.3 wt% in KL to 44.6 wt% in the product (see Table S2).


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OH HO GA=

OH

Step 1

O CH2OH

o GA

GA

GA

GA

O2 O

O

GA GA

GA

GA

O

GA -

GA

GA

O

O

GA O

O

O-

-

Tannic acid (TA)

O

GA

O

GA O

O

GA GA

O

nH2O2

O O

H3CO

O

O

O-

-

O-

O

O

Oxidized tannic acid (OTA)

Step 2 Transesterification

COOCOO-

COO-

CH2OH

CH2OH

O2, H2O2 Step 3 H3CO

-

H3CO

H3CO

O

O

O

O

O

OOC OOC

-

O

O

H3CO O

O

O

-

O

O

O

COOCOO-

O

-

O

O

O

O O

O O

O

CO

-

-

O

-

O

CO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Proposed mechanism for reaction of TA and KL under alkaline condition. H2O2 analysis. TA and GA have been extensively studied for their autooxidation properties in alkaline condition.43 The role of hydroxyl groups presented in the polyphenolic structure was investigated in the past, and it was illustrated that the molecule should have at least two hydroxyl groups in ortho and para positions of each other to undergo electron oxidation for forming quinone.44 The experimental results on the production and consumption of H2O2 during the reaction are illustrated in Figure 1. It is evident that hydroxyl group oxidation occurred very fast, which resulted in the generation of H2O2 in the first 15 min of the reaction. The maximum 11 ACS Paragon Plus Environment


amount of 256.7 mg/L was observed after 45 min. Reduction in H2O2 concentration after 45 min can be attributed to the oxidation of quinone and aliphatic hydroxyl groups of lignin in the 3th step of reaction and H2O2 degradation during the reaction.45 300 250

H2O2 mg/L

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 150 100 50 0 0

100

200

Time, min

300

400

Figure 1. Concentration of H2O2 during the reaction time. NMR Analysis. Figure 2(a) shows the 1H-NMR spectra of KL in DMSO-d6 and KL-TA in D2O. The existence of sharp peaks at 2.5 ppm in the KL spectrum and 5 ppm in the KL-TA spectrum correspond to the DMSO-d6 and D2O, respectively.32 The wide phenolic signal at 8-9.2 ppm for KL was reduced compared with that for KL-TA, indicating the KL’s phenolic group participation in the transesterification reaction. Another broad peak at 6.2-7.4 ppm can be related to the lignin’s aromatic structure.32 The breakage of TA’s aromatic rings led to the formation of new peaks in the aliphatic region (1-5 ppm) of lignin. The peak appeared at 3.5 ppm is assigned to the indicated hydrogens next to the carboxylate groups (1 and 1'). Furthermore, the peaks corresponding to the singlet hydrogen (2 and 3) appeared around 2.5 and 2, respectively. The signal at 4.4 ppm is also assigned to the hydroxyl group (4) next to the carboxylate group.46 New signals are singlet, which elucidates the absence of coupling between hydrogens; supporting the proposed structure in Scheme 1. 12 ACS Paragon Plus Environment

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The 13C-NMR spectrum in Figure 2(b) also confirms the proposed structure in Scheme 1. The characteristic peaks at 56 ppm for CH3O, 60.8 ppm for Cα in G unit, 112-130 ppm for aromatic carbons in C-C=C benzene ring, 148 ppm for aromatic C-O and 164 ppm for C=O in CH3COO appeared in KL.47 The new peaks appeared at 171 ppm for C-O (in ester b), 168 ppm for C-O in COOH c, 145 ppm for etherified aromatic C-O (a), 72 ppm for alcoholic C-O (4), 63.6, 43, 34 and 29 ppm for aliphatic C in vinyl structure correspond to KL-TA structure (Figure 2(b)).48 The appearance of etherified C-O peak in 13C-NMR supports the participation of phenol groups in the esterification reaction. The disappearance of the peak at 164 ppm in KL-TA illustrates the breakage of acetylated phenol groups during reaction, which could increase free phenol groups.

(a)



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(b)

Figure 2. a) 1H-NMR and b) 13C-NMR spectra of KL and KL-TA Functional groups. Table S3 in the supporting information tabulates the phenolic hydroxyl and carboxylate contents of KL and KL-TA. Comparing to KL, KL-TA had fewer phenol groups and higher carboxylate groups. By increasing the ratio of TA in the KL and TA reaction, the carboxylate group was increased, while its phenolate group decreased (Table S3, KL-TA-1). These results confirmed the occurrence of TA’s aromatic ring degradation (i.e., ring opening). The increase in the carboxylate group can also be attributed to the oxidation of aliphatic hydroxyl groups of lignin in the presence of H2O2.49 FTIR analysis. Figure 3 shows the FT-IR spectra of KL and KL-TA. As can be seen, both KL and KL-TA had a broad band at 3400 cm-1, corresponding to the stretch vibration of -OH groups.32 A comparison between FTIR spectra of KL-TA and KL showed that there was a strong absorption at 1600 cm-1, which is evidence for C=O of quinone and carboxylate groups in KL-TA.50 The band at 1379 cm-1 was also assigned to the symmetric carboxylate C-O vibration.51


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1.09

1600 1379

1.07 Transmittance, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.05 1.03 1.01 0.99 0.97

KL-TA KL

0.95 4000

3500

3000

2500 2000 Wavenumber, cm-1

1500

1000

500

Figure 3. FTIR spectra of KL and KL-TA. Reaction Optimization. A series of experiments was carried out to identify the optimal reaction conditions (details of the reaction optimization were provided in supporting information). Impacts of time, pH, TA/KL molar ratio and temperature on the charge density of samples were presented in Figure S2. Compared with the charge density of KL (0.7 meq/g), an enhancement in the charge density for KL-TA was observed. The outcomes revealed that the reaction under the conditions of TA/KL molar ratio of 0.2 mol/mol, 7 h, pH 11 and 80 °C generated KL-TA with the maximum charge density of 2.86 meq/g. Water solubility and contact angle analysis. The water solubility and contact angle data of KL-TA and KL in water was presented in supporting information (Figures S3 and S4). KL-TA was 100% soluble in a broad pH range of 6 and 12. Sample solubility was maintained up to 85% at pH 5, but it dropped to less than 13% at pH 3. KL-TA was able to be dissolved in water in a broader pH range than KL. Also, the contact angle analysis revealed a dramatic reduction in the 15 ACS Paragon Plus Environment


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contact angle of KL-TA sample compared to the KL sample; indicating the strong hydrophilic characteristics of the KL-TA sample (Figure S4). Zeta potential and hydrodynamic size characteristics. The stability of the samples depends on the repulsive and attractive forces between the particles, and the presence of high repulsion between particles introduces particle stability.52 The negative zeta potential of -46, -42 -39 and 34 mV was obtained for the KL-TA solutions at pH values of 10,7, 5 and 3, respectively. The zeta potential of KL solutions was -45, -40, -32 and -26 at pH values of 10,7, 5 and 3, respectively. Compared to KL, KL-TA introduced more repulsion force between particles. Details of hydrodynamic size analysis of KL-TA solutions are presented in supporting information (Figure S5). The hydrodynamic size for samples at pH ≥ 5 was in the range of 8-15 nm, while the sample at pH 3 had two different size ranges of approximately 100 and 500 nm (i.e., lignin cluster). A previous study reported a size of 5-20 nm for water soluble polymergrafted lignin surfactants.29 It has been also reported that lignin clusters can form under acidic conditions.53 Surface tension. A set of experiments was conducted to monitor the surface tension variations as a function of the concentration for KL-TA samples in Figure 4. The surface tension of water was 72.8 mN/m. With an increase in the concentration of KL-TA, the molecules spontaneously adsorbed on the surface, reducing the surface tension. Reduction in the surface tension was no longer observed with KL-TA loading higher than 1.1 wt%. Similar behavior was reported for surfactants.54-57 The KL-TA showed a reduction in surface tensions as a function of pH. The pKa of carboxylate groups is known to be around 4.7,58,59 indicating that the carboxylic acid groups are protonated at pH 3. Therefore, KL-TA was less effective at this low pH. At higher pH, the surface tension dropped due to the dissociation of carboxylate groups. The higher the pH, the


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higher the degree of deprotonation of functional groups would be. Further increase in pH to 10 lowered the surface tension to 53.6 mN/m, where the deprotonation of phenolate group with pKa of 9-10 was also occurred.60

75 Surface tension, mN/m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 65 60 55

pH=3

50

pH=5 pH=7

45

pH=10

40 0.1

1 Concentration, wt.%

10

Figure 4. Surface tension of KL-TA as a function of concentration (Error bars presented for graphs are very small) Interfacial tension. Typically, a net hydrophilic layer is oriented towards the oil phase in O/W emulsion.61 The polymer oriented/adsorbed at the oil-water interface is expected to substantially reduce the interfacial tension of the system, consequently producing stable emulsions. The pH dependence of interfacial tension values for samples containing KL-TA was reported in Figure 5. In comparison with the interfacial tension of cyclohexane-water system (47.9 mN/m, data was not presented in Figure 5), the sample with KL-TA displayed effective reduction in interfacial tension to 23.6 mN/m (at 25 °C and pH 7). The increase in pH lowered the interface tension. Notably, the results were reminiscent of the functional groups dissociation in which the obvious enhancement of interfacial tension at pH 3 is an indication of unionized functional groups. A



similar trend was also reported by Saien et al.62 for the interfacial tension of toluene/water/sodium dodecyl sulfate. 50 Interfacial tension, mN/m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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45 40 35 30 25 20 15 10 1

3

5

7 pH

9

11

13

Figure 5. Interfacial tension of emulsions as a function of pH. Stability analysis of KL-TA emulsions. The stability of emulsions was evaluated by means of visual inspection (see Figure S6). Interestingly, there was no clear phase separation for KL-TA sample at pH 3, even though the results of surface and interfacial tensions presented lower surface activity at this pH. Therefore, another mechanism is involved in the KL-TA emulsion stability. Prior studies have interpreted that water soluble lignin products acted as polymeric surfactants at various pHs.31,56 According to the solubility and DLS data (Figures S4 and S5), KL-TA was soluble at pH≥5 and thus we propose that KL-TA acted as polymeric surfactant at pH≥5. At pH 3, KL-TA was not water soluble and it could adsorb and be deposited at the interface of O/W emulsions, which facilitated the emulsion stability. However, further studies are required for identifying the mechanism of KL-TA in stabilizing O/W emulsion. The stability measurement of emulsions at different pH were conducted at room temperature over 18 h after preparation. After sample preparation, the sample was turbid entirely, which was considered as a fully dispersed sample (Figure S7). The turbidity of the samples dropped over 18 ACS Paragon Plus Environment

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time, indicating variations in the emulsion height. The emulsion fraction after 18 h was presented in Figure S8. The emulsion fraction was 100% for all samples upon preparation and dropped to 98.0%, 90.4%, 80.2% and 78.5% for samples at pH of 3, 5, 7 and 10 after 18 h, respectively. The concentration of lignin in emulsion fraction was also presented in Figure S9. Figure 6 exhibited the variation in the stability index values of the emulsion sample over time. For the samples under acidic conditions (pH 3 and 5), it was observed that TSI value was almost constant after 500 min, revealing the high stability of the sample. At pH 7, the obvious phase separation was occurred for the first 400 min after preparation, where the speed of separation was almost linear afterward. At a higher pH (pH 10), sample underwent a rapid phase separation. The results indicated that the samples having lower pH values were not only more stable, but they also reached to the equilibrium condition faster.

Figure 6. TSI variations for KL-TA-emulsions after 18 h. Comparison with LS. Of commercially available lignins, LS is a well-utilized technical lignin in various applications, such as dispersing agents and surfactants.63 The stability performance of



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KL-TA emulsions with those prepared with LS was compared. Figure S10 presents the stability characteristics of O/W emulsions containing LS at pH 3 and 10. The emulsion fractions for LSemulsion at pH 3 and 10 were 73.5% and 71.4%, respectively, which were lower than the emulsions stabilized by KL-TA (see Figure S10(a)). These results might be due to the higher charge density of KL-TA than LS. We found that LS was less effective than KL-TA to keep the emulsion stabilized. The TSI values for emulsion at acid and basic conditions were very close to each other in which a weak dependency of LS emulsion stability to the pH was observed (Figure S10b). To explain the difference between emulsion efficiency, one should look into the LS properties (see Table S4). LS is comprised of three functional groups; carboxylate, hydroxyl and sulfonate groups, in which sulfonate groups have lower pKa compared to the other ones. The pKa of sulfonated groups in LS was reported to be ≤2.63 Therefore, the presence of the sulfonate group, which was fully deprotonated at all pH values in this study, provided less pH responses. Droplet size analysis. Figures 7(a) and 7(b) displayed the distribution and droplet size of emulsions at different pH. The emulsion droplets were fairly monodispersed at pH >7. Below pH 7, the distribution of droplets moved slightly to the larger size range. In general, the droplet sizes were in the range of 1-100 μm, which is consistent with the previous studies.29, 64A lower mean size was observed at higher pH values. At pH 3, lignin was less water soluble and tended to form small aggregates. Therefore, oil droplets containing larger lignin particles presented larger droplet sizes at a higher pH.


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Volume density, %

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6.5

6

6

4

5.5

2 0

5 1

3

5

pH 7

9

11

Figure 7. a) droplet size distribution and b) size of emulsion droplets as a function of pH at room temperature after one day of preparation. Rheological characteristics. The viscosity of the emulsions was shown as a function of shear rate in Figure 8(a). In general, the shear thinning behavior was observed for all samples. At pH 3, the emulsion possessed a higher viscosity than other samples, presenting a dense and gel-like structure. A decrease in the viscosity of samples is attributed to the continuous rupture of microstructure, i.e., aggregates of oil droplets.65 The stress applied on the droplets results in the dissociation of droplets. If the stress is smaller than interaction forces, the emulsion will present elastic behavior and the energy will be stored as an extension of the bonds between the dispersed droplets.66 The elastic behavior of emulsions was investigated in terms of storage modulus (G') 21 ACS Paragon Plus Environment


and loss modulus (G'') over the frequency sweep (Figure 8(b)) representing the magnitude of the energy stored in the elastic structure and energy lost through viscous dissipation in a cycle of deformation, respectively. All samples exhibited higher storage modulus (G') than loss modulus (G'') over the entire frequency range (Figure 8(b)). It was stated that G' was affected by the strength of droplet-droplet attraction.67 Therefore, the assumption of strong interaction between droplets can be interpreted for the systems with G'> G'', which reflects the elastic behavior and presence of three-dimensional network in emulsion. Moreover, the frequency independence characteristic of G'' variation indicated the dissipation of energy.67 The viscoelastic characteristics of the emulsions is further explained by using the term of tan δ, which is defined as G''/G'. The emulsion with solid behavior shows tan δ lower than 1, while emulsion having tan δ higher than unity is characterized as a liquid like.68 Figure 8(c) illustrated the variation in tan δ as a function of angular frequency. The results confirmed the elastic behavior (solid like) of emulsions. Increased pH would reduce tan δ, resulting fewer elastic properties and thus less resistance to flow. 1000

Viscosity, Pa.s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a)

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0.1 0.1

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10000

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10 0.1

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(c)

Tan (δ)

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0.1 pH=3 pH=5 pH=7 pH=10

0.01 1

10 100 Angular frequency, rad/s

1000

Figure 8. Rheological characteristics of emulsions at different pH values a) viscosity variation versus shear rate, b) variation of elastic modulus G′ and viscous modulus G′′ versus angular frequency (G′ and G′′ are respectively represented as solid and open circles) and c) variation of tan (δ) as a function of angular frequency. CONCLUSIONS A simple and efficient process for lignin modification was reported using TA as a green reagent, resulting in a water-soluble lignin with a high anionic charge density. The present work elucidated that lignin and TA underwent both oxidation and transesterification reactions. We



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noted that the possibility for TA re-oxidation afforded a ring-opened product and consequently enhanced carboxylate group content and hydrophilicity of modified lignin. The optimal conditions were TA/KL molar ratio of 0.2, 80 °C, pH of 11 and 7 h of reaction, which generated KL-TA with the charge density of -2.8 meq/g and 10 g/L water solubility. The resultant lignin showed a potential application as a stabilizer for O/W emulsions. KL-TA reduced interfacial tension between water and cyclohexane from 47.9 to 21.7 mN/m, but the response was pH dependent. Emulsion droplets possessed a smaller mean size at a higher pH. Visual inspection along with data extracted from vertical scan analyzer revealed that the emulsion fraction of the O/W emulsion can be maintained up to 98.0 and 78.5 for pH 3 and 10 via using 1.5 wt% of KLTA, respectively. Although the sample with the lower pH of 3 had a higher interfacial tension, it showed the greatest stability against phase separation. A comparison between the performance of KL-TA stabilized emulsion with LS stabilized emulsion indicated better stability for KL-TA emulsion than LS one. It was observed that KL-TA emulsion was pH responsive, but LS emulsion was not, which was due to the presence of different charged groups attached to LS (i.e., sulfonate) and KL-TA (i.e., carboxylate). Acknowledgements The authors would like to thank NSERC, Canada Foundation for Innovation, Canada Research Chairs, Ontario Research Fund and Northern Ontario Heritage Fund Corporation programs for supporting this research. Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Supporting information contains extra information about preliminary reaction analysis, elemental and functional group analysis, reaction optimization, water solubility and contact angle,


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hydrodynamic size analysis, stability analysis, vertical scan analysis as well as lignin concentration measurement in emulsion systems. It contains 10 figures (Figures S1-S10) and 4 tables (Table S1-S4). Notes The authors declare no competing financial interests. REFERENCES (1) Numan-Al-Mobin, A. M.; Kolla, P.; Dixon, D.; Smirnova, A. Effect of water–carbon dioxide ratio on the selectivity of phenolic compounds produced from alkali lignin in sub-and supercritical fluid mixtures. Fuel. 2016, 185, 26-33, DOI 10.1016/j.fuel.2016.07.043. (2) Wu, W.; Dutta, T.; Varman, A. M.; Eudes, A.; Manalansan, B.; Loqué, D.; Singh, S. Lignin valorization: two hybrid biochemical routes for the conversion of polymeric lignin into valueadded chemicals. Sci. Rep. 2017, 7 (1), 8420. DOI 10.1038/s41598-017-07895-1. (3) Kong, F.; Wang, S.; Price, J. T.; Konduri, M. K.; Fatehi, P. Water soluble kraft lignin–acrylic acid copolymer: synthesis and characterization. Green Chem. 2015, 17 (8), 4355-4366, DOI 10.1039/c5gc00228a. (4) Konduri, M.; Fatehi, P. Production of water-soluble hardwood kraft lignin via sulfomethylation using formaldehyde and sodium sulfite. ACS Sustain. Chem. Eng. 2015, 3 (6), 1172-1182, DOI 10.1021/acssuschemeng.5b00098. (5) Couch, R.; Price, J.; Fatehi, P. Production of flocculant from thermomechanical pulping lignin via nitric acid treatment. ACS Sustain. Chem. Eng. 2016, 4 (4), 1954-1962, DOI 10.1021/acssuschemeng.5b01129.



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nanocrystals. ACS Appl. Mater. Interfac. 2018, 10 (23), 19318-19322, DOI 10.1021/acsami.8b05067.

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Synopsis Water-soluble and highly anionic charged lignin-tannic acid was produced as a green stabilizer for oil in water emulsions.


Water Emulsion

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