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High-throughput Synthesis of Lignin Particles (~30 nm to ~2 µm) via Aerosol Flow Reactor: Size Fractionation and Utilization in Pickering Emulsions Mariko Ago, Siqi Huan, Maryam Borghei, Janne Raula, Esko I Kauppinen, and Orlando J. Rojas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07900 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 21, 2016

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ACS Applied Materials & Interfaces

High-throughput Synthesis of Lignin Particles (~30 nm to ~2 m) via Aerosol Flow Reactor: Size Fractionation and Utilization in Pickering Emulsions Mariko Ago1,*, Siqi Huan1,2, Maryam Borghei1, Janne Raula3, Esko I. Kauppinen3, Orlando J. Rojas1,3,* 1

Bio-Based Colloids and Materials and Centre of Excellence on “Molecular Engineering of

Biosynthetic Hybrid Materials Research” (HYBER), Department of Forest Products Technology, Aalto University, FIN-00076, Espoo, Finland 2

College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, China

3

Department of Applied Physics, Aalto University School of Science, FI-00076, Espoo, Finland

* Authors for correspondence: (M.A.) [email protected], (O.J.R.) [email protected]. Telf. +358-50 5124227

ABSTRACT

An aerosol flow reactor was used for the first time for high-throughput, high yield synthesis of spherical lignin particles with given inherent hydrophilicity, depending on the precursor biomolecule. In-situ fractionation via Berner type impactor afforded populations with characteristic sizes ranging from ~30 nm to 2 m. The as-produced, dry lignin particles displayed 1 ACS Paragon Plus Environment


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excellent mechanical integrity, even after re-dispersion under high shear in either mineral oil or water. They were effective in the stabilization of oil-in-water (O/W) Pickering emulsions with tunable droplet size, depending on the dimension of the lignin particles used for emulsification. The emulsion stability correlated with particle concentration as well as the respective lignin type. For the O/W emulsions stabilized with the more hydrophilic lignin particles, negligible changes in phase separation via Ostwald ripening and coalescence were observed over a period of time of more than two months. Together with the fact that the lignin particle concentrations used in emulsification were as low as 0.1%, our results reveal a remarkable ability to endow emulsified systems with high colloidal stability. Overall, we offer a new, high-yield, scalable nanomanufacturing approach to producing dry spherical lignin particles with size control and high production capacity. A number of emerging applications for these organic particles can be envisioned and, as a proof-of-concept, we illustrate here surfactant-free emulsification. KEYWORDS: Lignin nanoparticles; microparticles; aerosol flow; particle fractionation; Pickering emulsions.

INTRODUCTION

Nanoparticles are among the most valuable materials in industrial and consumer product formulations.1 However, inorganic nanoparticles, especially those synthesized from metals and semiconductors may entail a given environmental impact, depending on their persistence in the environment.2 Recent interest is focused on the utilization of natural biomaterials to develop green nanoparticles with diverse advanced functionalities.3 Lignins, which represent an abundant and underutilized aromatic bioresource, are excellent candidates in the synthesis of such nanoparticles.

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Among others, they are harmless to humans and also environmentally-benign.4-5 Submicron ligninbased particles have proven distinct properties for applications in metallic ion absorption6 and emulsions.7 This is mainly due to their characteristic structure, suitable size, large surface area, and surface activity.6 Many approaches have been developed throughout the recent years to prepare lignin-based nanoparticles; however, most of the methods so far have used chemicallymodified lignins8-9 or synthetic materials similar to lignin10 that may involve solvents different than those used as suspending medium. Recently, a water-based, flash precipitation method based on pH-shift was developed to produce lignin particles.11 Moreover, it has been reported that the size of the lignin particles can be controlled.12 Lievonen et al. proposed a method for the preparation of colloidally-stable lignin nanoparticles.13 Our earlier efforts included a condensation method to obtain lignin nanoparticles of different sizes via crosslinking in a microemulsion system.14 The few reports available on the preparation of lignin-based nanoparticles are summarized in Table 1.

Table 1. Reported methods utilized for synthesis of lignin-based particles Lignin source (ref.)

Particle synthesis method Autohydrolysis treatment of wood

Yield, % 25-37

Remarks

Pinus radiata fibers16

In situ organosolv process

19-35

Exploration of lignocellulosic structures.

Furfural residues from kraft process17

Adjust pH of lignin aqueous solutions

26

pH-responsive Pickering emulsion and manufacture by emulsion polymerization.

Lignosulfonate and alkaline lignins6

Crosslinking and solidification

-

Formation of porous lignin-based spheres.

Kraft lignin14

Solvent-exchange

-

Industrial kraft lignin13

Condensation

-

Produced spherical particles with different size. Crosslinked lignin particles were synthesized.

Eucalyptus globulus wood chips15

Work in the context of cellulose accessibility in wood pretreatment.



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Wheat straw and eucalyptus wood chips15 Low-sulfonated lignin11

Kraft black liquor18

Digestion and washing

-

Lignin particles precipitation was observed on fibrous substrates.

Precipitation of lignin from an ethylene glycol solution and high-pH aqueous solution Suspension polymerization

-

Highly porous and small lignin domains

77

Smooth, spherical and crosslinked beads.

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Dry and wet milling, solvent-based processes (emulsion-solvent evaporation, emulsionsolvent diffusion, as well as precipitation) are methods often used to synthesize nanoparticles.19-22 However, milling usually results in uncontrollable changes in particle size distribution, morphology, crystallinity and also is prone to contamination. Solvent-based methods most often required organic or toxic fluids and, in some cases, may need incorporation of surfactants or stabilizers to prevent coagulation. Here, in general, there is also a need to remove the solvent during drying. Methods for solvent removal include evaporation, solvent exchange, lyophilization and spray-drying.23-24 Unfortunately, in these cases the particles may agglomerate or fuse, limiting the full exploitation of the expected features of such nanoscale materials, including their surface area and activity. In this work, we introduce an aerosol flow reactor25 for the in-situ synthesis of spherical lignin particles that can be fractionated by size. The process involves a single step to continuously atomize the lignin precursor solution, which forms an aerosol that then passes through a heating tube with the aid of a gas carrier. Dry lignin particles are then collected downstream in the system. The broad use of aerosol technologies stems from their distinct advantages over wet and solid-phase chemistry processes,26-27 namely, (1) absence of liquid byproducts; (2) simple and less expensive particle collection; (3) few processing steps; (4) continuous operation and (5) high product yield. Moreover, transport phenomena, including diffusion in gases, afford more rigorous treatment, which facilitates process design towards products of high purity


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and non-equilibrium metastable phases.26 The aerosol technique used here was recently applied in the synthesis of self-assembled nanoparticles from block-copolymers to encase pharma-molecules for drug delivery.28-29 The above described aerosol flow reactor platform was used towards the synthesis of nanoand micro-scale lignin particles. Besides the various distinct aspects discussed, the proposed method departs from efforts reported so far by the fact that we obtain dry, perfectly spherical, nonfused lignin particles that also present very high structural integrity. Moreover, the organic particles can be fractionated to produce a range of sizes and with a high yield. The self-assembly of sub-micron sized particles was tested in the stabilization of oil-water interfaces; particularly, stable Pickering emulsions were formulated, as has been also attempted recently by us and others.30,14 Traditionally, soluble or partially soluble lignins, including highand low-molecular-weight lignosulfonates,31 kraft lignins and its derivatives,32-33 as well as alkali lignins17 have been used as emulsifiers and foaming agents. In order to improve the emulsifying ability of lignin, chemical grafting has been applied to adjust the hydrophilic-hydrophobic balance,34 which can enhance the emulsion stability. A special case is that of fuel emulsions with high-internal phase ratio,7 which take advantage of lignin amphiphilic properties and also its inherent high calorific value. In this case, low sulfur, carboxymethylated lignins were found to contribute in the reduction of SO2 and NOx when fired as a fuel emulsion.35-36 At the center of interest in this paper, and as a validation of the utility of the generated particles, Pickering emulsions were obtained as a starting point towards novel structures and advanced functions37,38 including CO2/N2-switching.39 In sum, we use a new method for the synthesis of spherical lignin particles with selected sizes (in the range from ~30 nm to ~2 m), via the aerosol flow, and take advantage of the inherent



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chemical characteristics of the lignin source to tune their hydrophilicity. Finally, oil-in-water (O/W) emulsions were formulated with the as-produced, dry lignin particles.

MATERIALS AND METHODS

Kraft lignin (Indulin AT, Mead Westvaco, USA) and alkali lignin (Sigma-Aldrich, USA) were used as received. Organosolv lignin originated from beech wood was kindly supplied by the Fraunhofer Institute, Germany. Dimethylsulfomamide and Nile red (analytical grade) were used as supplied. Synthesis of spherical lignin particles. The alkali lignin was dissolved in water. Kraft and organosolv lignins were dissolved in dimethyl formamide (DMF), which is a better solvent in these cases. The concentrations of the lignin precursor solution ranged from 0.5 to 2 %. The experimental set up used to synthesize the particles consisted of a Collison-type jet atomizer with nitrogen gas.23 The generated droplets were suspended at a nitrogen gas flow rate of 3 l/min and carried to a heated laminar flow reactor that was kept at 100 or 153 oC, depending on the type of the solvent used. The reactor tube was made with stainless steel with the inner diameter and length of 30 and 80 mm, respectively. During flow-through, the droplets were dried into solid particles, which were subsequently cooled and diluted at the reactor downstream with the turbulent air flow volume of 30 l/min prior to the collection (Figure 1). The generated solid particles were collected and fractioned with a Berner-type lowpressure impactor40 that comprised 11-stages with a nominal cut-off diameter (D50) from 31 nm to


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7.817 m, providing differential size fractionation. In this study, ten size fractions with given average diameter were collected and referred to as “Fi ” where the subscript i is an even number varying from 1 to 10 to describe qualitatively the size range of the obtained aerosol samples, from smaller to larger average diameter. The abbreviations “KL”, “OL”, and “AL” are used here to indicate the type or origin of the lignin particle, namely, kraft, organosolv, or alkali lignin, respectively.

Figure 1. Simplified experimental aerosol-flow reactor set-up for the synthesis of lignin particles. Lignin particle characterization. The morphology of the lignin particles was analyzed with a field emission scanning microscopy (SEM, Zeiss Sigma VP, Germany) using an acceleration voltage of 1.6 kV. The samples for SEM were coated with platinum with 3 nm thickness. Transmission electron microscopy (TEM) imaging of the lignin particles was performed with a JEOL JEM-3200FSC TEM under zero-loss conditions at liquid nitrogen temperature. The particle size and size distribution were determined with a dynamic light scattering (DLS, Zetasizer NanoS90, Malvern). Zeta potential of the particle was also examined by using Zetasiser ZS. The 7 ACS Paragon Plus Environment


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average values of the size and zeta potential were determined from at least six measurements and the average is reported. Preparation of O/W Pickering emulsions. The spherical lignin particles were used to stabilize Pickering emulsion produced by mixing kerosene and an aqueous phase. Two of the lignin sources, KL (hydrophilic) and OL (less hydrophilic), were used to generate spherical particles for the formulation of the emulsions. For this purpose we selected KL and OL particles with a relatively small diameter (-s) (KL-s±nm andOL-s nm, respectively) and also with a relatively large diameter (-l) (KL-l 1019 ± 144 nm and OL-l =1897 nm, respectively). Emulsification was performed by sonication for 1 min, for most case, including cycles of 10 s sonication followed by 5 s pause time with 10 % amplitude (Branson S-450D microtip sonicator). The liquid droplets in the Pickering emulsions were imaged by confocal laser scanning microscope (Leica DMRXE, Germany) with or without Nile red staining. A drop of the emulsion was placed on a slide glass and covered with another glass with a spacer in between to avoid compression of the emulsion. Particle size of the droplets was determined with a Hydro 2000MU (Malvern, UK) after diluting the emulsion with a given amount of water. The Pickering emulsions were centrifuged at 2000 rpm for 2 min to measure the volume that phase-separated in the emulsion, as a measure of the relative stability. For SEM imaging purposes, emulsion samples were freezedried for 48 h followed by freezing in liquid nitrogen. Contact Angle Measurements. Solution of KL and OL in DMF (4 % w/w) were prepared to spin-coat smooth, thin lignin films onto silicon wafers, which were previously cleaned thoroughly in a UV ozone chamber (Bioforce Nanosciences Inc., California, USA) for 30 min, and rinsing with ethanol and Milli-Q water. The solution was spin-coated onto the silicon surfaces for 90 s at 3000 rpm. In order to dry the spin-coated lignin films, the samples were placed into an 8 ACS Paragon Plus Environment

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80 °C oven for 15 min. The contact angle with water was measured with a KSV CAM200 contact angle goniometer (KSV Instruments, Finland). The static sessile drop method was employed in the measurements. The tests were performed with Milli-Q water at room temperature. The full Young–Laplace equation was used to determine the contact angle from the shape of the sessile drop. The contact angle of water on two or three different substrates obtained from each type of lignin film was measured.

RESULTS AND DISCUSSION

The spherical particles synthesized by the aerosol flow reactor were obtained from the respective precursor solution (kraft, organosolv, and alkali lignin) at 0.5, 1, and 2 % concentration. Figure 2a shows the fractional yield of the lignin particles collected at each stage from F1 to F10, corresponding to nominal cut-off diameters (D50) from 31 nm to 7.8 m. The total yield was generally > 60 % based on consumed precursor solution after running the reactor for the given time. The fractional yield of the larger particles, i.e., those obtained from the stages with larger cut-off collector, namely, F5 to F10, increased with increasing lignin (KL or OL) solution concentration (Figure 2a). This increase in the fractional yield is related to the variation in apparent viscosity with precursor solution concentration, which increases the atomized droplet size.41 The particle size and the distribution of the lignin particle from 1% KL precursor solution were analyzed for the particles collected from each fraction, from F1 to F10 (Figure 2b). For this concentration, the mean particle diameter ranged from ~230 to ~1900 nm and, as was noted before, it increased with the fraction number. Finally, it can be noted that the particle size distribution 9 ACS Paragon Plus Environment


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became wider as the mean particle size was reduced. The KL or OL precursor solutions of 1% lignin concentration were selected for further experiments. This was on the basis of particle output (mass/h), particle size as well as the distribution collected from each stage.


(a)

(b) 40

Intensity, %

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Mean particle diameter, nm

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30

KL 1%

2000 1500 1000

20

500 0 0.01

0.1

1

10

CollectorD50, m

F1 F2 F3 F4 F5 F6 F7 F8 F9 F10

10

0 10

(c) OL, F5

100 1000 Particle diameter, nm

OL, F8

(d)

Figure 2: (a) Histograms with fractional yield of kraft (KL, top) and organosolv (OL, bottom) lignin particles produced in the aerosol flow reactor unit from lignin solution of different concentration. (b) Particle distribution and mean diameter of the 1 % kraft lignin precursor solution. (c) SEM 11 ACS Paragon Plus Environment


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micrographs are compared for solid lignin spheres fractions synthesized by aerosol flow of OL: F5 (left) and F8 (right). Scale bar is 2 m. (d) SEM micrographs of KL particles to illustrate the change in particle size with fraction number Fi (also shown as a plot for average size in b). Scale bar is 2 m

The surface charge of the KL particles in the fractions F4 and F8 was equivalent to a zeta potential of -39.4 ± 2 and - 42 ± 1 mV, respectively, slightly more negative than those measured for the respective OL particles: -34.1± 1 mV (F4) and -38 ± 1 mV (F8). Figure 2c includes SEM images of the KL and OL particles collected as F5 and F8 from 1 % lignin solution concentration. A sequence of SEM images with the full range of KL particle fractions is displayed in Fig. 2d. The particles distribution is inherently polydisperse but there is a clear evolution in the average size, as can be appreciated from the figure. The KL and OL solid particles are spherical with non-porous, smooth surfaces. A similar spherical morphology was obtained for particles from AL solution (not shown). The surface morphology of the particles obtained via aerosol-flow synthesis was not affected by the lignin source (kraft, organosolv or alkali lignin). Prior studies on the particle morphology after aerosol flow of synthetic polymer solutions24, 42 indicated a distinct influence of the solvent, i.e., the solubility of the polymer as well as the volatility of the solvent used.24 In our system, KL and OL were dissolved in DMF, which is a good solvent for these lignins. The spherical particle formation can be explained by a sequential process that involves firstly the evaporation of the solvent from the surface of the droplet. Concurrently, a macromolecular shell is formed on the surface of the droplet, while the solvent remaining in the interior of the droplet gradually diffuses radially outwards. Consequently, the solute condenses as the droplets evolve into dried, solid particles. Figure 3 shows a TEM micrograph of the KL solid particles (fraction F5


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in Figure 2c), indicating a uniform interior structure caused by isotropic shrinking that leads to solidified and perfectly spherical shapes.

200 nm

Figure 3. TEM micrograph of KL solid particles from the F5 fraction. Scale bar = 200 nm. One of the key characteristics of the particles synthesized is their high stability in polar and non-polar solvents. The morphology of the particles was preserved after re-dispersion in different media under heating or shear. As an example, high energy sonication (1 minute) followed by freeze-drying and final freezing at -196 oC did not affect the KL particles (Figure S-1). Lignin



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packing and cohesiveness may be reasons for such remarkable integrity, highly desired in most applications. O/W Pickering emulsions. We describe here one possible use of the lignin spheres synthesized by using the aerosol flow reactor, namely, the formulation of surfactant-free Pickering emulsions. Two critical factors influence emulsion stability, which were explored in this work: (i) the concentration of the lignin particles initially dispersed in the aqueous phase and, (ii) their size and polydispersity. In addition, we assessed the effect of emulsification time on the size of the droplets comprising the dispersed phase (oil). The oil-to-water volume ratio was fixed to 1:1 (oil volume fraction= 0.5) and two different particle types (KL and OL) were considered given their different hydrophilicity. Qualitatively, the particles were categorized as “small” ( KLs±nm

andOL-s nm) or “large” (KL-l 1019 ± 144 nm and OL-l =1897 nm) in

order to determine the effect of size on the properties of the emulsion. The particle concentration with respect to the emulsion’s aqueous media was also varied, from 0.1 to 0.6 % (w/v). Figure 4a includes photographs of a set of the Pickering emulsions stabilized by the KL and OL particles. Taking advantage of the auto-fluorescence of lignin, confocal images of the emulsions indicate particles located or adsorbed at the interface between oil (internal or dispersed phase) and water (external phase) (Figure 4b), effectively reducing the total free energy of the system. This is because the surface energy of the oil-water interface is substantially larger than that obtained from the difference between particle-oil and the particle-water.43 In case of O/W emulsions stabilized by the KL particles, negligible changes were observed in the volume that phase-separated over a period of >2 months (Figure S-2). This reveals highly stable emulsions despite the low KL particle concentration used (as low as 0.1 %) and irrespective of their size.


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(a)

KL-s

(c)

KL-l

0.8

KL-s

KL-l

0.6

%: 0.1

0.3

0.6

0.1

0.3

0.6

(b)

Retention volume

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0.4

0.2

0 0.1

40 m

0.3

0.6

KL concentration, wt%

Figure 4: (a) Photograph of kerosene-in-water Pickering emulsions (oil volume fraction= 0.5) stabilized with KL particles of two different sizes (KL-s = and KL-l =1019 nm) at particle concentrations from 0.1 to 0.6 %, as noted. (b) Auto-fluorescence images of KL particles adsorbed at the oil-water interface in a Pickering emulsion containing 0.3 % of KL-s particles (no fluorescence dye was applied). The circles are drawn as a guide to the eye. (c) Fraction of emulsion volume retained after centrifugation of the indicated Pickering system initially stabilized with small (open bars) and large particles (filled bars) and at different particle concentrations. The O/W Pickering emulsion stability against coalescence was examined by centrifugation, providing a quantitative assessment on the volume that phase-separated under the centrifugal field. Figure 4c shows the change in volume fraction after centrifugation (2000 rpm for 2 min) of emulsions stabilized with KL particles (see also Figure S-2). The emulsion volume fractions retained with the smaller KL particles (KL-s) were 0.48, 0.7, and 0.7, for particle concentrations of 0.1, 0.3 and 0.6 %, respectively. It was apparent that the smaller KL particles (KL-s) applied at high 15 ACS Paragon Plus Environment


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concentrations (0.6 %) produced emulsions with the largest retention volume of the cream layer. This indicates a greater emulsion stability as explained by the fact that at a sufficiently high particle concentration better interfacial (oil/water) coverage is produced, which in turn limits drainage between drops. Moreover, it is possible that excess lignin particles form a network in the continuous phase, surrounding the droplets, preventing drainage from the oil droplets and hindering coalescence. Typically, the smaller the size of the stabilizing particles, the lower the energy required for their removal from the oil-water interface and, thus, the lower the emulsion stability.44 In our system, however, the opposite effect was observed; namely, the smaller particles afforded higher emulsion stability. The reasons for this observation are still not clear but it is possible that size polydispersity plays a role, making particle desorption from the interface more difficult in the case of particles with smaller average size and, as a result, emulsion droplet coalescence is prevented. As anticipated, the concentration of the KL particles was found to affect the size of the oil droplets in the emulsion, which formed an upper cream layer. A reduction of the oil droplet size was noted with the increase in KL particle concentration. More specifically, the size of the droplets in emulsions stabilized with the smaller particles (KL-s) were 17.2 ± 2.1, 5.7 ± 0.2, and 5.3 ± 0.3 µm for lignin particle concentration of 0.1, 0.3 and 0.6 %, respectively. Similarly, the KL particles of larger size (KL-l) produced droplet sizes of the order of 14.2 ± 1.7, 7.8 ± 0.1, and 6.0 ± 0.1 µm, again, for particle concentration of 0.1, 0.3 and 0.6 %, respectively (Figure 5a and 5b). As can be observed, the droplet size distributions were monomodal except for the KL particles of the larger size, which were better described by a bimodal distribution (in this case a small peak at around >90 µm was observed, possibly originated by aggregation).


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The effect of the particle concentration on the stabilization at the oil-water interface can be explained by the kinetics of adsorption. During the emulsification process, particle transport to the oil-water interface becomes faster at increased particle concentration. In turn, emulsions of higher stability and smaller droplet size are obtained. It is also shown, consistently, that the volume fraction of the emulsion increases with the concentration of the lignin particles. Irrespective of the size (KL-s or KL-l) or concentration of KL particles, the average size and the polydispersity of the oil droplets increased by decreasing the emulsification time (Figure 5c illustrates emulsions obtained after 18 s emulsification, in contrast to those in Fig. 5a for 1 min emulsification). This behavior can be explained in terms of the kinetics of adsorption. During emulsification, agitation enhances transport of the lignin particles to the oil-water interface, which causes greater exposed interfacial area and reduces the interfacial tension. As a result, smaller droplets sizes are observed with emulsification time.



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(a)

KL-s

0.1 %

0.3 %

0.6 %

0.3 %

0.6 %

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40 m 0.1 %

KL-l

(b)

KL-l

KL-s m

m

(c)


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KL-s 0.1%

KL-s 0.3%

KL-l 0.1%

KL-l 0.3%

Figure 5: (a) Confocal micrograph of kerosen-in-water Pickering emulsions (oil volume fraction = 0.5) stabilized with KL particles with different size (upper, KL-s±nm and bottom, KLl

1019 ± 144 nm) and given particle concentration (0.1–0.6 %) after 1 min emulsificaton. Scale

bar=40 m. (b) The oil droplet size distribution is also displayed for emulsions stabilized by the KL particles, left, KL-s and right KL-l (0.3 and 0.6 % concentration). (c) Decreasing the emulsification time (18 s) resulted in an increased size of oil phase droplets and a wider size distribution.

As shown in Figure 6 that displays images of the emulsion cream layer after freeze drying, the polydisperse KL particles assemble as densely-packed layers around the droplets and, also, bring a characteristic roughness to the interface.45-46 The surface morphology might favor a large, attractive capillary interaction between neighboring solid particles at the oil-water interface.47-48 This results in the observed remarkably high emulsion stability against creaming, coalescence and drainage.



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1 m

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1 m

Figure 6: SEM micrographs of freeze-dried KL particle-stabilized O/W Pickering emulsions after freezing in liquid nitrogen. The micrographs were taken from the same sample at different locations. The emulsion was stabilized with the KL particles (KL-s±nm) at 0.3 % concentration. The O/W Pickering emulsions stabilized by the organosolv OL particles were found to be less stable compared with those stabilized by the KL particles. Emulsion flocculation was observed with the OL particles, which tended to remain in the oil phase soon after preparing the emulsion, as can be explained by their lower hydrophilicity (Figure S-3). In addition, compared to OL particles, the aqueous phase that separated from the emulsions stabilized with KL particles were more clear (Figure 4a). Therefore, there is indication of a limited driving force for the OL particles to adsorb at the oil-water interface, leading to a sparser interfacial packing (favoring inter-droplets coalesce). The low stability of the Pickering emulsion prepared with the OL particles is explained by considering the adsorption energy of a single particle at the O/W interface. Most relevant to the nature of the stabilizing particles is the three-phase contact angle that they assume at the interface (contact angle is defined relative to the water phase): for  < 90 o , the particles position 20 ACS Paragon Plus Environment

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preferentially in the water phase and for  > 90o the particles adsorb preferentially in the oil phase.44 Thus, in our systems, the hydrophilicity (and thus the contact angle) plays an important role in relation to the emulsion stability: interfacial rupture is favored in the case of the less hydrophilic OL particle. In turn, particle flocculation is enhanced, revealing oil surfaces that coalesce or are released into the continuous phase.

CONCLUSIONS

We demonstrate a high-throughput nano-manufacturing method that uses an aerosol flow reactor for the synthesis of spherical lignin particles. With this method, the particles can be separated in the size range between ~30 nm to ~2 m, depending on the collector number (fraction), lignin source and concentration of precursor solution. The particle synthesis is flexible as far as the lignin source and therefore the surface energy and charge of the particles can be varied depending on the precursor used. Moreover, the integrity of the synthesized lignin particles was confirmed in re-dispersion tests with mineral oil or water under high shear and upon heating or freezing. As a proof of possible uses of the lignin particles, they were shown to be effective in stabilizing O/W Pickering emulsions, the properties of which depended on particle size, concentration and water contact angle. Droplet coalescence was facilitated in the case of the less hydrophilic organosolv lignin particles, which was explained by the interplay of surface energy and interfacial packing.

SUPPORTING INFORMATION



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SEM micrograph of KL particles after high energy sonication in organic solvent, drying and freezing to test the integrity of the particles. Images of kerosene-in-water Pickering emulsions soon after preparation and centrifugation and after two months at rest. Pictures of kerosene-in-water Pickering emulsions stabilized with OL particles of different size and given concentration. Water contact angle on spin-coated lignin thin films of kraft and organosolv lignins. Corresponding Authors * Mariko Ago ([email protected]), Telf. +358-50-3841759 and Orlando J. Rojas ([email protected]), Telf. +358-50-5124227

ACKNOWLEDGMENT

This work was financially supported by the Academy of Finland through its Centres of Excellence programme (2014-2019), “Molecular Engineering of Biosynthetic Hybrid Materials Research” (HYBER). We are thankful to The Cyber-Physical Microsystems project (Academy of Finland) and the MOPPI project of the Aalto University Energy Efficiency (AEF) Program. The provision of facilities and technical support by Aalto University at OtaNano – Nanomicroscopy Center are acknowledged. We are grateful to M.S. Tero Kämäräinen for the illustration of the flow reactor.

ABBREVIATIONS

KL, kraft lignin; OL, organosolv lignin; AL, alkali lignin,

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FOR TABLE OF CONTENTS USE ONLY High-throughput Synthesis of Lignin Particles (~30 nm to ~2 m) via Aerosol Flow Reactor: Size Fractionation and Utilization in Pickering Emulsions Mariko Ago1, Siqi Huan, Maryam Borghei, Janne Raula, Esko I. Kauppinen, Orlando J. Rojas

200 nm

Lignin particle water oil


via Aerosol Flow Reactor - ACS Publications - American Chemical

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