Dual Functions of TEMPO-Oxidized Cellulose Nanofibers in Oil-in

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Dual Functions of TEMPO-Oxidized Cellulose Nanofibers in Oil-in-Water Emulsions: a Pickering Emulsifier and a Unique Dispersion Stabilizer Yohsuke Goi, Shuji Fujisawa, Tsuguyuki Saito, Kenichi Yamane, Katsushi Kuroda, and Akira Isogai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01977 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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Dual Functions of TEMPO-Oxidized Cellulose Nanofibers in Oil-in-Water Emulsions: a Pickering Emulsifier and a Unique Dispersion Stabilizer Yohsuke Goi,†,‡ Shuji Fujisawa,† Tsuguyuki Saito,† Kenichi Yamane,§ Katsushi Kuroda,§ and Akira Isogai*,† †Department

of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The

University of Tokyo, Tokyo 113-8657, Japan ‡Rheocrysta

R&D Group Life Sciences R&D Department, R&D Headquarters, DKS Co. Ltd., 5

Ogawara-cho, Kisshoin, Minami-ku, Kyoto 601-8391, Japan §Forestry

and Forest Products Research Institute, Tsukuba 305-8687, Japan

KEYWORDS: nanocellulose, Pickering emulsion, stabilization, oil/water interfacial free energy

ABSTRACT: The emulsifying and dispersing mechanisms of oil-in-water emulsions stabilized by 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-oxidized cellulose nanofibers (CNFs) have been investigated. The emulsifying mechanism was studied by changing the oil/water interfacial tension from 8.5 to 53.3 mN/m using various types of oils. The results showed that the higher the oil/water interfacial tension, the greater the amount of CNFs adsorbed at the oil/water interface, making the CNF-adsorbed oil-in-water emulsions thermodynamically more stable. Moreover, the

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amount of CNFs adsorbed on the surfaces of the oil droplets increased with increasing interfacial area. The dispersion stability of the oil droplets was dominated by the CNF concentration in the water phase. Above the critical concentration of CNFs (0.15% w/w), the CNFs formed network structures in the water phase, and the emulsion was effectively stabilized against creaming. Emulsion formation and the CNF network structures in the emulsion were visualized by cryoscanning electron microscopy.

INTRODUCTION Nanocellulose has attracted attention as a plant-derived sustainable material with high performance and functionality.1F3 Many routes to produce nanocelluloses from plant bodies, such as wood cellulose fibers, have been proposed, and various nanocelluloses are currently available in terms of the width and length, with or without branching, and different surface functional groups.4F7 A representative example is 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-oxidized cellulose nanofibers (CNFs). TEMPO oxidation can selectively convert the C6-primary hydroxy groups exposed on the approximately 3-nm-wide crystalline fibril units of cellulose in biologically structured cellulosic materials to highly dissociable carboxylate groups. TEMPOoxidized celluloses can be readily disintegrated in water under mechanical shearing to form carboxylated CNFs with uniform widths of ~3 nm.8,9 The obtained CNFs possess high strengths, high stiffnesses, low coefficients of linear thermal expansion, and large specific surface areas. To exploit these excellent properties, great effort has been devoted to development of CNF-based or -containing materials to be used as reinforcing fillers of plastics, gas-barrier packaging films, thermal insulators, air filters, and thickeners.10F14 Another function of nanocelluloses is their ability to emulsify oils in water. CNFs, cellulose


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nanocrystals (CNCs), bacterial cellulose, and their hydrophobic derivatives have been studied as stabilizers for oil/water emulsions.15F23 In such nanocellulose-induced emulsions, solid cellulose nanoparticles can form stable oil droplets by adsorbing on the droplet surfaces. These systems are called Pickering emulsions.24F32 However, there have been few attempts to gain a theoretical understanding of the formation of Pickering emulsions using nanocelluloses. A previous report suggested that the hydrophobic (2 0 0) plane of CNCs is responsible for the wettability of CNCs at the oil/water (O/W) interface, and its accessibility to oil droplets thus dominates the formation of thermodynamically stable oil droplets.33 Interestingly, TEMPOoxidized CNFs are also good Pickering emulsifiers.34F36 A common specification of these CNFs is that they should have a high density of sodium carboxylate groups on their surfaces so that the CNFs are not stained with a (2 0 0) plane-adsorbing hydrophobic dye, such as Congo red.37 However, the reason why TEMPO-oxidized CNFs can emulsify oils better than the other nanocelluloses is not known. TEMPO-oxidized CNFs are thinner and longer than CNCs, and their surface chemical structures are very specific. From previous studies, TEMPO-oxidized CNFs can function as a Pickering emulsifier with different adsorption and emulsifying mechanisms. In the present study, TEMPO-oxidized CNFs with high carboxylate content and high aspect ratio were used, as described in the following Experimental section. And oils with various interfacial tensions to water were emulsified with TEMPO-oxidized CNFs. The amounts of the CNFs adsorbed on the surfaces of the oil droplets were measured, and the relationships between the adsorbed CNF amount and the oil/water interfacial tension, interface area, and interfacial free energy were investigated. The dispersion stabilities of the resultant emulsions were also evaluated for different CNF concentrations, and the spatial distributions of the CNFs in the


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emulsions were analyzed by cryo-scanning electron microscopy (cryo-SEM).

EXPERIMENTAL SECTION Materials. TEMPO, sodium bromide, 12% sodium hypochlorite solution, sodium borohydride, n-octanol, methyl n-decanoate, n-decanedioic acid diethyl ester, n-decane, nhexadecane, cyclohexane, and the other chemicals were purchased from Nakalai Tesque, Inc. (Japan) and used as received. Preparation of the CNF/Water Dispersions. TEMPO-oxidized cellulose was prepared using the TEMPO/NaBr/NaClO oxidation system with 6.5 mmol NaClO per gram of cellulose.8 The oxidized cellulose was further treated with NaBH4 to reduce the aldehyde groups formed as the intermediate structure during oxidation.38 The carboxylate content of the oxidized cellulose was determined to be 1.8 mmol gI by conductivity titration.37,40 The TEMPO-oxidized cellulose was suspended in water at 1.0% w/w solid content. The suspension was passed twice through a highpressure homogenizer (Microfluidizer M-110EH, Microfluidics Corp., USA) at 150 MPa, resulting in production of a 1.0% w/w transparent TEMPO-oxidized CNF/water dispersion. The average width and length of the resultant TEMPO-oxidized CNFs were ~3.4 nm and ~820 nm, respectively.39,40 Preparation of the TEMPO-Oxidized CNF-Stabilized Oil-in-Water Emulsions. The 1.0% w/w CNF/water dispersion was diluted to 0.01–0.3% w/w with water. Oil was then added to the diluted CNF dispersion, and the oil-in-water (O/W) emulsions were prepared by mixing with a homomixer (3000–10000 rpm, Mark II Model 2.5, Primix Corp., Japan) for 10 min. The ratio of O/W is 20/80 by volume. The prepared emulsions were stored in graduate test tubes at room


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temperature for one week before use. All experiments were carried out (n=3) to obtain standard deviations. Critical Concentration of the CNF/Water Dispersion. The critical concentration of the CNF/water dispersion c*, or the boundary concentration between the dilute and semi-dilute regions at which the CNFs start to interact, was determined by a stress-controlled rheometer (MCR 302, Anton Paar Inc., Austria).41 Emulsion Stability Index. The amount of the white emulsified phase in the CNF-stabilized O/W emulsions was measured by reading the graduation. The emulsion stability index (ESI) was determined from the volume of the emulsified phase VE (mL) and the total volume VT (mL) using the following equation, (%) =

E T

(1)

× 100

Confocal Laser Scanning Microscopy. An n-hexadecane/water (20/80 by weight) emulsion prepared with a 0.2% w/w CNF dispersion was subjected to confocal laser scanning microscopy (CLSM) analysis. Calcofluor solution (100 N,, 0.1% w/w, calcofluor white stain, Sigma-Aldrich Co. LLC) was added to the emulsion sample (1 mL) for fluorescence staining of the CNFs. The emulsion was the diluted 4-fold with distilled water and kept at rest for one day to separate it into the white emulsion and water phases. After separation, the water phase was removed with a pipette, and distilled water was added to the remaining emulsion phase. By repeating this procedure three times, the CNFs in the water phase were completely removed. A series of these emulsion samples was observed with a confocal laser scanning microscope (Fluoview FV3000, Olympus Corporation, Japan). Measurement of Size Distributions of the Oil Droplets in Emulsions. The average


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diameters of the oil droplets in the emulsions were determined with a laser diffraction particle size analyzer (SALD-2200, Shimadzu Corp., Japan) equipped with a flow cell unit. Optical Microscopy. The emulsions were dropped onto glass plates and then observed with an optical microscope (VC 1000, Omron Corp., Japan). Determination of O/W Interfacial Tension and Calculation of Interfacial Free Energies of O/W Systems. The O/W interfacial tension "OW was measured with an interfacial tensiometer (Drop Master 500, Kyowa Interface Science Co., Ltd., Japan) by the pendant drop method. A glass container was half filled with oil and then a syringe with a stainless-steel needle was immersed in the oil phase to form a water droplet. The O/W interfacial free energy G was calculated using the following equation (2) using the value of "OW of each oil determined above and the surface area of oil droplets measured as described in the above section; G = "OW × A

(2)

Amount of CNFs Adsorbed on the Oil Droplets. The emulsion samples were centrifuged for 10 min at 3200 g. The CNF concentration in the water phase was determined by the phenol– sulfuric acid method.42,43 In brief, 5% w/w phenol solution (0.5 mL) was added to the separated portion of the water phase (0.5 mL). After mixing, concentrated H2SO4 (2.5 mL) was added and the mixture was vortexed and then allowed to stand for 30 min at room temperature. The light absorbance of the mixture at 490 nm was measured with a spectrophotometer (U-3900H, Hitachi High-Technologies Corporation, Japan). The CNF concentration in the sample was determined from the absorbance based on a calibration curve constructed using CNF/water dispersions with known CNF concentrations. The amount of CNFs adsorbed on oil droplets AA was calculated from the original and residual CNF concentrations:


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(g) =

0

100

W

×

(3)

W

where C0 is the original CNF concentration before emulsification, CW is the residual CNF concentration in the water phase after emulsification, and Mw is the weight of the total water phase. Cryo-SEM Analysis. The CNF-stabilized O/W emulsions were prepared with cyclohexane or n-octanol and a 0.2% w/w CNF/water dispersion at an O/W ratio of 20/80 (w/w). These emulsions were subjected to cryo-SEM analysis. Using a specimen holder, a droplet of the emulsified sample was cryofixed with liquid nitrogen at I196 °C. After exposing the inside of the droplet by cutting the middle part of the droplet using a cryostat (CryoStar NX70, ThermoFisher Scientific, Tokyo) at I50 °C, the specimen holder was transferred to a cryo-SEM system (JSM6510A, JED-2300, JEOL, Tokyo, Japan). The secondary electron images were obtained at an acceleration voltage of 3 kV in the cryo-SEM system after freeze-etching treatment to enhance the contrast of the cryo-SEM images and successive shallow gold coating.

RESULTS AND DISCUSSION Dilution Stability of the Oil Droplets. An n-hexadecane/water emulsion was prepared by mixing n-hexadecane with a 0.2% w/w CNF/water dispersion at an O/W ratio of 20/80 (w/w). The emulsion was homogeneous and stable after one week. When the emulsion was diluted 4fold with distilled water, the emulsion was still homogeneous immediately after dilution. However, after one day, the emulsion separated into white emulsified and transparent water phases, which is called creaming (Figure 1aFc).


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sample after 4-fold dilution (Figure 2e), the fluorescence intensity in the water phase was weak, but the CNFs were clearly present at the interface. It is interesting that even after water-phase replacement (Figure 2f), CNF elements were still localized at the O/W interface. The size distribution of the oil droplets at each stage is shown in Figure 2g. The sizes of oil droplets were almost unchanged after dilution and water-phase replacement. These results indicate that the CNFs stably adsorb on the surfaces of the oil droplets and cover each oil droplet, that is, a Pickering emulsion with hydrophilic CNFs. Effect of the O/W Interfacial Tension. To evaluate the effect of "OW, oils with different O/W interfacial tensions were used. The rational formulas of the oils and their "OW values determined by the pendant drop method are given in Table 1. The O/W interfacial tensions of the emulsified oils are in the range 8.5–53.3 mN/m. These emulsions were prepared with oil/water at volume ratio of 20/80 with a 0.05% w/w CNF/water dispersion, and were stored for one week. The AA values were then determined from the residual CNF concentration in the water phase after emulsification and subsequent centrifugation. The relationship between "OW and AA is shown in Figure 3. The results show that higher O/W interfacial tension results in higher AA. The wettability of solid particles on oil surfaces is an important factor to stabilize the oil emulsion particles through adsorption of solid particles on the oil surfaces. The desorption energy of solid particles adsorbed on the surfaces of oil emulsion particles E is given by; =

2

OW(1

cos )2 ( < 90°)

(4)

where R is the radius of the solid nanoparticle (CNF in this case) and ' is the contact angle at the O/W interface.44 Equation (4) suggests that higher O/W interfacial tension "OW results in a more thermodynamically stable emulsions caused by CNF adsorption. It has been reported that


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relationship between the rotational speed of the homomixer during emulsification and the size oil droplet size is shown in Figure 4a. There was an inverse relationship between the rotational speed and the oil droplet size. Therefore, the interfacial area of the emulsion increases with decreasing rotational speed. 80

a)

70 Size of oil droplet ( m)

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60 50 40 30 20 10 0 2000

4000

6000

8000

10000

Rotational speed (rpm)

b)

c)

10 m

d)

10 m

10 m

Figure 4. (a) Relationship between the rotational speed of the homomixer during emulsification and size of the oil droplets. Optical microscope images of oil/CNF/water emulsions prepared at rotational speeds of (b) 3000 rpm, (c) 5000 rpm, and (d) 10000 rpm.

The interfacial area A is calculated by =

E O

×

(5)

O

where VE (m3) is the oil volume in the emulsion phase, VO (m3, 4/3( 3) is the average volume of the oil droplets determined from the average radius of oil droplets r (m), and AO (m2, 4( 2) is the


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surface area of the oil droplets determined from r. r is half of the average diameter of the oil droplets determined by the laser diffraction particle size analyzer. The coverage ratio of oil surfaces with CNFs was calculated according to the following equation (6) based on a previously reported method;27 (6)

Surface coverage ratio (%) = +, × 100 where AA (g) is the adsorbed amount of CNF onto the oil droplets, ) is the density of the CNF

0.010

50

0.008

40

0.006

30

0.004

20

0.002

10

0.000

Coverage (%)

(1.7 g/cm3), and w is the width of the CNF.

AA (g)

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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0 0

2

4 6 Interfacial area A (m2)

8

10

Figure 5. Relationship between the interfacial area (A) and the amount of CNFs adsorbed on oil droplet surfaces (AA) or the surface coverage ratio of each oil droplet with CNFs.

The relationship between the interfacial area (A) and the amount of CNFs adsorbed on oil droplet surfaces (AA) or the surface coverage ratio of each oil droplet with CNFs are shown in Figure 5. The AA increased as the A was increased. During formation of a Pickering emulsion, the solid particles contributing to emulsification efficiently adsorb on the oil droplets to decrease the interfacial area.44 The O/W interfacial free energy decreases with decreasing interfacial area, and the Pickering emulsion system becomes thermodynamically more stable. Therefore, the CNF elements with large specific surface areas effectively and more quantitatively adsorb on the surfaces of the oil droplets with increasing interfacial area.


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As shown in Figure 5, the coverage ratios of oil droplet surfaces with CNFs were only ~5% regardless of the interfacial areas. When the CNCs were used in place of CNFs, the corresponding coverage ratios reached to 40F100%.27 The CNCs have sulfate ester contents of ~0.3 mmol/g and aspect ratios of

Dual Functions of TEMPO-Oxidized Cellulose Nanofibers in Oil-in

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