Chitin Nanofibrils to Stabilize Long-Life Pickering Foams and Their

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Chitin Nanofibrils to Stabilize Long-life Pickering Foams and Their Application for Lightweight Porous Materials Yao Huang, Jingqi Yang, Lingyun Chen, and Lina Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01883 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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ACS Sustainable Chemistry & Engineering

Chitin Nanofibrils to Stabilize Long-life Pickering Foams and Their Application for Lightweight Porous Materials Yao Huang1, Jingqi Yang1, Lingyun Chen1*, Lina Zhang2 1. Department of Agricultural, Food and Nutritional Science, 3-18 M Agriculture/ Forestry Ctr, University of Alberta, 116 St. and 85 Ave, Edmonton, AB, T6G 2P5, Canada 2. College of Chemistry and Molecular Sciences, 419 Southern Chemistry Building, Wuhan University, 299th BaYi Road, Wuhan 430072, China

*

Corresponding author. E-mail address: [email protected] (L. Chen). 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract

2

The demand of sustainable development is challenging researchers to convert

3

renewable resourced biomass into functional materials via environmentally friendly

4

and sustainable pathways. This work introduces a long-life Pickering foam stabilized

5

by chitin nanofibers (CNFs) as colloidal rod-like particles, and a facile method for

6

fabricating lightweight porous solid foams that recycles biomass materials derived

7

from seafood waste. These foams were formed by combining nonionic surfactant

8

Tween 20 (T20) and CNFs, with the CNFs being irreversibly adsorbed at the air–

9

water interface to provide Pickering stabilization. At a concentration of 7.5 mg/mL,

10

the foams could be stable for over a week without any apparent drainage. The

11

rheological data indicated the formation of gel networks by self-aggregated CNFs at

12

the air–water interface, which provided long-term stabilization by preventing foam

13

coalescence and disproportionation. This long-term stability of CNF-T20 wet foam

14

has permitted the fabrication of solid porous matrix by removal of the water through

15

simple air drying. The air-dried chitin foams were ultra-light weight porous materials

16

with porosity of 99.4% and a density of 8.84 kg/m3. In addition, they exhibited

17

significantly improved mechanical performance (Young’s modulus of ~290.2 kPa)

18

compared to porous chitin materials of comparable densities prepared by a traditional

19

freeze-drying method. Therefore, this research has provided a convenient pathway for

20

scalable processing of macroporous material from renewable biomass for potential

21

applications in packaging, pollutant treatment, catalysis, tissue engineering and other

22

related fields. 2


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1

Keywords:

2

Chitin nanofibers, foam stabilization, ultra-lightweight, porous materials

3



1

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Introduction

2

Foams are of great practical and research interest due to their wide spread

3

applications in food, pharmaceutical and cosmetic industries as well as for oil

4

recovery, fire-fighting processes1. A limitation of aqueous foams formed and

5

stabilized by low molecular weight surfactants or proteins is that they lack long-term

6

stability as a result of bubble coalescence and disproportionation2. Surfactant

7

molecules form thin films at the air-water interface with inadequate strength and

8

elasticity to withstand the increasing Laplace pressure or capillary forces during

9

drainage3. In recent years, Pickering foams have emerged as a superior approach to

10

enable wet foams of long-term stability4, 5. Similar to Pickering emulsion, Pickering

11

foams refer to foams

12

the interface between air and liquid. The potential of various solid particles has been

13

evaluated and show promise as foam stabilizers, such as silicon nanoparticles6, 7,

14

hydrophobic calcium carbonate particles8, hypromellose phthalate particles9,

15

octylamine or decylamine modified cellulose nanofibrils10,

16

anhydride modified starch particles12. These particles can adsorb at the air bubble

17

surface either alone or with surfactants to reduce the gas–liquid interfacial energy and

18

hinder bubble coalescence. Among them, cellulose nanofibrils and other nano-rods

19

from natural polymers have shown promising potential due to their high aspect ratio.

20

It has been suggested that anisotropic particles may be more efficient foam and

21

emulsion stabilizers than spherical particles owing to their higher surface coverage

22

and the possibility of forming intertwined networks of good mechanical strength13.

stabilized

by

solid

particles

4


which

adsorb

onto

11

, octenyl succinic

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1

Due to the outstanding foam stability and the nanofibrillar structure, foams stabilized

2

by cellulose nanofibrils could be further utilized as an intermediate structure to

3

produce lightweight macroporous materials. This would expand their applications in

4

areas such as thermal insulation, absorption materials, biodegradable packaging, drug

5

delivery and tissue engineering14-16. The increasing demand of a sustainable world

6

highly motivates the replacement of petroleum-based polymers with natural polymers

7

from renewable resources. Chitin (poly-N-acetyl-D-glucosamine) is the second most

8

abundant natural polymer after cellulose and one of the main skeletal components in

9

crustaceans, algae and fungi. However, unlike cellulose, chitin has long been

10

underdeveloped and discarded as seafood waste17 until recent decades when unique

11

structure and properties as well as the great potential of chitin as natural nitrogen rich

12

polymer in various fields has aroused global attention18. Chitin has some

13

advantageous properties over cellulose, such as in vivo biodegradability19, immune

14

regulation capacity20, wound healing properties21 and so on. In addition, it is the only

15

naturally positively charged polysaccharide with acetamido and amino groups. All

16

these properties have enabled great potential of chitin to be used in various fields like

17

pharmaceuticals, cosmetics, textiles, biosensors and water treatments22. In living

18

organisms, chitin forms nano /micro fibrillar bundles embedded in proteinous matrix,

19

with single fibril diameters varying from 2.5 to 25 nm, depending on different

20

biological origins23. Chitin nanocrystals (ChN) and chitin nanofibers (CNF) can

21

therefore be isolated from chitin extracts via chemical or physical methods and

22

applied as good candidate for functional materials24. Compared to ChN which is 5



1

generally prepared by acid hydrolysis, CNF is prepared by partial deacetylation and

2

physical grinding and exhibits higher aspect ratio and higher content of -NH2 groups

3

with better adsorption capacity25-27. Thus, CNFs have good potential to be used a

4

novel nanoparticle for Pickering emulsion and foam stabilization. Although cellulose

5

nanofibrils have been well studied in this area, few works have been reported

6

regarding Pickering foams stabilized by chitin nanofibrils.

7

On the other hand, porous chitin materials have attracted interest as heat/sound

8

insulator, catalyst supports, tissue scaffolds, filtration absorbents due to their excellent

9

biocompatibility and biodegradability19,

28, 29

. Methods have been developed for

10

producing porous chitin materials, including supercritical drying of wet gels with

11

network structure or freeze-drying of the solution/suspension28, 30. Although efficient

12

in retaining porous structures, these methods however are not suitable for mass

13

production due to time-consuming solvent exchange processes and high equipment

14

cost11, 31. If chitin nanofibers could form Pickering foams with good stability, it is

15

possible to further convert the wet foams into lightweight porous materials by simple

16

air drying. It is also expected such materials would have good mechanical properties

17

since air drying may result in more compact and homogeneous structure with strong

18

hydrogen bonding developed between the nanofibrils when compared to freeze

19

drying32.

20

Limited previous work by Tzoumaki (in 2015) suggested that chitin nanocrystals

21

(ChN) alone had poor foamability and the formed foam showed limited stability. The

22

foam could only be formed by a high energy input treatment (ultrasonication) with 6


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1

ethanol to enhance initial ChN adsorption33. This is common for colloidal particles

2

stabilized foams because they are much larger than common surfactants and therefore

3

their diffusion rate towards the interface is lower and the adsorption process takes

4

longer time34. In this work, to achieve both adequate foamability and good foaming

5

stability, a nonionic surfactant, Tween 20 (T20) was introduced to assist the foam

6

formation. T20 belongs to the sorbitan ester ethoxylates (Tweens) and have been

7

safely and extensively used in food, cosmetics and many pharmaceutical as approved

8

additives35. With T20 the surface energy was reduced to facilitate CNF adsorption on

9

the air-water interface, providing long term stabilization. The effect of the CNF

10

content on the foam stability was studied. The stabilization mechanism of CNFs was

11

further investigated by confocal microscope and rheological tests. Furthermore, the

12

CNFs stabilized T20 foams were converted into lightweight solid chitin foams by a

13

convenient approach of air drying and their physical and mechanical properties were

14

evaluated.

15

Experimental

16

Materials

17

Raw chitin powder was purchased from Zhejiang Golden-Shell Biochemical Co.

18

Ltd. (China) with a degree of acetylation (DA) of 90%. The weight-average molecular

19

weight (Mw) was determined to be 5.96 × 105in 5% (w/v) LiCl–DMAc by dynamic

20

light scattering (DLS, ALV/GGS-8F, ALV, Germany). All Other chemical reagents

21

were purchased from Sigma-Aldrich Canada Ltd.

22

Preparation of partially deacetylated chitin nanofibers (CNF) 7



1

The CNF suspension (CNFs) was prepared according to our previous work36.

2

Briefly, partially deacetylated chitin with a degree of deacetylation of 19.8%

3

(determined by titration with alkali according to previous report37)were prepared from

4

the commercial chitin powder by reacting with 33 wt% NaOH solution and used as

5

the starting materials for CNFs preparation. Partially deacetylated chitin powder (2.5g)

6

was suspended in 500 mL dilute acetic acid (HAc) solution (pH 3–4). The resultant

7

suspension was stirred at room temperature for 3 d and then treated with IKA

8

ultrasonic homogenizer for 10 min, followed by sonication on an ultrasonic cell

9

disruptor (JY98-IIIDN, Ningbo Scientz Biotechnology Co., Ltd., China) at a power of

10

800W for 1 h in an ice bath. During this process, the pH value was maintained in

11

range of 3–4 to facilitate the nanofibrillation. Subsequently, the suspension was

12

centrifuged at 10,000 g to remove any visible precipitate. The supernatant was

13

collected and concentrated by rotary evaporation. The excessive acid in the CNFs

14

suspension was removed by dialysis against distilled water until the pH was about 6.0.

15

The concentration of the aqueous CNFs suspension was measured by the solid content

16

upon oven drying at 60oC.

17

CNF-T20 composite foam preparation

18

To prepare the wet composite foam, 0.5 mL of 5 wt% Tween 20 (T20) aqueous

19

solution was added to 9.5 mL of CNFs with different concentrations

20

(0mg/mL,1mg/mL, 2.5mg/mL, 5mg/mL, 7.5 mg/mL). The resultant mixture was

21

stirred for 30 min on a magnetic stirrer, degassed for 5 min on an ultrasonication

22

before a foaming process of 4 min on a high-speed homogenizer (PowerGen 1000, 8


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1

Fisher Scientific, Fairlawn, NJ, USA) at speed six. The resultant foams were denoted

2

as T20, CNF1, CNF 2.5, CNF 5, CNF 7.5, respectively.

3

Solid Foam preparation

4

For solid composite foam preparation, 10 mL composite solution (9.5mL 7.5

5

mg/mL CNFs with 0.5 mL 5 wt% T20) was foamed in a circular mold. The foam was

6

neutralized with the atmosphere of liquid ammonia deposited in a desiccator for 12 h

7

and then either freeze dried or air-dried (on top of a filter paper) at ambient

8

temperature (23oC). The resultant solid foam was denoted as SF-FD and SF-AD

9

according to different drying processing. For comparison, 10 mL pure CNFs

10

suspension with a concentration of 8 mg/mL was freeze dried and the resultant porous

11

sponge was denoted as CF-FD.

12

Characterization

13

wet foam properties

14

The morphology of the chitin nanofibrils was observed by transmission electron

15

microscopy (TEM) on a JEOL JEM-2010 (HT) high-resolution electron microscope,

16

with an accelerating voltage of 200 kV. Foaming capacity and foam stability was

17

determined as in previous study3. The composite suspension of CNFs and T20 was

18

deposited in a cylinder container with scale for foam formation and the volume of

19

suspension and foam was measured at different time. The foaming capacity (FC) and

20

foam stability (FS) was calculated as:

21 22

FC (%) = Vf/Vs ×100 FS (%)= (Vt-- Vs)/ (Vf - Vs) ×100 9


(1) (2)


1

where Vf and Vs are the volume of the formed wet foam and the initial volume of the

2

suspension, and Vt refers to the volume of foam at time t.

3

The foam bubble size was observed by an optical microscope equipped with a

4

digital camera (OPTIKA, Italy) immediately after foaming process. Using an image

5

analysis software Nano Measurer (Fudan University, China), the size distribution of

6

the bubbles was calculated (at least 300 bubbles were counted for each sample). The

7

foam morphology was further observed by confocal microscope with FITC labeled

8

CNF. For FITC labeling, 5 mg FITC was added to 1 mg/mL or 7.5 mg/mL CNFs

9

suspension. The mixture was stirred for 3h in dark and then dialyzed against distilled

10

water for 1 week. The foaming process was consistent with mentioned above and the

11

corresponding confocal images were taken on an inverted Zeiss LSM710 laser

12

scanning confocal microscope(Carl Zeiss, Jena, Germany) using 20× objectives at

13

488 nm. The CNFs composite foam was also imaged by a digital camera at different

14

time after foaming.

15

Rheological measurements of the wet foam were analyzed on a DHR3 rheometer

16

(TA Instruments, Delaware, USA) fitted with a cone plate geometry with a diameter

17

of 35 mm and angle of 2° according to previous reports38, 39.The foam was prepared

18

according to the procedure mentioned in part 2.3 and immediately transferred to the

19

cone plate for test. Strain sweep tests with strain amplitudes ranging from 0.01%

20

to100% were performed at 1Hz in order to establish the linear viscoelastic range.

21

Based on these results, oscillatory frequency sweep measurements (0.1-100 Hz) were

22

carried out at a strain amplitude of 1%, and the storage modulus G’, loss modulus G’’ 10


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1

and loss tangent tan (δ)G’’/G’ were recorded. Time sweep tests were also carried out

2

for 3000s at 1% strain and a frequency of 1 Hz for analyzing the stability of foam

3

texture as a function of time. In addition, shear rate dependent apparent viscosity data

4

were collected as shear rate was increased linearly between 1 and100 s-1 over a total

5

run time of 5 min. During the analysis, the environment temperature was kept at 25oC.

6

These tests were aimed to detect the evolution of the foam structure resulted from the

7

destabilization mechanisms. Each sample measurement was replicated at least three

8

times for statistical purpose. Coalescence of droplets was not considered when the

9

rheology measurements were done.

10

Solid foam properties

11

The volume of the dried foams was measured with a caliper and the shrinkage

12

and density (dp) were estimated from the volume of the wet foam and the mass of the

13

added components. The porosity of the obtained materials was estimated by the

14

following equation:

15

Porosity (%) = (1-dp/db) ×100

16

where dp is the density of the porous material and db is the density of the bulk chitin

17

(1.425 g/cm3)28. Five individual tests were performed and data were averaged. The

18

mechanical properties of the dry foams were evaluated by compression tests on an

19

Instron 5967 universal testing machine (Instron Corp) equipped with the “Bluehill

20

Lite” analysis software. The temperature was 23oC and the relative humidity was 33%.

21

The compressing speed was set at 1mm/min. Tests were performed on foams with

22

circular diameters and heights of 20 mm and 10 mm, respectively. Morphology 11


(3)


1

observations of the air dried and freeze-dried foams were carried out with a Zeiss

2

EVO M10 scanning electron microscope (SEM) at an acceleration voltage of 5 kV.

3

Samples of dry foams were prepared by cutting the foams into several slices by a

4

scalpel and coating with a thin layer of gold prior to observation and photographing.

5

Statistical analysis

6

All experiments were performed in at least three independent batches. For all the

7

characterizations, at least three parallel measurements were taken for each sample and

8

the result was presented as average value ± standard deviation (SD).

9

Results and discussion

10

Effects of CNFs on foamability and foam stability of T20 wet foam

11

Through partial deacetylation and mechanical disintegration, catanionic chitin

12

nanofibers with diameters of around 20 nm and lengths from hundreds nanometer to

13

several micrometers (TEM images shown in Figure 1b) were obtained. As a result of

14

the increased -NH2 groups caused by deacetylation, the CNFs were easily dispersed in

15

acidic solution and formed a stable suspension. However, due to the relatively large

16

size of the nanofiber, the foamability of sole CNFs suspension was rather poor (data

17

not shown) and no stable foams can be formed by conventional foaming process such

18

as shaking or homogenization at tested concentrations. This, is similar to the

19

previously reported chitin nanocrystal suspension which was not capable of forming

20

stable foam at low concentration without the aid of high energy input and ethanol

21

assistance33. Therefore, T20 was used in combination with CNFs for foam formation

22

and stabilization. In a preliminary experiment, we tested CNFs in combination with 12


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1

different types of surfactant solution (0.25wt%) including nonionic surfactants

2

Pluronic F-68, Triton X-100 and Tweens (Tween 20, Tween 40, Tween 80), cationic

3

surfactant cetyltrimethylammonium bromide(CTAB) and anionic surfactant sodium

4

dodecyl sulfonate (SDS). The corresponding results are shown in Figure S1. After 3h

5

post foaming, CNFs in combination with almost all surfactants showed improved

6

foam stability, except that CNFs aggregated in SDS solution since they are oppositely

7

charged. By increasing the time to 18h, however, only foams formed by nonionic

8

surfactants with CNFs showed good stability, among which those stabilized by

9

Tweens and CNFs behaved the best. Thus, T20 was chosen as a model surfactant in

10

this work. As we hypothesized in Figure 1a, with the initial aid of T20, CNFs would

11

be adsorbed at the air-water interface during the foaming process. For small molecule

12

surfactant, the adsorption and desorption are usually a dynamic and reversible process.

13

While in most Pickering stabilization system, the solid particle adsorption is generally

14

irreversible and the desorption is not a rapid and automatic process40. In this work,

15

after being adsorbed at the air-water interface during the foaming process, the

16

self-aggregation and gelation tendency of CNFs made this adsorption an irreversible

17

process since the CNFs cannot be desorpted and re-dispersed automatically after

18

being immobilized in the gelation network. This irreversible process would

19

significantly improve the foam stability against drainage and coalesce, contributing to

20

the solid porous structure upon water removal by drying.

21

For the wet foams preparation, composite suspension of T20 with different

22

concentration of CNFs (1-7.5 mg/mL) was foamed on a high-speed homogenizer and 13



Page 14 of 37

1

observed by a microscope. The corresponding photographs and the bubble size

2

distributions are shown in Figure 2. With the addition of CNFs, the spherical shape of

3

the bubbles formed by T20 was retained but the mean size decreased with the

4

increasing CNFs concentration. For foams stabilized only by T20, the air bubbles

5

have a mean diameter of 94.5±36.8 μm, while the diameter of foams with CNF7.5

6

were decreased to 49.4±16.5μm. It is also obvious that distribution range of the air

7

bubble size became narrower with increasing CNFs content, indicating that the

8

addition of CNFs facilitates the formation of foam with smaller and more uniform air

9

bubbles. On the other side, due to the foam formed by sole T20 has poor stability, the

10

coalesce of air bubbles and drainage immediately occurred upon preparation (Figure

11

4b). During this process, the size of the air bubbles kept increasing and the

12

distribution became irregular. While with the introduction of chitin nanofibers, the air

13

bubbles could be stabilized and protected at an early stage and therefore the smaller

14

and uniform sizes were retained. It is similar to the nano silica particles stabilized

15

foam, where such trends are explained by the fact that an increase

16

concentration

17

for the particles to diffuse and adsorb on to the surface of the air bubble7.

and a

decrease

in particle

size

reduce

14


the

in

time

particle required

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1

2 3

Figure 1. Schematic illustration of the CNFs-T20 stabilized foam and its conversion

4

into solid foam by air drying (a); TEM images of diluted CNFs suspension in water (b)

5

and photographs of CNF7.5 foams at wet state (c) and after being air-dried (d).

6

15



1

Figure 2. Microscopic photographs of T20, CNF1, CNF 2.5, CNF 5, CNF 7.5 under

2

different magnifications (scale bar: 200 μm for top and 50 μm for middle) and the

3

statistical distribution of the bubble diameter (bottom).

4

At the same time, the effects of CNFs addition on the foamability and foam

5

stability were further studied. As shown in Figure 3, when CNFs concentration

6

increased from 0 to 7.5 mg/mL, the foamability of the composite suspension was

7

decreased from 283.5±23.6 % to 207.1±6.6 %. T20 is a low molecular weight

8

surfactant which has lower adsorption energies and can diffuse rapidly to the surface

9

within a limited time40. The addition of colloidal particle like CNFs would inevitably

10

reduce the foamability because they have larger size and higher adsorption energies,

11

therefore their diffusion rate towards the interface is lower, which means at the same

12

condition, they might require longer time to be adsorbed when compared to small

13

molecular surfactant33, 34. Nevertheless, the foamability of CNF7.5 is still much higher

14

than the that of the sole chitin nanocrystal (lower than 75%)33. The evolution of the

15

foam volume of different samples as a function of time is plotted in Figure 3b. In

16

contrast to the reduced foamability, the foam stability was largely enhanced by the

17

addition of CNFs. Like most foams stabilized by sole surfactant, the T20 foam

18

displayed a short lifetime and the foam collapsed entirely within 12 h. There was a

19

significant increase of foam stability with the increase of CNF concentrations. For

20

CNF1 and CNF2.5, the foam volume was around 50% at 12h but collapsed by the end

21

of the test time (3d). For CNF5, the foam volume retained above 66% after 3d and the

22

percentage further increased to 96% with CNF concentration reached 7.5 mg/mL, 16


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1

which is comparable to foams stabilized by octylamine modified cellulose nanofibrils

2

at the concentration of 10 mg/mL10. The outstanding stability of T20 foams stabilized

3

by CNFs was further recorded by photographs over different time in Figure 4. For

4

T20 foam, the drainage of water from between the bubbles began to appear

5

immediately after the foaming process, with a clear drainage solution at the bottom

6

and a concentrated foam at the top. In the case of CNFs stabilized foam, the drainage

7

took place relatively slowly and was only obvious after several hours (Figure 4c-d).

8

Especially, the drainage solution at the bottom was transparent, different from the

9

original opalescent CNFs suspension. It is likely that the after the CNFs were

10

adsorbed at the air-water interface, the self-entanglement and gelation behavior

11

confined the mobility of CNFs thus they could not be automatically re-dispersed in

12

water even after the foam drainage. When the CNFs concentration was high enough

13

for sufficient surface coverage (7.5 mg/mL), the drainage was almost negligible.

14

Bubble coalescence and disproportionation lead to a direct consequence of coarsening

15

of T20 foam. The adsorbed CNFs, however, clearly prevented the occurrence of both

16

phenomena, and thus enhanced the foam stability. Unlike most foams stabilized by

17

small molecular surfactant or protein, which generally can only be stable for

18

minutes40, 41, the foams stabilized by T20/CNF exhibited long-term stability up to

19

weeks. As shown in Figure 4f, after 1 week of preparation, the CNF5 and CNF7.5

20

foams both displayed good stability with gel like behavior. This was due to the

21

entanglement and gelation of CNF suspension in neutral solution, which have been

22

elaborated in previous works

42-44

. By partially deacetylation, cationic CNFs with 17



1

increased content of -NH2 groups are easily self-aggregated and gelated at a pH near

2

or higher than the pKa of chitosan (6.2-6.8) due to their large aspect ratio and reduced

3

surface charge caused by deprotonation of its -NH3+groups42, 45. The reduced surface

4

charge decreased the electrostatic repulsion, leading to intertwined fibrillar networks10.

5

As we measured the ξ-potential of the CNF aqueous suspension, it decreased from

6

51.9±1.6 mV (pH 3.3 upon preparation) to 33.5±0.8 mV (pH 6.0 after dialysis) and

7

24.0±1.7 mV (pH 6.4 after pH adjustment). When the pH was higher than 6.4, no

8

homogeneous dispersion of CNFs could be obtained as a result of the

9

self-aggregation.

10 11

Figure 3. Foamability (a) and foam stability (b) of T20-CNFs foams with different

12

concentration of CNFs.

13

Since the main foaming agent in the present work was T20, it was necessary to

14

show that there is indeed an accumulation of CNFs around the air bubbles. This was

15

done by using confocal microscopy for observation of the foams stabilized by FITC

16

labeled CNFs. The result in Figure 5 shows that NFC formed a lamellar around the air

17

bubbles and the layer thickness increased obviously as the CNFs concentration

18

increased from 1 mg/mL to 7.5 mg/mL, indicating that for CNF7.5 there was 18


Page 18 of 37

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1

sufficient amount of CNF adsorbed and accumulated at the air−water interface to

2

protect the thin film formed by T20. That’s in accordance as what we hypothesized in

3

Figure 1a. The presence of self-aggregated CNFs between the foam films (Figure 5b)

4

increased the stability of the film by providing steric stabilization and also reduced the

5

drainage out of the films, both stabilizing the foams against coalescence and

6

coarsening46. This was consistent with the Pickering stabilization where particles

7

irreversibly adsorbed at the interface to sterically impede the coalescence of

8

neighboring bubbles and form a coating layer to restrict bubble shrinkage and

9

expansion, minimizing Ostwald ripening and creating long-lasting stable foams2.

10 11

Figure 4. Photographs of T20, CNF1, CNF 2.5, CNF 5, CNF 7.5(from left to right)

12

suspension before foaming (a), immediately after foaming (b), 3h (c), 24 h (d), 3 d (e)

13

and 7 d (f) after foaming. 19



Page 20 of 37

1 2

Figure 5. Confocal images of CNF1 (a) and CNF7.5 composite foams stabilized by

3

FITC-labeled CNFs. Scale bar: 50 μm

4

Bulk rheological properties of the wet composite foam

5

Foam stability and texture was further investigated by rheological testing of the

6

bulk foam. The

7

the continuous

8

the

9

the drainage rate is inversely proportional to the apparent viscosity, which means that

10

a greater apparent viscosity may represent a greater stability against drainage48. The

11

dependence of apparent viscosity on shear rate is shown in Figure 6. Yet all foams

12

stabilized with either T20 alone or T20-CNFs mixtures exhibited shear thinning

13

behavior, which has been also observed in aqueous dispersion of chitin nanocrystals49

14

and other foams stabilized by surfactants or proteins with or without polysaccharide

15

additives38, 50, 51. The shear thinning behavior has played an important role for foams

16

to be processed and applied in food or cosmetics field, as well as in oil recovery52. At

17

the same time, with the increase of CNFs concentration, the apparent viscosity of the

18

foam increased prominently, especially at low shear rate, indicating enhanced stability

rate

of

viscosity

of

phase around foam

the

bulk

the foam

phase bubbles

affects

the

mobility

and therefore

of

influences

drainage47. Previous work regarding foam rheology found that

20


Page 21 of 37 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

of the foam against drainage. Viscosity at low shear rate may generally be related to

2

long-term stability in foams since shear rate range of 0.01s

3

gravity induced drainage occurs38.

-1

to 1s

-1

is where typical

4 5

Figure 6. Apparent viscosity T20-CNFs stabilized foams as a function of shear rate.

6

Dynamic oscillatory tests have long been utilized to estimate the texture of

7

emulsions and foams in food and cosmetic industry53. Figure 7a shows the storage

8

modulus (G’) and loss modulus (G’’) of T20, CNF1 and CNF 7.5 foams at different

9

frequency. The tan(δ) value (ratio of G’’ /G’) is presented in Figure S2. The

10

equilibrium between elastic (G’) and viscous (G’’) contribution in the foam modulus

11

was greatly changed with addition of CNFs. For T20, G’’ was slightly greater than G’

12

with a tan(δ) above 1 for most of the frequency range, suggesting the viscous nature

13

of the T20 foam. While with addition of CNFs, G’ was definitely higher than G’’,

14

showing that elasticity became the main contributor in modulus. With low CNFs

15

concentration of CNF1, G’ was highly dependent on frequency just like that of T20.

16

While increasing the CNFs to 7.5 mg/mL led to a near frequency-independent storage 21



1

modulus with G’ higher than G’’ and a tan(δ) below 0.4 for almost the entire

2

frequency range, corresponding to a gel structure with solid-like behavior54, which

3

confirmed the gelation tendency of chitin nanofibrils. This gelation behavior confined

4

the air bubbles within the network composed of CNFs, which hindered the drainage

5

and collapse of the foam further. This concentration dependent gelation has also been

6

reported in a previous study regarding the suspension of chitin nanocrystal (ChN), but

7

at a much higher concentration55. With larger aspect ratio and increased -NH2 contents,

8

the gelation of CNFs used in this work occurred at lower concentration43. That’s why

9

CNFs were able to stabilize the foam at such low concentration such as 5 mg/mL,

10

whereas in other systems like fatty acid modified alumina particles stabilized foam, a

11

minimum particle concentration of 15 v/v% would be required to form long-life

12

foams7.

13

Changes in storage moduli G’ also help to understand some of the under-lying

14

mechanisms of foam stability since G’ is dependent on bubble diameter and liquid

15

fraction in the foam. During drainage, if the bubbles get bigger, faster drainage will

16

occur, implying a drier foam and then even faster coarsening. In such case, G’ will

17

continuously decrease as the foams coarsen and collapse56. Figure 7b shows the aging

18

behavior of the foams presented by G’ as a function of time. Within 1 hour after

19

foaming, G’ of the T20 foam dropped continually. This tendency was slowed down by

20

addition of CNFs, but still presented at lower concentrations in CNF1 and CNF2.5.

21

For CNF5, this decrease was almost negligible. When the concentration reached 7.5

22

mg/ml, G’ even increased over the time range (from 130.8 Pa to 206.6 Pa), which 22


Page 22 of 37

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1

further contributed to the long-term stability of the CNF7.5 foam. Foam stability

2

against drainage mainly depends on the initial solution viscosity and the presence of

3

stabilizing agents in the liquid films and at the Plateau border57. A greater storage

4

modulus resulted from the gelation of CNFs would intensify jamming of liquid

5

confined to a narrow Plateau border (liquid-carrying channels formed where three

6

lamellae meet) between droplets50. Under this jamming effect, the foam will change

7

from exhibiting a liquid-like behavior towards a solid-like behavior and thus an

8

increase of the G’ value could be observed56.

9 10

Figure 7. Storage modulus (G’) and viscous modulus (G’’) of T20, CNF1 and CNF

11

7.5 foams at different frequency (a); Evolution of G’ over time for T20 and composite

12

foams (b).

13

3.3 Preparation and Properties of the solid composite foams

14

In recent years, macroporous polymeric materials have been extensively applied

15

in areas such as opto-electronic devices, sorbent materials, catalysis, chromatography

16

and tissue engineering58-60. Based on above results, the CNFs were absorbed on the

17

air-water interface with the aid of T20 and then self-aggregated and gelated,

18

contributing to the long-term stability of the composite foams. Thereafter, the 23



1

outstanding stability of the foam has allowed us to fabricate bulk macroporous

2

structures by drying the wet foams. We fixed the initial CNFs concentration at 7.5

3

mg/mL and tried two different drying methods: freeze drying and air drying, and the

4

obtained foams are coded as SF-FD and SF-AD, respectively. The CNFs suspension

5

without T20 (8 mg/mL) was also freeze dried (CF-FZ) for comparison since there are

6

very few reports revealing the mechanical performance of chitin aerogel fabricated

7

from such low concentration. Even though supercritical drying has been applied to

8

fabricate lightweight aerogel from chitin nanofiber suspension with concentrations

9

less than 10 mg/mL, no specific information regarding their mechanical performance

10

have been clearly revealed29, 61. Table 1 shows the physical properties of the resultant

11

porous materials. The freeze-dried foam (SF-FD) exhibited the lowest density (5.6

12

kg/m3) and highest porosity (99.6%) owning to the foaming and subsequent

13

freeze-drying process. To our findings, one inevitable result for air dried foam is a

14

considerable volume shrinkage after drying, which has also been mentioned in

15

previous reports. To minimize this shrinkage, the wet foams were treated with an

16

ammonia atmosphere before air drying at ambient temperature. And the effect of this

17

treatment on restraining the shrinkage was revealed in Figure S3a. The reason behind

18

this was ascribed to the self-entanglement and gelation of CNFs under neutralized

19

condition which formed rigid network structure to prevent drastic dehydration and

20

shrinkage during drying. This pH-dependent gelation behavior of CNF suspension

21

was clarified by Mushi42 and Fan43 et al. Since the packing of the CNFs are slow

22

processes, it takes almost 7 days to obtain a self-standing foam gel with tolerable 24


Page 24 of 37

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1

strength at the foaming pH of 6.0. When subjected to an ammonia atmosphere the

2

gelation of CNFs was accelerated. As shown in Figure S3b, after treatment of 12 h, a

3

self-standing translucent hydrogel was formed from the aqueous suspension of CNFs.

4

As for the CNF7.5 wet foam, a similar gel with foam appearance could be obtained

5

(Figure 1c) which almost retained its cylinder shape but with decreased volume after

6

being air dried (Figure 1d). The resultant foam SF-AD (air drying after ammonia

7

atmosphere treatment) exhibited a density of 8.84 kg/m3 and porosity of 99.4%,

8

similar to that of CF-FD (freeze drying of sole CNF suspension without T20 addition).

9

It is worthy of mentioning that for the sole CNFs hydrogel without T20 and foaming,

10

only a translucent thin film with dense aggregation of CNFs could be obtained (Figure

11

S3c) by air drying, instead of the porous materials.

12

Table 1. Physical properties of solid porous CNFs materials prepared by different

13

drying methods. Sample

dp

Porosity

Young’s

Yield

Yield

kg/m3

(%)

modulus (kPa)

Strain(%)

Strength(kPa)

CF-FD

8.67±0.72

99.4±0.05

79.6±13.1

53.4±2.1

30.5±8.3

SF-FD

5.60±0.74

99.6±0.04

40.0±11.4

76.8±3.1

44.7±5.9

SF-AD

8.84±0.94

99.4±0.06

290.2±55.1

70.4±4.0

57.9±8.8

Ref16

13±2

99.1±0.1

200±80

N/A

24±6

Ref16

6±0.7

99.6±0.1

200±30

N/A

6.0±0.1

14

The mechanical properties of the foams are also listed in Table1 and the

15

representative compressive stress-strain curves are shown in Figure 8. As an aerogel

16

by direct freeze-drying of sole CNF suspension without T20 and foaming, CF-FD

17

exhibited a moderate modulus (79.6 kPa) but the least yield strain (53.4%) and yield 25



1

strength (30.5 kPa), indicating the rigidity and brittleness of pure CNF materials. As

2

reported in a previous work involving a freeze-dried chitin nanocrystal (ChN) aerogel,

3

the strain value would further decrease with increasing concentration of ChN62. As for

4

SF-FD (freeze dried CNF/ T20 foams), the yield strain and strength were improved to

5

76.8% and 44.7 kPa by addition of T20, while the modulus was reduced to 40.0 kPa.

6

This plasticization effect of T20 has also been observed on starch film with possible

7

explanation that T20 is small hydrophilic molecule that can interact with water to

8

facilitate its presence between polymer chains, increasing chain mobility and

9

enhancing the initial plastic effect63. By air drying, however, the mechanical

10

performance was largely enhanced. SF-AD exhibited a Young’s modulus of 290 kPa

11

with a yield strength of 57.9 kPa, which were even higher than the reported air-dried

12

foams (before crosslinking) stabilized by modified cellulose nanofibrils at comparable

13

densities16 (as reference 16 listed in Table 1). At the same time, the yield strain was

14

above 70%, showing good elasticity of the solid foam. On the other side, SF-AD also

15

exhibited good water resistance and retained its intergrity when immersed in water

16

with the aid of a tweezer ( Figure S4). That could be ascribed to compact packing of

17

chitin nanofibers and formation of strong hydrogen bond between them during

18

air-drying, which prevents them from disintegration in water64, 65. As shown in the

19

insert picture of Figure 8, a 1cm3 cubic cut from bulk SF-AD foam could stand on the

20

slender leaf tip of a bracket-plant without crushing it down, indicating its ultra-light

21

weight property and good mechanical integrity against external shear.

26


Page 26 of 37

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1 2

Figure 8. Compressive stress-strain curve of solid porous CNFs materials prepared by

3

different drying method; insert shows photograph of a piece of 1cm3 SF-AD on the

4

leaf of bracket-plant.

5 6

Figure 9. SEM images showing the cross section of CF-FD (a), SF-FD (b), SF-AD (c)

7

and their corresponding enlarged view in (d), (e), (f), respectively. Scale bars: 20 µm

8

in a, e, f; 100 µm in b, c and 5 µm in d.

9

To further demonstrate the effect of different drying methods on the morphology

10

of the solid foams, SEM images for the cross-section of various foams are shown in

11

Figure 9. For foams prepared by freeze drying the CNFs suspension, small submicron 27



1

pores could be observed (Figure 9a), similar to previous studies66. With the addition

2

of T20 and the foaming process, a hierarchical porous structure formed (Figure 9b).

3

Besides the small pores induced by ice crystallization, larger pores with diameters of

4

50-80 µm were observed. This pore size was consistent with the air bubble sizes

5

revealed in Figure 2, which was preserved during the freeze drying. The foam walls of

6

both freeze dried samples appeared as thin films consisted of nanofibrous CNFs,

7

which are clearly presented in an enlarged view (Figure 9d-e) around the edges of the

8

pore walls. Upon air drying, this nanofibrous structure and the nanopores became

9

invisible due to a more compact packing induced by the drastic dehydration of air

10

drying64. The pore size further increased to 100-200 µm (Figure 9c and f) due to the

11

coalesce of bubbles during the longer period of drying. This structure is similar to of

12

air-dried foams prepared from modified cellulose nanofibrils16. Lamellae of the foam

13

were composed of densely packed CNFs, consistent with the confocal images in

14

Figure 5. Compared with the freeze-dried foams, the lamellae of SF-AD showed more

15

solid texture and greater thickness. This can be explained by an initial adsorption and

16

accumulation of CNFs in the wet foam, followed by a further gelation and packing of

17

the CNFs during the drying16. This is in accordance with the mechanism we proposed

18

for the good stability of the wet foam, furthermore, it also explains the good

19

mechanical performance of the solid foam, naturally related to the excellent

20

mechanical performance of the native α-chitin nanofibrils with an estimated elastic

21

modulus of ~59.3 GPa67 and tensile strength value of (~1.6 GPa)68. On the other hand,

22

strong intra- and inter- hydrogen bonding between the nanofibrils as well as the high 28


Page 28 of 37

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1

crystallinity of chitin could be well maintained in the compact packing structure of the

2

pore wall observed for the air-dried samples32. These structures could further

3

contribute to the better mechanical performance of the air-dried foams, which would

4

have been impaired by the ice crystal formation and growth during freeze drying69.

5

Conclusion

6

For the first time, chitin nanofibers were applied as Pickering stabilizer in a

7

nonionic surfactant aqueous foam, which was further utilized for the fabrication of

8

ultra-lightweight porous chitin materials by simple air drying. With the addition of

9

CNFs, the foam tended to compose of smaller and more uniform bubbles. Here, the

10

formability was slightly decreased but the foam stability was greatly improved. When

11

the CNFs concentration reached 7.5 mg/mL, the foam could keep stable for at least a

12

week without obvious drainage and collapse. This long-term stability was ascribed to

13

the irreversible adsorption and gelation of CNFs at the water-air interface. By freeze

14

drying or air drying the CNF-T20 wet foam, ultra-lightweight solid foams were

15

successfully fabricated. The air-dried foam exhibited good mechanical performance

16

with a Young’s modulus and yield strength much higher than those of the freeze-dried

17

foams from CNF suspension at similar concentration, as a result of the compact

18

packing and restored hydrogen bonding induced by air drying. In addition, the

19

air-drying processing is convenient for industrial processing on mass production of

20

macroporous materials. This work put forward a promising simple and

21

environmentally friendly method for the fabrication of porous chitin material with

22

good mechanical performance, showing potential applications in degradable 29



1

packaging materials, pollutant absorbent and so on.

2 3

ASSOCIATED CONTENT

4

Supporting Information

5

Supporting Information Available: [Figure S1. Photographs of foams from different

6

surfactants solution (0.25 wt%) with and without CNFs (7.5 mg/mL) at different time

7

after foaming; Figure S2. Tangent delta of wet composite foams as a function of

8

frequency; Figure S3. Photographs of a self-standing hydrogel obtained by CNFs

9

suspension (8 mg/mL) treated with ammonia atmosphere for overnight (a) and the

10

membrane obtained by direct air drying of this hydrogel; Solid foams obtained by air

11

drying the CNF7.5 wet foams (initial solution volume of 10 mL) with (left) and

12

without (right) ammonia atmosphere treatment (c).; Figure S4. Photographs of SF-AD

13

floated on water (a) and after being immersed in water with the aid of a tweezer for 30

14

min (b)]. This material is available free of charge via the Internet at http://pubs.acs.org

15

Corresponding Authors

16

* Lingyun Chen. E-mail: [email protected]

17

ORCID

18

Yao Huang, 0000-0001-9071-4389

19

Jingqi Yang, 0000-0002-5762-5031

20

Lina Zhang, 0000-0003-3890-8690

21

Lingyun Chen, 0000-0002-8956-7358

22

Notes 30


Page 30 of 37

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1

The authors declare no competing financial interest.

2

Acknowledgements

3

This work was supported by the Major International (Regional) Joint Research Project

4

of National Natural Science Foundation of China (21620102004). The authors are

5

grateful to the financial support from Natural Sciences and Engineering Research

6

Council of Canada (NSERC), Alberta Crop Industry Development Fund Ltd. (ACIDF)

7

and Alberta Innovates Bio Solutions (AI Bio). Lingyun Chen would like to thank the

8

Natural Sciences and Engineering Research Council of Canada (NSERC)-Canada

9

Research Chairs Program for its financial support.

10

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1

Table of contents

2 3

Long term stabilization effect of chitin nanofiber on a nonionic foam was studied and

4

utilized for the fabrication of lightweight chitin solid foam with good mechanical

5

performance.

6

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1

Table of contents

2 3

Long term stabilization effect of chitin nanofiber on a nonionic foam was studied and

4

utilized for the fabrication of lightweight chitin solid foam with good mechanical

5

performance.

33


Chitin Nanofibrils to Stabilize Long-Life Pickering Foams and Their

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