Impact of Divalent Cations on the Rheology of Cellulose Nanofibrils

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Impact of Divalent Cations on the Rheology of Cellulose Nanofibrils

Rahmini

¹

, Soyoung Juhn

²

, Hyun-A Seong

^3‡

, Soo-Jeong Shin

^4†

Received December 4, 2019; Received in revised form April 17, 2020; Accepted April 20, 2020

ABSTRACT

Cellulose nanofibrils (CNFs) hydrogels have been used in various applications due to their ability to form gels via entanglement networks in water. In this study, the rheological properties of CNF gel with various concentrations of divalent cations (Mg

²⁺

, Ca

²⁺

, and Ba

²⁺

) are investigated using a rheometer (MCR 102, Anton Paar, Austria). The addition of divalent cations significantly increased the viscosity, storage modulus, and loss modulus as the concentration of cation increased from 100 to 8,000 ppm that contributed to the formation of stronger gels. This phenomenon indicates that higher cation concentration leads to fibril aggregation and a stiffened network. However, when an excessive amount of cations (10,000 ppm) is added, the CNF starts to coagulate and lose strength. Com- pared to Ca

²⁺

and Ba

²⁺

, Mg

²⁺

has a greater effect on hydrogel strength at 100 ppm, and thus exhibited maximum strength at 5,000 ppm. The maximum strength of Ca

²⁺

occurred at 8,000 ppm, and Ca

²⁺

produced the strongest gel (Mg

²⁺

and Ba

²⁺

were second and third, respectively) between 500 and 1,000 ppm. Ba

²⁺

produced the highest-strength gel with the maximum strength achieved at 8,000 ppm. The impact of divalent ions on the rheo- logical properties of CNF strongly correlated with the kind of cation used and the cation concentration.

Keywords: Cellulose nanofibril, rheology, barium, magnesium, calcium

Printed in Korea http://dx.doi.org/10.7584/JKTAPPI.2020.04.52.2.78

1 Department of Wood and Paper Science, Chungbuk National University, Cheongju, 28644, Republic of Korea, Student 2 Nature Costech Inc., Cheongju, 28578, Republic of Korea, Director for R&D

3 Department of Biochemistry, Chungbuk National University, Cheongju, 28644, Republic of Korea, Professor

4 Department of Wood and Paper Science, Chungbuk National University, Cheongju, 28644, Republic of Korea, Professor

† Corresponding Author: E-mail: [email protected] (Address: Department of Wood and Paper Science, Chungbuk National University, Cheongju, 28644, Republic of Korea)

‡ Co-Corresponding Author: E-mail: [email protected] (Address: Department of Biochemistry, Chungbuk National Uni- versity, Cheongju, 28644, Republic of Korea)

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1. Introduction

Hydrogels have received a lot of attention in var- ious fields due to their unique advantages, such as biocompatibility, higher water content, non-toxic characteristics, flexibility, soft material properties, and ease of handling.

^1,2)

Hydrogels, including cel- lulose nanofibrils (CNFs) hydrogels, are based on natural materials and have been commonly used in the biomedical sector for a variety of purposes, such as drug delivery, tissue engineering, and bio- sensors.

^1-6)

CNFs are manufactured from cellulose fibers through mechanical treatments (blending, grinding, and homogenization), chemical pretreat- ments (carboxymethylation and TEMPO-mediated oxidation), and enzymatic hydrolysis.

^7-12)

Recently, studies have focused on CNF hydrogels that are introduced into charge screening by using metal cations to crosslink carboxylated CNFs.

^13,14)

CNF gels are delicate and have low mechanical strength. However, the mechanical properties of CNF gels can be improved by adding metal salts.

CNF gels form stronger networks when in an entangled structure due to their ability to form flexible nanofibrils. A bulk investigation is per- formed on the rheological properties of nanocellu- lose gel in combination with salt to control the mechanical properties of CNF gels. Tandjawa et al.

¹⁵⁾

reported an increase in the viscosity and mechani- cal strength of cellulose gels by adding 50 mM sodium chloride (NaCl) and calcium chloride (CaCl

2

). However, the mechanical properties of carboxymethylated cellulose gradually decreased from 0.1 to 10 mM.

¹⁶⁾

CNF gelation rapidly occurs with the addition of divalent (Ca

²⁺

, Zn

²⁺

, or Cu

²⁺

) and trivalent cations (Al

³⁺

, Fe

³⁺

).

¹⁴⁾

A higher cation valency enables a greater tendency for crosslinking between fibrils, creating a stronger nanofibril net- work structure. Furthermore, the same concentration of CNF (ranging from 0.01 to 3 M) containing CaCl

2

shows a higher viscosity and storage modulus than

when containing NaCl.

¹⁷⁾

The mechanical strength of the hydrogel can be controlled by adding salt.

However, there is a research gap regarding the rheological properties of CNF gels prepared using divalent cations.

This study investigates the rheological properties of CNFs by using various concentrations of diva- lent cations to understand how divalent cations influence the CNF gel strength and the maximum concentration of cations that can be added to form the strongest gel.

2. Materials and Methods

2.1 Materials

In this study, dried kraft pulp (M. Company) was used to prepare CNFs. 95% Ethanol (EtOH; Sam- chun Pure Chemical Co., Ltd., Korea), 98% sodium hydroxide (NaOH; OCI Company Ltd., Korea), 98%

monochloroacetic acid (MCA; Denak Co., Ltd., Japan), 99% barium chloride anhydrous (BaCl

2

; Junsei Chemical Co., Ltd., Japan), 98% magnesium chloride hexahydrate (MgCl

2

; Showa Co., Ltd., Japan), and calcium chloride dihydrate: extra pure (CaCl

2

; Junsei Chemical Co., Ltd., Japan) were used.

2.2 Preparation of CNF

CNFs were prepared according to the method

reported by W ågberg et al.

¹⁸⁾

The fiber obtained

from the dried kraft pulp (100 g) was suspended in

a NaOH solution (14.11 g NaOH in EtOH) at room

temperature for at least 1 h. The 0.6 mmol/g cel-

lulose MCA solution was prepared by mixing

monochloroacetic acid (28.9 g) and EtOH. The MCA

solution was added to the suspension and reacted

at 75℃ for 2 h. After a reaction time of 2 h, the

fiber suspension was washed thoroughly using

distilled water and filtered several times until neu-

tral. The CNF was prepared by passing the cellu-

lose fiber (2% wt) through a supermasscolloider

(3)

(MKZA10-15IV; Masuko Sangyo, Japan) twice, and then homogenizing the fiber twice by using a GEA Homogenizer processor (Panda Plus, GEA, Italy).

2.3 Preparation of CNF gels

The CNF suspension (2%) was diluted using a divalent salt solution to obtain 1.5% CNF gel. The CNF gel was mixed with ionic crosslinking BaCl

2

, CaCl

2

, and MgCl

2

to obtain various concentrations (100, 500, 1,000, 5,000, 8,000, and 10,000 ppm) for Ba

²⁺

, Ca

²⁺

, and Mg

²⁺

. After the salt was added, the mixtures were homogenized by hand, shaking approximately 5 times/s.

2.4 Rheological properties of CNF gels

The rheological properties of all the samples were determined using a rheometer (MCR 102; Anton Paar, Austria). A parallel plate (25-mm diameter) was used and the gap between both plates was set to 1 mm. The viscosity of the samples was mea- sured at a shear rate ranging from 1 s

^-1

to 100 s

^-1

. The strain sweep was used to determine the linear viscoelastic region at a frequency of 10 rad/s and over a strain range of 0.01-100%. The flow point (strain γ

f

) was evaluated where the storage modu- lus equaled the loss modulus (G′=G″). The mea- surements were performed at 25°C.

3. Results and Discussion

The rheological measurements were studied to

observe how the divalent salt content affected the properties of the CNF gels. The CNF gels were produced by introducing metal cations as cross- linkers to bind with the carboxylic groups, starting the gelation process. The bulk investigation was performed by adding metal salts to increase the strength of the hydrogel.

11,15,17,19,20)

The effects of divalent salts were investigated using various divalent salts and concentrations.

Detailed CNF gel compositions and divalent ions are listed in Table 1. The change in viscosity because of the influence of the divalent ions added to the CNF is summarized in Figs. 1-3. The diva- lent ions significantly affect the gel strength by increasing the viscosity. The viscosity of the hydrogel was increased because of the network structure formed by crosslinking the positively charged cation (added) and the negatively charged CNF.

²¹⁾

All the samples show typical shear thinning behavior with a decrease in viscosity when the

Table 1. Characteristics and compositions of cations in the hydrogel Cations Cation charge

number

Cation radius

Å

Solubility of cation in gel (mg/L)

10,000 ppm 8,000 ppm 5,000 ppm 1,000 ppm 500 ppm 100 ppm

Ba

²⁺

2+ 1.36 15,200 12,160 7,600 1,520 760 152

Mg

²⁺

2+ 0.72 40,000 32,000 20,000 4,000 2,000 400

Ca

²⁺

2+ 1.00 28,000 22,400 14,000 2,800 1,400 280

Ionic radii were taken from a previous study.

²²⁾

Fig. 1. Viscosity as a function of shear rate for

1.5% CNF with Ba

²⁺

at various concen-

trations.

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shear rate increases. An increase in the shear rate leads to a gradual breakdown in the network, which results in a higher viscosity. However, the viscosity is relatively low when the shear rate is reduced because the interfibril network does not change as quickly.

²³⁾

Fig. 1 shows the viscosity of CNF at various con- centrations of Ba

²⁺

; the viscosity gradually increases by increasing the concentration of Ba

²⁺

from 100 to 8,000 ppm. However, the continuous addition of Ba

²⁺

from 8,000 to 10,000 ppm, decreases the viscosity. The viscosity increases from 19.89 Pa·s (100 ppm Ba

²⁺

) to 572.16 Pa·s (8,000 ppm Ba

²⁺

). Fig. 2 displays the viscosity of CNF and Mg

²⁺

at concentrations ranging from 100 to 10,000 ppm. Fig. 2 indicates that the viscosity increases as the concentration of Mg

²⁺

increases from 100 to 5,000 ppm and then decreases as the concentration of Mg

²⁺

increases from 8,000 to 10,000 ppm. This trend shows that the highest viscosity of CNF containing Mg

²⁺

is 5,000 ppm.

Mg

²⁺

allows for the generation of higher viscosities than Ba

²⁺

between 100 and 1,000 ppm; however, at 5,000 ppm, Ba

²⁺

has a higher viscosity than Mg

²⁺

.

The apparent viscosities of CNF with various concentrations of Ca

²⁺

are shown in Fig. 3. Fig. 3 shows a trend similar to that observed for Ba

²⁺

. The increase in Ca

²⁺

from 100 to 8,000 ppm

increases the viscosity of the gel; after the increase to 10,000 ppm, the viscosity decreases. This also indicates that 8,000 ppm of Ca

²⁺

leads to a stron- ger hydrogel. These results show that the viscosity of CNF containing a divalent ion improves pro- gressively when the concentration is increased from 100 to 8,000 ppm and, thereafter, starts to decrease when adding cations up to 10,000 ppm.

Overall, an increase in the cation concentration leads to an increase in viscosity until the CNF gel ceiling is reached, causing the CNF to start to coagulate and lose viscosity. A similar result is observed while gradually increasing the CaCl

2

con- centration from 0 to 3 M; the viscosity of CNF gel gradually increases too.

¹⁷⁾

Liu et al.

²¹⁾

reported that a significantly higher viscosity is obtained when using a more concentrated crosslinked hydrogel.

However, an excessive cation content in hydrogel reduces the strength of the hydrogel.

In this study, Mg

²⁺

had the highest viscosity, Ca

²⁺

the second-highest, and Ba

²⁺

the lowest, at 100 ppm. This might be attributed to the difference in cation radii, where Mg

²⁺

has a smaller radius com- pared to Ca

²⁺

and Ba

²⁺

. CNF containing Ca

²⁺

has a higher viscosity than Mg

²⁺

and Ba

²⁺

(Ca

²⁺

> Mg

²⁺

>

Ba

²⁺

) at concentrations of 500 and 1,000 ppm. At higher concentrations (5,000 to 8,000 ppm), Ba

²⁺

had the highest viscosity, Ca

²⁺

the second-highest, Fig. 2. Viscosity as a function of shear rate for

1.5% CNF with Mg

²⁺

at various concen- trations.

Fig. 3. Viscosity as a function of shear rate for

15 % CNF with Ca

²⁺

at various concen-

trations.

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and Mg

²⁺

the lowest. These trends indicate that the impact of divalent ions on the CNF viscosity is directly correlated with the kind of cation used, shear rate, and concentration of cation. In addi- tion, the critical ionic strength of the cellulose suspension depends on the charge content, mass fraction and dimension of the fibril.

¹¹⁾

Similar to what was observed for viscosity, add- ing divalent ions increases the storage modulus and loss modulus of the CNF gels (Figs. 4-6). The effect of ionic strength on the storage modulus and loss modulus of 1.5% CNF caused by adding Ba

²⁺

is shown in Fig. 4. The G′ value increased from 417.9 to 17,334 Pa as the concentration of Ba

²⁺

increased from 100 to 8,000 ppm, confirming that a stronger network of CNF gel is created at 8,000 ppm. How- ever, when Ba

²⁺

surpassed 8,000 ppm, the G′ value decreased. This suggests that by increasing the concentration of cations, the gel stiffens and becomes stronger until the CNF gel ceiling is

reached; further increase in the concentration decreases the strength of the hydrogel. The stiff- ening of the gel is caused by the intimate contact between nanofibrils due to the increased cation content in the CNF gel. This removes water from the network, making it stronger.

¹⁶⁾

A similar trend is observed when Ca

²⁺

is added. The gel strength increases as the concentration of cations are increased from 100 to 8,000 ppm, after which the strength decreases as the concentration is increased to 10,000 ppm (Fig. 5). The presence of Ca

²⁺

ions in the CNF induces screening on the charged surface of the fibrils because the Ca

²⁺

ions are closely associated with the carboxyl groups.

²⁴⁾

Gelation of CNFs occurs when 50 mM of Ca(NO

3

)

2

is added. This has a significant effect on the viscoelastic proper- ties, caused by network formation.

¹⁴⁾

Sim et al.

¹⁷⁾

reported that storage modulus and yield stress increase with increasing CaCl

2

concentration, due to flocculation that results in the formation of a

Fig. 4. Storage and loss modulus as a function of strain amplitude at a frequency

of 10 rad/s for 1.5% CNF with Ba

²⁺

concentrations of (a) 500, (b) 1,000,

(c) 5,000, and (d) 10,000 ppm.

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Fig. 5. Storage and loss modulus as a function of strain amplitude at a frequency of 10 rad/s for 1.5% CNF with Ca

²⁺

concentrations of (a) 500, (b) 1,000, (c) 5,000, and (d) 10,000 ppm.

Fig. 6. Storage and loss modulus as a function of strain amplitude at a frequency

of 10 rad/s for 1.5% CNF with Mg

²⁺

concentrations of (a) 500, (b) 1,000,

(c) 5,000, and (d) 10,000 ppm.

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strong network in the CNF gel. Maestri et al.

¹⁹⁾

reported that the storage modulus increases as the CaCl

2

concentration of cations increases from 10 to 100 mM in cellulose nanocrystals gel.

¹⁹⁾

Fig. 6 shows the storage and loss modulus of the CNF gel as a function of strain amplitude for vari- ous concentrations of Mg

²⁺

. As shown in Fig. 6, the gel strength increases as the Mg

²⁺

concentration is increased from 100 to 5,000 ppm, and reaches a maximum of 12,888 Pa, followed by decrease to a low of 10,134 Pa at 8,000 ppm. The Mg

²⁺

concen- tration of 5,000 ppm formed the strongest gel (12,888 Pa). Storage moduli G′ as a function of dif- ferent cation concentrations of Ba

²⁺

, Ca

²⁺

, and Mg

²⁺

at a frequency of 10 rad/s is summarized in Fig. 7. As shown, Mg

²⁺

has a more significant effect on the hydrogel strength than Ca

²⁺

and Ba

²⁺

(Mg

²⁺

> Ca

²⁺

> Ba

²⁺

) at low concentrations (100 ppm).

This result can be explained by the difference in the ionic radii (Mg

²⁺

< Ca

²⁺

<Ba

²⁺

), as shown in Table 1, in which smaller ionic radii leads to a higher strength. Small ionic radii of Mg

²⁺

implies a hard cation, which works better when increasing stiffness than soft cations such as Ca

²⁺

and Ba

²⁺

. However, this observation is in contrast to the work presented by Chau et al.,

²⁰⁾

who reported that storage and loss modulus of the CNF gels increase

as the ionic radii of cation increase (Ca

²⁺

>Mg

²⁺

).

Compared to Ba

²⁺

and Ca

²⁺

, the increase in Mg

²⁺

concentration from 5,000 to 10,000 ppm resulted in a lower-strength hydrogel. The strongest CNF gel containing Mg

²⁺

was formed at 5,000 ppm Mg

²⁺

concentration. The gel containing Ca

²⁺

was stron- ger than the gel containing Mg

²⁺

and Ba

²⁺

between 500 and 5,000 ppm, and exhibited a maximum strength of 16,843 Pa at 8,000 ppm. Ba

²⁺

produced the strongest gel (17,334 Pa) at 8,000 ppm (Ba

²⁺

>

Ca

²⁺

> Mg

²⁺

). This was attributed to a stronger bridging of Ba

²⁺

in CNF at high concentrations when compared to that of Mg

²⁺

and Ca

²⁺

. This also indicated that MgCl

2

is the most effective at increasing the hydrogel strength at low concentra- tions. A higher concentration of BaCl

2

was required to trigger gelation in comparison with MgCl

2

and CaCl

2

. The presence of cations as ionic crosslinking enhanced the mechanical properties of car- boxymethylated CNF by creating intra/interfibril repulsion.

^14,25)

The flow point (γ

f

) for all the samples at a fre- quency of 10 rad/s is shown in Fig. 8. The higher the crossover point value, the stiffer the sample. In Fig. 8, the value of the flow point increased as the concentration of the cation is increased, indicating that the sample becomes more rigid and strong as

Fig. 7. Storage moduli G′ as a function of dif- ferent cation concentrations (Ba

²⁺

, Ca

²⁺

, and Mg

²⁺

) at a frequency of 10 rad/s and a strain of 0.1%.

Fig. 8. The flow point as a function of different

cations concentrations (Ba

²⁺

, Ca

²⁺

, and

Mg

²⁺

) at a frequency of 10 rad/s.

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the concentration increases. However, a decrease is observed at 10,000 ppm after maximum strength is achieved. A cross over point at the strain sweep can be used to describe the flexibility of the sam- ples.

²⁶⁾

4. Conclusions

CNFs were modified by adding divalent cations (BaCl

2

, MgCl

2

, CaCl

2

) and using ionic crosslinking to form stronger gels at various concentrations (100, 500, 1,000, 5,000, and 10,000 ppm) of Ba

²⁺

, Mg

²⁺

, and Ca

²⁺

, respectively. As expected, the addition of cations and the increase in cation con- centration significantly increased the viscosity, storage modulus, and loss modulus of the gels.

However, the gel strength decreased after the maximum concentration was reached. Mg

²⁺

had a greater effect on hydrogel strength when compared to Ca

²⁺

and Ba

²⁺

at 100 ppm (Mg

²⁺

>Ca

²⁺

> Ba

²⁺

), and achieved a maximum strength at 5,000 ppm.

Ca

²⁺

produced the greatest strength (Ca

²⁺

> Mg

²⁺

>

Ba

²⁺

) between 500 and 1,000 ppm, and achieved a maximum strength at 8,000 ppm. Lastly, Ba

²⁺

exhibited the highest strength at 8,000 ppm. Rhe- ological properties of CNFs with divalent ions showed a positive correlation among viscosity, storage modulus, and loss modulus of the gels, which increased and decreased following the con- centration trends of cations.

Acknowledgement

This research was supported by the National Research Foundation of Korea government (NRF) funded by Ministry of Education (NRF-2019R1I1 A3A01058481).

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