Driving Forces for Accumulation of Cellulose Nanofibrils at the Oil

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Interface-Rich Materials and Assemblies

Driving Forces for Accumulation of Cellulose Nanofibrils at the Oil/Water Interface Hua-Neng Xu, Ying-hao Li, and Lianfu Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02310 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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Driving Forces for Accumulation of Cellulose Nanofibrils at the Oil/Water Interface Hua-Neng Xu1, 2*, Ying-Hao Li 2 and Lianfu Zhang2 1

State Key Laboratory of Food Science and Technology, Jiangnan University, 1800

Lihu Avenue, Wuxi, Jiangsu 214122, People’s Republic of China 2

School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue,

Wuxi, Jiangsu 214122, People’s Republic of China * To whom correspondence should be addressed. E-mail: [email protected]

ABSTRACT Understanding the adsorption and organization of nanocelluloses at oil/interfaces is crucial to develop a promising route to fabricate functional materials from the bottom-up. Here we prepare acetylated cellulose nanofibrils (CNFs) with two degrees of substitution and investigate their assembly behavior at the oil/water interface. We study the adsorption process by tracking the dynamic interfacial tension using pendant drop tensiometry, and further characterize the viscoelasticity of the CNF interfacial films as a function of ionic strength. The results show the adsorption of the CNFs to the interface is dominated by energy barriers associated with electrostatic repulsion. With the addition of NaCl, the fibrils are rapidly accumulated at the oil/water interface and jammed into a solid-like film. The overall accumulation of the fibrils is related to the competition between van der Waals attractive forces and electrostatic repulsive forces according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. By 1

ACS Paragon Plus Environment

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screening on the fibril-fibril and fibril-interface electrostatic repulsive forces, the salt addition facilitates the formation of packed fibril clusters and the development of the clusters to into a sold-like film. Moreover, the salt addition is assumed to trigger an abrupt density fluctuation in the vicinity of the interface (the formation of locally dense clusters and voids), leading to an increase in brittleness of the film.

INTRODUCTION The efficient assembly of colloidal particles at oil/water interfaces can be helpful to generate the so-called Pickering emulsions that have found various applications in food, cosmetics and pharmaceuticals.

1-6

Controlling the adsorption dynamics of

colloidal particles at oil/water interfaces not only paves a way to develop reliable emulsions, but also offers a great opportunity to study arrested states in two-dimensional (2D) colloidal systems.

7-12

Recently, nanocelluloses including

cellulose nanofibrils (CNF, long) and cellulose nanocrystals (CNC, short), have been used as ideal candidates for preparing emulsions with outstanding stability.13-25 Despite the importance of nanocellulose-laden interfaces, the adsorption dynamics and underlying driving forces of nanocelluloses still remain poorly understood. The formation of particle-laden interfaces is a time process including the migration and adsorption of particles from the bulk to the interface, concomitant with an overall reduction in interfacial energy. The interfacial adsorption energy for rod-like nanoparticles ( ∆G ) is expressed as follows: 26 ∆G = −lbγ (1 − cos θ )

(1)

2


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where l is the length, b is the width of the rods in contact with the interface, γ is the interfacial tension and θ is the contact angle. The adsorption energy can easily exceed the thermal energy by orders of magnitude and thus make particle attachment to the interface practically irreversible. It suggests that the particles with appropriate wettability can adsorb into the interface, and the particles’ wetting properties provide a convenient way to estimate their packing density at the interface. However, in reality the assumptions may not fit in some systems as the dynamic adsorption equilibrium can not be easily achieved. It means that even if energetically favorable, the particle adsorption process may be limited with a significant energy barrier to adsorption.

27

The origin of the energy barrier can be traced to the particle-interface

and particle-particle interactions. Whereas the dynamical adsorption is governed by the particle-interface interactions, the ultimate coverage of the interface at steady state is controlled by the interactions among already adsorbed particles. It was found that interfacial films of some particles were sculpted into a variety of nonequilibrium structures, which might be locked and rendered permanent by the jamming of the particles. 28-34 Nanocelluloses in aqueous phase tend to carry a negative charge due to dissociable surface groups. When the fibrils are highly charged, a high energy barrier may prevent the particle adsorption to the oil/water interface. Whereas the electrostatic energy barrier was mainly focused on electrostatic double layer repulsion, it has been recently shown that image charge repulsion plays an important role in inducing energy barrier when colloidal particles approach the interface.

35, 36

The origin of the image charge

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effect is related to the difference in the dielectric constants of the two liquid phases across the interface.

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Hence the pair interactions of nanocelluloses may include not

only van der Waals attraction, but also electrostatic double layer repulsion and image charge repulsion. According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, if the total pair interaction is attractive, nanocelluloses will adsorb and aggregate at the interface, resulting in the formation of closely packed films. By screening the particle-particle electrostatic repulsion, salt addition was found to facilitate the partitioning of charged nanocelluloses at the oil/water interface and improve their abilities to stabilize emulsions.15 Since native nanocelluloses bearing a large amount of hydroxyl groups are often regarded as not surface active,

23, 37

methods involving hydrophobicity modifications are also required. By varying the hydrophobic/hydrophilic balance of the nanocellulose surface, direct (O/W) or inverse (W/O) emulsions might be prepared selectively. It was reported that the nanocelluloses esterified with lauroyl chloride are able to stabilize water/oil emulsions as a result of the introduction of hydrophobic alkyl chains, and an oil/water/oil double emulsion is also formed by combing the modified and unmodified nanocelluloses. 18 It was also suggested that sulfated CNCs can stabilize oil-in-water emulsion, preferentially absorbed at the oil/water interface with the hydrophobic 200 edge orientated toward the oil phase.

21

Furthermore, the importance of the physical

characteristics of nanocelluloses such as high aspect ratio and flexibility for creating a high complex viscoelastic modulus of the interface was demonstrated.22 Although significant progress has been achieved in preparing stable emulsions and foams by the

4


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nanocelluloses, there is still an open question as to how the nanoparticles effectively cover the interfaces, and how they pack together to maximize the interfacial coverage. Our recent work has showed that acetylated CNFs exhibited different aggregated states in aqueous solutions when their electrostatic repulsions were screened by salt addition.

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The occurrence of two-step yielding observed in the shear experiments

was related to the establishment of cross-linked clusters on a fibril level and the arrested phase separation on a cluster level. Possibly, the clustering extent of the CNFs also plays a significant role in their assembly behavior at the oil/water interface. The present study is motivated by a demonstration of the ability of the CNFs to stabilize emulsions and focuses on their accumulation and subsequent mechanical response at the interface. We examine the evolution in the fibril packing during the adsorption process using a pendant drop tensiometer, and evaluate the effective diffusion coefficients following the changes in interfacial tension at the early and late stages of the adsorption process. Moreover, we characterize the viscoelasticity of the CNF interfacial films as a function of ionic strength. This research gives a new insight on particle-particle and particle-interface interactions near the oil/water interface, which could be one of the keys to enable a generalized approach towards the development of cellulose-stabilized emulsions.

EXPERIMENTAL SECTION Preparation and characterization of acetylated CNFs and their suspensions The acetylated CNFs were prepared and characterized according to the method

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previously described.

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Cotton microcrystalline cellulose (ALDRTCH Chemistry,

Shanghai, China) was dispersed into a mixture of acetic acid (1:10, w/v) and sulfuric acid (5%, w/w cellulose), and then a desired amount of acetic anhydride was added in order to obtain different acetylated samples. The reaction mixture was stirred vigorously at 50oC for 1 h. After acetylation, 10-fold deionized water was added, and the resulting sample was centrifuged at 4000 rpm for 10 min to remove excess acid. Then the sample was collected and passed 10 times through a high-pressure homogenizer (AH-2010, ATS Engineering Inc., Germany) under value pressure of 1000 bar to obtain acetylated CNF samples. For further purification, the acetylated CNF was dialyzed with membranes against deionized water for several days until the pH of the water from successive washes stayed constant. The degrees of substitution (DS) defined as the average number of acetyl groups substituted on a glucose unit with a maximum possible value of 3, can be determined by a titration method. 39 The obtained three CNF samples gave DS values of 0, 1.32 and 2.01. According to their DS values, the CNF samples are named as CNF0, CNF1.32 and CNF2.01, respectively. More details about the properties of the CNFs (CNF0, CNF1.32 and CNF2.01) are shown in the supporting information of reference 38. The CNF suspension was concentrated by immersing the dialysis tube in polyethylene glycol 20000, and then diluted with deionized water in the concentration range of 0.1~1 wt%. For studies on the influence of salt addition, NaCl of 0, 10 and 100 mM was added to the CNF suspensions of 1 wt%. Measurement of dynamic interfacial tension

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The dynamic interfacial tension (DIT) of the CNFs at the soybean oil/water interface was measured using an OCA15EC tensiometer (Dataphysics Ltd., Germany) with a CMOS camera for drop-image processing. The soybean oil (purity > 99%) was purchased from a local supermarket and purified with Florisil (60-100 mesh, Sigma Aldrich) to remove surface-active impurities as described elsewhere.

40

Prior to each

experiment, the syringe was cleaned attentively and loaded the CNF samples for 20 min to balance. The initial drop of CNF suspensions was formed in the soybean oil, and then the time evolution of interfacial tension was acquired by rapid acquisition of the drop image and edge detection and fitting the droplet’s suspended shape to the Young-Laplace equation. In all case, the drop image was collected before and after measurement.

Measurement of interfacial shear rheology

The shear-rheological properties of the CNF-laden interfaces were characterized using a DHR-3 rheometer with a Du Noüy ring (radius R= 20 mm), similar to that described elsewhere.

41, 42

The ring was placed concentrically within the gap of a

PTFE cup to provide a double Couette effect of interfacial shear on either side of the ring. The inner radius of the cup is 12.7 mm and the outer radius is 25.4 mm. The individual CNF suspension of 40 mL was slightly poured into the PTFE cup and the bubbles were removed from the interface carefully. Afterwards the ring was placed directly at the water/air interface and then covered with a thin soybean oil layer. The data were recorded after the films were equilibrated. An oscillatory strain amplitude

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sweep was done in the strain range from 0.001 to 1 at fixed frequency of 1 rad/s, and a frequency sweep done over a range from 0.1 to 10 rad/s at 0.002 strain (within the linear viscoelastic domain, as determined from the strain amplitude sweep). As a result, the mechanical spectra for storage modulus (G’) and loss modulus (G’’) have been recorded to determine the viscoelastic properties of the interfacial films. An important quantity to consider in interfacial rheology experiments is the Boussinesq number, which is given by Bo =

ηs ηL

(2)

where ηs is the interfacial shear viscosity, η is the shear viscosity of subphase liquid, and L is a geometrical parameter characterizing the measurement geometry.

43

As the

Boussinesq number is essentially the ratio of interfacial and subphase drag, the measured response can represent the interfacial rheology only when Bo is high enough (Bo

1). Here we examined the contribution of subphase drag effect and

found that the subphase contribution is negligible for the system studied.

RESULTS AND DISCUSSION The acetlyated CNFs (CNF1.32 and CNF2.01) are shown to stabilize the oil/water interface and to produce emulsions with outstanding stability, whereas there are no stable emulsions obtained for the CNF0 (Figure 1). It is obvious that the acetlyation makes the fibrils more hydrophobic, and it is expected that the fibrils behave as amphiphilic surfactants at the interface, which is probably the major explanation for their emulsifying activities. However, the CNF0 is also found to produce structurally 8


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stable emulsions in the presence of NaCl (Figure 2). It seems that there might be another mechanism to drive the fibrils to the oil/water interface and to form a robust interfacial film, and therefore the stable emulsion drops are achieved mechanically.

Figure 1. Stability of the emulsions stabilized by CNF0, CNF1.32 and CNF2.01 with different concentrations of 0.5~2% after preparation for 2 weeks. Note that the CNF1.32 and CNF2.01 are shown to stabilize the oil/water interfaces and produce emulsions with outstanding stability, whereas no stable emulsion is obtained for the CNF0.

Figure 2. Stability of the emulsions stabilized by CNF0, CNF1.32 and CNF2.01 with NaCl concentrations of 0, 10 and 100 mM after preparation for 2 weeks. Note that the emulsions are macroscopically stable, except for the occurrence of visible phase separation for the CNF0 at 0 mM NaCl.

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Dynamic interfacial tension (DIT) is a fundamental quantity that relates to the assembly properties of adsorbed species at interfaces and plays a crucial role in the process of emulsion formation and stabilization. A typical surfactant reduces the interface tension continuously before reaching an equilibrium value because the adsorbed species tend to migrate to the interface. It shows that the DIT of the pure oil/water remains constant, and the DIT in the presence of the CNF0 keeps no difference (Figure 3), which confirms the CNF0 is not interface active. However, in the cases of the CNF1.32 and CNF2.01, the DIT is markedly reduced, reflecting the surfactant-like nature of the acetylated fibrils. It should be noted that the CNF1.32 and CNF2.01 have very similar DIT in spite of having different DS. With the DS increasing from 1.32 to 2.01, higher amount of charge groups are introduced to the fibrils (the zeta potential of the CNF1.32 and CNF2.01 is -35.70 and -42.78 mV, respectively).38 The interfacial activity of the fibrils thus depends on the amount of hydrophobic groups as well as that of charge groups on their surface. Although increasing DS makes the fibrils more hydrophobic, higher charges would make them more hydrophilic. As a result, a negligible change in the interface tension is observed. The DIT of the CNF1.32 and CNF2.01 at different concentrations is shown in Figure 4. In general, the interfacial tension decreases rapidly with time during the early stage of self-assembly. Subsequently, the decrease in interfacial tension slows down, and eventually the interfacial tension approaches a plateau, where the interfacial maximum coverage of the fibrils is obtained.

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28 Interfacial tension (mN/m)

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24 20 16

Water CNF0 CNF1.32 CNF2.01

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600

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Ages (s) Figure 3. Dynamic interfacial tension (DIT) measured by pendant drop tensiometry for an droplet of aqueous CNF dispersions (CNF0, CNF1.32 and CNF2.01) with the concentration of 0.5 wt% suspended in soybean oil.

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Interfacial tesion (mN/m)

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25 0.1 wt% CNF1.32 0.25 wt% CNF1.32 0.5 wt% CNF1.32 0.75 wt% CNF1.32 1 wt% CNF1.32

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25

0.1 wt% CNF2.01 0.25 wt% CNF2.01 0.5 wt% CNF2.01 0.75 wt% CNF2.01 1 wt% CNF2.01

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Age (s) Figure 4. Dynamic interfacial tension (DIT) measured by pendant drop tensiometry for an droplet of aqueous CNF dispersions with the concentration range of 0.1~1 wt% suspended in soybean oil. (a) CNF1.32 and (b) CNF2.01.

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At low CNF concentrations the interface is not saturated by the fibrils, and the equilibrium interfacial tension decreases with increasing CNF concentration. A careful observation of the droplet images also confirms that the droplets become darker with increasing CNF concentration due to interface modification by the fibrils (Figure 5). When the CNF concentration is higher than 0.5 wt%, a densely packed state of the fibrils at the interface is observed.

a

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b

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0.5%

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0.75%

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1%

Figure 5. Photographs of pendant droplets of aqueous CNF dispersions with the concentration range of 0.1~1 wt% suspended in soybean oil for a period of 25 min. (a) CNF1.32 and (b) CNF2.01. The overall feature of this evolution is an increasing opaqueness of the images.

As shown in Figure 4, the decrease in interfacial tension slows down gradually. Hence we assume that the diffusion of new fibrils is increasingly hindered by the rearrangement of fibrils already adsorbed at the interface. Taking into account these particularities, here we apply a reported diffusion-controlled adsorption model to the DIT results.

44

The adsorption process is characterized by D0 , effective diffusion

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coefficients at short time (t → 0), and D∞ , effective diffusion coefficients at long time (t → ∞), which can be obtained as follows:

π  1  dγ   D0 =     4  RTc  dt1/ 2 t →0 

2

(3)

4

 dγ    d ln c   D∞ = π 2   dγ   RTc  dt −1/ 2      t→∞ 

(4)

where γ is the interfacial tension, R is the universal gas constant, T is the temperature, c is the particle concentration in the bulk phase and t is the adsorption time. The γ- t1/2 and γ- t-1/2 dependencies for various CNF concentrations are shown in Supporting Information, Figure S1. It can be seen that there are linear dependencies for both cases (t→0 and t→∞), and therefore the procedure for determining the effective diffusion coefficients as described above should be applicable. The effective diffusion coefficients of the fibrils obtained at different fibril concentrations are shown in Figure 6. With increasing CNF concentration, the diffusion coefficients decrease by two orders of magnitude. For the fibrils at each concentration, the diffusion coefficients at short time are larger than those at long time.

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1E-7 CNF 1.32 CNF2.01 CNF 1.32 CNF2.01

1E-8

1E-9

1E-9

1E-10

1E-10

1E-11 0.00

2

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1E-8

D∞ (m /s)

1E-7

D0 (m /s)

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1E-11 0.25

0.50

0.75

1.00

c (wt%) Figure 6. Calculated diffusion coefficients of the CNFs (CNF1.32 and CNF2.01) with the concentration range of 0.1~1 wt% for t→0 and t→∞ (eqn (3) and (4)).

The DIT for the CNF1.32 and CNF2.01 of 1 wt% at 10 mM NaCl is shown in Fig. 7. It should be mentioned that the DIT keeps almost constant with a value as low as 18 mN/m, while the droplet surface appears rough with dense clusters and some void spaces. The significant decrease in the DIT suggests that the salt addition facilitates the adsorption of the CNFs to the interface. Moreover, it seems that the droplets might present an appreciable amount of elastic character and a hysteresis in deformation, which turns out to result in rather small change in interfacial tension.

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Interfacial tension (mN/m)

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20 15 10 5

1% CNF1.32 10mM NaCl 1% CNF2.01 10mM NaCl

0 0

500

1000

1500

Age (s) Figure 7. Dynamic interfacial tension (DIT) measured by pendant drop tensiometry for an droplet of aqueous CNF suspensions of 1 wt% at 10 mM NaCl. Arrows correspond to the photographs of pendant droplets suspended in soybean oil.

To further understand the mechanism that governs the accumulation of the fibrils at the interface, we characterize shear rheological properties of the interfacial films. The oscillatory sweep measurements were performed, and the storage modulus (G’) and loss modulus (G’’) of the interfacial films were recorded as a function of strain amplitude (γ) or frequency (f). Figure 8 shows the dependence of G’ and G’’ over a wide range of strain amplitudes at the CNF concentration of 1 wt% but increasing NaCl concentration from 0 to 100 mM. This strain sweep test allows us to probe the transition from linear to nonlinear viscoelastic behavior. The G’ and G’’ are approximately constant at low strains (linear viscoelastic regime) and then both 16


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moduli decrease rapidly with G’ dropping below G’’ at high strains. The increase of linear moduli with increasing NaCl concentration indicates that the salt addition can contribute to the formation of solid-like and cohesive interfacial films. A local maximum in G’’ exhibits at the beginning of the nonlinear regime, which becomes sharper at higher NaCl concentration. The decrease in the moduli with increasing strain might be related to a progressive breakdown of the interfacial film under shear, and the local maximum in G’’ might be related to some dissipation processes induced by rearrangements of the interfacial structure. At a critical strain with the crossover of G’ and G’’, the film breaks with a transition from solid-like (G’ > G’’) to liquid-like (G’ < G’’) response. The critical strain can be taken as a measure of brittleness.37, 45 It can be found that the critical strain decreases with increasing NaCl concentration, which means the interface films with a higher viscoelasticity are more brittle than those with lower viscoelasticity. The G’ and G’’ measured during the frequency sweep experiments in the linear domain are plotted in Figure 9. In the presence of salt, the G’ and G’’ are almost independent of frequency, and no crossover is observed for them. Consistent with the expected solid-like properties, we find that the G’ dominates over the G’’ in the measured frequency range.

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0mM NaCl 10mM NaCl 100mM NaCl 0mM NaCl 10mM NaCl 100mM NaCl

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-1

-1

10

0

10

1

γ (%)

10

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2

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Figure 8. Storage modulus G’ (solid symbols) and loss modulus G’’ (open symbols) as a function of strain for the CNF interfacial films (a) CNF1.32 and (b) CNF2.01 at different NaCl concentrations (0, 10 and 100 mM ). The strain sweep done at a fixed frequency of 1 rad/s.

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10

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DS 1.32 DS 1.32 DS 1.32 DS 1.32 DS 1.32 DS 1.32

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0

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-1

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Figure 9. Storage modulus G’ (solid symbols) and loss modulus G’’ (open symbols) as a function of frequency for the CNF interfacial films (a) CNF1.32 and (b) CNF2.01 at different NaCl concentrations (0, 10 and 100 mM ). The frequency sweep done at 0.002 strain (within the linear viscoelastic domain, as determined from the strain amplitude sweep). 19


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Based on the above experimental results, it can be found that the salt addition helps the fibril adsorption to the interface and the fibrils are effectively trapped at the oil/water interface and jammed into a solid thin film in the presence of salt. The acetylated CNFs are highly charged in water (the zeta potential of the CNF1.32 and CNF2.01 is -35.70 and -42.78 mV, respectively), 38 and hence the adsorption behavior of the fibrils depends on the amount of charge groups as well as that of hydrophobic groups on their surface. The negatively charged fibrils in the aqueous phase may induce an image charge in the oil phase, and the resulted image charge repulsion thus can affect the adsorption behavior of the fibrils to the interface. With the addition of NaCl, the fibril surface charge is screened (the zeta potential of the CNF1.32 and CNF2.01 at 100 mM NaCl reduces to -10.16 and -15.73 mV, respectively), resulting in reduced image charge repulsion and fibril-interface repulsion. As a result, the net energy barrier decreases considerably and the attraction forces between the fibrils become dominant, and hence the fibrils readily adsorb to the interface. The way in which the salt addition could be used to actively manipulate the accumulation of the fibrils at the interface, is depicted in the cartoon in Figure 10. In the absence of salt the fibrils remain unassembled as the electrostatic repulsion dominates over the van der Waals attraction. The long-range electrostatic repulsion also provides a strong barrier to adsorption, overwhelming the short-range van der Waals attraction that becomes dominant at close contact. It appears that the hydrophobic surface of the fibrils might be the site that anchors them to the oil/water interface. The charge of the fibrils provides sufficient repulsion so that the fibrils do not aggregate in the bulk and

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they move as single objects to the interface. As the fibril coverage increases, the adsorbed fibrils at the interface create an energy barrier via electrostatic repulsion for the new fibrils to adsorb to the interface from the bulk. The salt addition greatly weakens the electrostatic repulsion relative to the van der Waals attraction, thereby facilitating the formation of interconnected fibrils as well as triggering a local density fluctuation in the aqueous phase. 38 The density fluctuation is related to the formation of locally dense clusters and voids by spinodal decomposition. The adsorption of the fibrils is thus apparently enhanced as a result of an easy access of the dense clusters to the interface. It means that the adsorbing species, which play a crucial role in defining the mechanical properties of particle-laden interfaces, appear to be the fibril clusters rather than single fibrils. The rapid adsorption proceeds via the formation of fibril clusters that subsequently merge to form a solid-like spanning network. This contrasts with the smooth fluidlike interfaces that found in the absence of salt. Moreover, the collective adsorption of the fibrils to the interface leads to an increase in brittleness of the film duet to jamming at short times.

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Figure 10. Schematic representation of two kinds of mechanisms for the CNFs accumulation at the oil/water interface. The fibrils are effectively trapped at the oil/water interface and jammed into a solid thin film via the formation of fibril clusters.

CONCLUSION The assembly behavior of the CNFs at the oil/water interface is probed by tracking dynamic interfacial tension using pendant drop tensiometry and characterizing viscoelasticity of their interfacial films as a function of NaCl concentration. Without the salt addition, the interfacial adsorption of the fibrils is diffusion-controlled and significantly suppressed due to energy barriers associated with electrostatic repulsion. There is a strong tendency for the fibrils to accumulate and jam at the interface through salt addition. The salt addition is proposed to initiate the adsorption by screening charges on the fibrils and weakening the image charge repulsion and fibril-interface repulsion. The adsorption of the fibrils follows a clustering-migration mechanism where the fibrils move as clusters toward the oil/water interface. The rapid 22


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adsorption via the formation of fibril clusters is quite different from the conventional diffusion-controlled adsorption, with the fibrils effectively filling the interface and ultimately merging to form a solid-like film.

ACKNOWLEDGMENTS

The work is supported by the National Natural Science Foundation of China (No. 21576116), the State Key Research and Development Plan (No. 2017YFD0400200), and China Scholarship Council.

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Driving Forces for Accumulation of Cellulose Nanofibrils at the Oil

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