ARTICLE pubs.acs.org/Langmuir
Zeta Potential Time Dependence Reveals the Swelling Dynamics of Wood Cellulose Nanofibrils Kojiro Uetani and Hiroyuki Yano* Division of Creative Research and Development of Humanosphere, Research Institute for Sustainable Humanosphere, Kyoto University, Uji 611-0011, Japan
bS Supporting Information ABSTRACT: In this paper, we present the swelling dynamics of individual wood cellulose nanofibrils (CNFs) following solvent substitution into various organic solvents and drying, by employing the time dependence of the zeta potential (ζ). We succeeded in smoothly redispersing the coaggregating CNFs dried in solvents, including acetone, acetonitrile, DMSO, ethanol, and t-butanol into water. ζt plots of the redispersed CNFs measured in a 1 mM KCl solution indicated different values of Δζ (volume fraction of hydration capacity), corresponding to the dielectric constant of the substituted solvents. Differential scanning calorimetry confirmed that the redispersed CNFs swell to different degrees, corresponding to Δζ. This swelling behavior is characterized by expansion of hemicelluloses, the amorphous polysaccharides located on the CNF surface, with a different degree of aggregation during drying. The specific swelling ratio, radius, and diameter of the CNFs in water were calculated using the values of ζ0 and ζ∞ by introducing surface chemical analysis. The calculated diameters of the CNFs at t = 0 coincided well with the median diameters measured directly by transmission electron microscope. Swellability of hemicelluloses exponentially increased with the decrease in dielectric constant of solvent during drying. The analysis method combining zeta potential time dependence and a surface chemical approach proved useful for specifically evaluating the swelling dynamics of polymers on a bulk surface.
’ INTRODUCTION Cellulose nanofibrils (CNFs) obtained mostly from abundant wood resources have been promoted extensively for use in various fields as one of the most attractive nanomaterials for the creation of a sustainable society and for solving environmental issues.1 Tiny nanofibrils, with just a 34 nm crystalline cellulose width, have excellent mechanical properties including a high Young’s modulus of 138 GPa,2 an estimated strength of 2 to 3 GPa,3 and a very low coefficient of thermal expansion of 107 K1.4 These nanofibrils have been used in various interesting materials as well as in transparent paper,5 self-assembled gels,6 high strength composites,7,8 high gas barrier films,9,10 chiral nematic structures,11 functional aerogels,12 and other products.13 The higher-order structures or significant properties of these materials are often characterized by huge surface areas and the interfacial areas of the colloidal nanofibrils. Understanding the interfacial interactions of nanomaterials with other substances is very important for producing composites with functional properties. It is also essential to evaluate the surface conditions or properties of wood CNFs as a starting material for chemical and physical applications. However, the hemicelluloses, amorphous complex polysaccharides contained within wood CNFs, make analysis difficult.14 These hemicelluloses predominantly consist of galactoglucomannan and arabino4-O-methylglucuronoxylan, which are not completely eliminated by pulping. On the other hand, the swelling behaviors of pulp and r 2011 American Chemical Society
paper with water retention and the effect of this on mechanical properties or dimensional stability have been discussed in the papermaking field.1517 However, the origin and the specific degree of swelling on the order of individual nanofibrils are still not determined. In general, amorphous polymers are known to swell in solvents and change the morphology or properties constitutively in response to the surrounding pH,18,19 ion concentration,20 temperature,21 polarity,22 and other external stimuli.23,24 In utilizing these fibrils directly or indirectly in various materials in which high homogeneity and precise control of the required properties are needed, both dynamics and analysis methods should be identified. On the other hand, CNFs easily coaggregate when they are dried in water because of strong hydrophilicity and as a result are difficult to redisperse after drying.25 In this study, we found that CNFs dried after solvent substitution into various organic solvents are able to be smoothly redispersed into water. Therefore, we considered evaluating the surface changes of CNFs after drying in organic solvents by measuring zeta potential time dependence. The zeta potential (ζ), the potential at the slip plane, which is the electrochemical border between the immobile layer of Received: August 31, 2011 Revised: October 19, 2011 Published: November 21, 2011 818
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absorbed counterions and the diffuse layer on the investigated surface, has been employed for discussions including suspension rheology26 and the effect on nanofibrillation.27 There are many techniques for measuring a single representative value of the zeta potential, including the streaming potential, a nondestructive technique for large particles, and the electroosmotic or electrophoresis method, a destructive technique for colloidal dispersions.28 In addition, it is known that measuring the time dependence of the zeta potential gives information on the swelling behavior of polymers. Kanamaru et al. pointed out that the zeta potential of “dry” polymer particles immersed in water is influenced by the hydration behavior derived from the swelling, and the absolute values of the potential decrease, depending on the immersion time, to the equilibrium value.29 The decrease of ζ as a function of time can be described as follows:
dζ ¼ kðζt ζ∞ Þ dt
surface and its own double layer in the solution.28,34 We believed it might be possible to evaluate the dynamic behavior of polymers located on the bulk surface by means of successive zeta potential measurements from the dry state. We therefore employed this method to evaluate the surface alteration of individual CNFs after treatments of solvent substitution and drying in combination with differential scanning calorimetry (DSC) measurement and surface chemical analysis. The validity of the analysis was then verified experimentally and theoretically.
’ EXPERIMENTAL SECTION Materials. A 3060 mesh wood powder of Japanese cedar (Cryptomeria japonica) was subjected to processing to produce pulp. Lignin was removed by the Wise method,35 consisting of a cyclic treatment performed 10 times with sodium chlorite (NaClO2) solution under acidic conditions (pH 4 to 5) at 80 °C for 1 h, after which hemicelluloses were partially removed by alkaline treatment with a 4 wt % KOH solution at room temperature overnight. The product was then thoroughly washed with distilled water, without drying. Never-dried holocellulose pulp was obtained, with an α-cellulose content36 of 82 wt % and a Klason lignin content37 of 0.1 wt %. A water suspension with a pulp content of 0.8 wt % was fibrillated by a single pass through a grinder (MSKA63, Masuko Sangyou Co., Ltd.) with two specially adjusted stone disks for supergrinding at 1500 rpm to obtain the disintegrated nanofibrils.38 Solvent Substitution and Drying. Three grams of 0.8 wt % CNFwater slurry was diluted by 200 mL of various aprotic polar solvents including acetone, tert-butanol, acetonitrile, ethanol, and DMSO for ∼12 h with stirring. The slurry was then filtered by a membrane filter, and the nondried mat containing ∼10 wt % of CNF was resuspended into the same 200 mL solvents with stirring. This cycle of dilution and filtration was repeated 34 times, and then the nondried CNF mat was dried in a 55 °C oven overnight. Only the DMSOCNF slurry was dried in a 110 °C oven. Additionally, a water-dried CNF was prepared by freezedrying a 2 g/L CNFwater slurry. Chemical Component Analysis. The carbohydrate composition of the CNFs before and after treatment by solvent substitution and drying was determined by gas chromatography analysis with alditol acetate, following the method described by Borchard and Piper.39 Water Content. The water content in the dried CNF sheets was determined by means of the Karl Fischer moisture titrator (MKC-610, KEM Co., Ltd.) by heating the sample to 130 °C with a moisture gasification instrument (ADP-611, KEM Co., Ltd.). The remaining amount of organic solvents in the dried CNF sheets was estimated by the difference in sheet weight before and after measuring and the measured water weight. X-ray Diffractometry. X-ray diffraction studies were performed to determine the change in crystallinity of the dried CNFs before and after solvent substitution. Cu Kα radiation, generated by an UltraX 18HF (Rigaku Corporation) operating at 30 kV, 100 mA, was irradiated onto specimens consisting of several layers of fibrillated fiber sheets about 200 μm in total thickness. The degree of crystallinity was calculated from each diffraction profile, detected using an X-ray goniometer scanning from 5 to 45°. After subtracting the air scattering, curve fitting of the diffraction profile was carried out to separate the noncrystalline part and the crystalline reflection. The degree of crystallinity was defined as the ratio of the area of crystalline reflection to the whole area of the scattering profile. The size (width) of the cellulose crystallites was estimated from the width of the peak at 2θ = 22.8° at half-height using Scherrer’s equation
ð1Þ
which leads to ln
ζ ζ∞ ¼ kt ζ0 ζ∞
ð2Þ
where ζt is the zeta potential at time t, and k is a structural constant. According to Kanamaru, ζ is estimated as a function of dielectric constants of solids and liquids as follows: εf ð3Þ ζ ¼ K 1 εw where K is the structural constant, εf is the dielectric constant of the solid, and εw is that of water. Meanwhile, εf for the case when the solid contains some water is described as follows: εf t ¼ εf 0 ð1 hÞ þ εw h
ð4Þ
where εf is the dielectric constant of the solid containing water at time t, and h is the volume fraction of water. Equation 4 is called the parallel mixing law and this determines the dielectric constant of a solid containing water by means of εf0 (dielectric constant in the dry state), εw, and the volume fraction h of water.30 ζ as a function of time was proven to depend on the change in εf. Furthermore, eqs 3 and 4 lead to t
Δζ ¼
ðζ0 ζ∞ Þ ¼ h∞ ζ0
ð5Þ
where h∞ is the hydration capacity of the solid at t = ∞. The swelling degree of the solid is likely to be estimated by the values of ζ. The sequence of derivations described above, which we call Kanamaru’s law here, has been employed to evaluate the properties of chemically modified pulps.31 Ribitsch et al. has extended the measurement of the zeta potential as a function of pH and discussed the relation between the dissociable groups on the surface and the surrounding ion concentration through ζpH plots.32 Thus, it has been concluded that the reduction of ζ as a function of time is caused by the shift of the slip plane with swelling of the fiber structure.3133 The zeta potential time dependence seems to trace the behavior of the slip plane of the investigated surface. This phenomenon can be understood by considering that the bulk surface forms an electric double layer, whereas the amorphous polymer chain itself has no electrochemical
Lð200Þ ¼ 819
K3 γ β 3 cos θ
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where K = 0.9, γ = 1.5418 Å, β is the half width after curve fitting, and θ is the Bragg angle for the 200 reflection.40 Morphology Observation. The dried CNFs after solvent substitution were dispersed into water with stirring for 12 min by a domestic hand blender. The redispersed CNF slurries were negatively stained using 2 wt % uranyl acetate and imaged by transmission electron microscopy (JEM-2000EXII, JEOL Ltd.). Zeta Potential. Successive measurements of the zeta potential were performed by electrophoresis with the laser Doppler method using a SZ-100 (HORIBA Co., Ltd.) in which electrode cells with Cu electrodes coated with Au were attached. The new cells required a preconditioning interim operation for the electrode surfaces by applying the voltage (34 V) in a 1 mM KCl solution for ∼30 min. These electrodes rapidly eroded in solutions of high electric conductivity (i.e., greater than 5 mM KCl solution) with an electric current, within just 4060 min, so successive measurements at high salt concentrations were technically difficult. The dried CNF samples were redispersed into 0.1 or 1 mM KCl aqueous solutions (pH fixed at a neutral value of 6.8 to 7) by a hand blender for 12 min to prepare dilute slurries of 0.0020.01 wt %, which is the proper range of sample concentration where the electric double layers on the particles should not overlap or interact with each other, i.e., the particles are highly individualized. Successive measurements of the zeta potential were then immediately carried out on 200 μL slurries at 25 °C every 1 min, with a quantity survey of 50 s obtained by applying AC voltage (3.4 V) and an interval of 10 s. The zeta potentials were then calculated according to the following equation: ζ¼
UE η εf ðkaÞ
Table 1. Chemical Compositions and Crystallinity of CNFs cellulose
glucomannan
xylan
cellulose I type
crystallite
(%)
(%)
(%)
crystallinity (%)
size (nm)
AO
82.29
10.41
7.30
63.09
3.38
TO
83.65
12.88
3.47
61.35
3.46
EO
78.27
10.17
11.56
62.44
3.30
ANO
78.90
14.46
6.63
62.17
3.46
DO
75.86
13.17
10.97
63.69
3.31
WFD
80.21
14.82
4.97
64.51
3.30
ND
77.36
9.27
13.37
65.24
3.31
Table 2. Bulk Density and Water Content of Dried CNFs water content
dielectric constanta of
(g/cm )
(%)
the solvent at 25 °C
AO
0.55
5.57
20.7
TO
0.51
5.96
12.4
EO
0.87
5.92
24.6
ANO
1.09
6.26
37.5
DO
1.24
5.13
46.7
ND
1.40
5.74
78.4
bulk density 2
a
Values cited from Riddick et al.42
and ∼80% cellulose, which have a crystalline region that changes little before and after treatment of solvent substitution and drying. We considered every CNF sample to have a similarly sized cellulose crystalline region and almost the same amount of hemicelluloses, which are amorphous polysaccharide chains with no bulk and a colloidal surface that is located on the colloidal microfibril surface. The states of dry matter are shown in Table 2. The water contents of dried CNFs were almost the same, regardless of solvent species, whereas the bulk density differed substantially between solvent species, especially depending on their dielectric constants. The estimated amount of residual solvent was 23 wt % in AO, TO, EO, and ANO; the boiling points of which are lower than water. DO appeared to contain slightly more residual DMSO than the residual solvent in other samples. The degree of aggregation in the macroscopic state of CNFs differed according to the solvent polarity. Apparently, CNFs can aggregate more weakly during drying in a polar solvent with a lower dielectric constant. The macroscopic dry matter then has a lower bulk density. The microscopic morphologies of CNFs redispersed into water following drying are shown in Figure 1. ND shows that the grinder treatment created pulp comprising individual nanofibrils with various widths, having a branched structure. The dried CNFs seem to be redisintegrated in a similar manner to ND fibrils by agitation using a hand blender for just several minutes. Here, we randomly extracted 100120 fibrils to investigate the fibril size distribution and obtained the arithmetic average, the median, and the mode diameters, as shown in Table 3. Every CNF sample had a large standard deviation. The median diameter of ND was in accordance with the 1520 nm diameter that has been reported for CNFs produced by the grinder treatment,38 and the mode diameters of the samples showed smaller values than the median. This indicates that each distribution places a disproportionate emphasis on small values. The distribution graphs of all CNFs are supplied in the Supporting
ð7Þ
where the electrophoretic mobility UE = (λΔν)/(2nE sin(θ/2)); η, ε, and n are the viscosity, dielectric constant, and refractive index of the dispersion media, respectively; f(ka) is the Henry’s function, where f(ka) = 1 when ka . 1 and f(ka) = 2/3 when ka , 1, k is the inverse of Debye length and a is the major particle radius; λ is the incident wavelength (532 nm); Δv is the frequency shift by Doppler effect; E is the voltage; and θ is the scattering angle (173°). Differential Scanning Calorimetry. To investigate the hydration behavior on the redispersed CNFs, DSC measurements were made on the redispersed CNFwater suspensions, which were concentrated to ∼10 wt % CNF content by filtration. Tests were carried out on a DSC-6100 (SII Co., Ltd.). The temperature of ∼4 mg samples was cooled from 20 to 40 °C at a rate of 3 °C min1, kept at 40 °C for 10 min, and then raised to 20 °C at a rate of 3 °C min1.
’ RESULTS AND DISCUSSION Characterization of Dried CNFs. Pulp purified from Japanese cedar powder was fibrillated using a grinder to obtain disintegrated CNFs.38 Dried CNFs after solvent substitution into acetone were named AO; into tert-butanol named TO; into acetonitrile named ANO; into ethanol named EO; and into DMSO named DO. CNFs dried by freezedrying in water were named WFD, while samples identified as ND refer to CNFs that have not been dried and for which there has been no solvent substitution from water. The chemical compositions and crystallinity of the CNFs before and after solvent substitution and drying treatments are shown in Table 1. The hemicellulose content was calculated from the monosaccharide composition using the following equations and assuming that the glucose/mannose ratio was 1:4 for softwood glucomannan.41 Cellulose = glucose (1 mol glucose/4 mol mannose); xylan = xylose + arabinose; glucomannan = galactose + mannose + (1 mol glucose/4 mol mannose). All samples consisted mainly of ∼20% hemicelluloses 820
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Figure 1. Transmission electron micrographs of (a) ND, (b) DO, (c) WFD, (d) ANO, (e) EO, (f) TO, and (g) AO. Note 500 nm scale bars in lower right corners.
Table 3. Arithmetic Average, Median, and Mode Diameters of Redispersed CNFs Measured on the TEM Images ND arithmetic average
DO WFD ANO
EO
TO
AO
32.91 40.65 39.97 37.72 36.96 34.15 34.01
diameter (nm) standard deviation (nm) 26.16 32.33 30.85 29.30 29.25 25.11 28.11 median diameter (nm)
19.23 24.75 21.35 22.64 24.70 21.46 18.76
mode diameter (nm)
19.23 14.85
6.73 29.70
6.93 13.23
6.00
Information. These CNFs were subjected to successive measurement of the zeta potential. Successive Zeta Potential Measurements. ζt plots of redispersed CNFs were first measured in 1 mM KCl solution by the electrophoresis method. The pHs of the slurries were fixed at neutral values of 6.8 to 7 in order to keep the degree of deprotonation of carboxyl groups on the CNF surface constant43 during the test and between the samples, and for the calculations described in a later section. In this condition, the Henry’s function f(ka) required in eq 7 was set to 1 by assuming the Debye length k1 = 9.6 nm34 and the major particle radius a g 10 nm, based on TEM observations. Equation 7 with f(ka) = 1 is known as the Smoluchowski’s equation. Figure 2 shows that all the patterns have three parts, including first a rapid increase of the potential (an increase in the absolute value) within 10 min, then gentle reduction curves for ∼100 min, and finally the equilibrium state where the potentials approach constant values. The rapid increase in the first 10 min is thought to be the effect of the thermal convection and electroosmotic flow inside the electrode cell, which in principle is unavoidable using the device. Thus, the reduction curves after ∼10 min and the equilibrium values are subject to discussion regarding time dependence. The equilibrium values (ζ∞) of redispersed CNFs are positively correlated with the dielectric constant of solvents and the bulk density (see Tables 2 and 4.). The CNF surface dried from the original water in solvents having different dielectric constants seems to denature according to the solvent polarity. The t-butanolCNF slurry seemed to retain a slight amount of water after the solvent substitution, and the dielectric constant
Figure 2. ζt plots of dried CNFs. Different reduction patterns and plateau values, ζ∞, are observed. The rapid increase (absolute value) during the first 10 min seems to be due to the thermal convection and electroosmotic flow inside the electrode cell.
of the actual solvent might not decrease to that of pure t-butanol. The dielectric constant of the solvent of WFD is unclear because it involves the phase transition of water. However, considering that the constant of ice is ∼4,42 a reduction of polarity is understood to occur during freezing. The existence of a plateau region following the gentle attenuation indicates that the ζ∞ value of once-dried CNFs did not return to the original potential shown as ND. This tendency of alteration and attenuation of the zeta potential corresponds well to the zeta potential time dependence caused by swelling of the material; the ζt plots were, therefore, introduced to Kanamaru’s law to specifically examine them. Figure 3 represents the attenuating part of all ζt plots, showing good linearity, resulting in the introduction into eq 2. The attenuation behaviors of each plot are thought to be consistent with the time dependence referred to in Kanamaru’s law. Namely, it is presumed that the redispersed CNFs showing different time-dependent plots have different values of Δζ or hydration capacities. However, the ζ0 values used for the 821
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Table 4. Values of Measured ζ∞, Estimated ζ0, and Δζ ζ∞/mV
approximately ζ0a/mV
Δζb
AO
33.64
55.43
0.39
TO
37.11
53.49
0.30
WFD
41.84
54.71
0.23
ANO
44.74
58.12
0.23
EO
44.04
53.49
0.17
DO
47.65
53.99
0.11
ND
50.95
55.81
0.08
Approximate ζ0 was calculated by measured ζ∞ value and the ln(ζ∞ ζ0) value, which is the ordinate intercept of the linear approximation in Figure 3. b Δζ was calculated by approximate ζ0 values using eq 5. a
Figure 4. DSC heating curves of ∼10% wet mat of the redispersed CNFs and water.
Thermal Analysis. To examine the different swelling behavior, a DSC measurement was carried out on a ∼10 wt % wet mat of redispersed CNFs from the viewpoint of absorption water on CNFs. Thermal analysis can characterize the water conditions, with a different molecular motion energy derived from the bound force on the absorbents. It is known that three types of water are defined in DSC, including free water (Wf), which freezes at the same temperature as the bulk water; the freezing bound water (Wfb), which freezes at a lower temperature from the bulk water and having a lower melting point peak on the heat flow; and nonfreezing bound water (Wnf), which is directly bound water with no peak of freezing or melting.44 The amount of each kind of water can be calculated by the peak areas.45 The DSC heating profiles displayed in Figure 4 show that the profiles of CNFs shift slightly to the low-temperature side, with smaller peak areas compared with that of water. This indicates the existence of absorption water. We presumed the boundary between Wf and Wfb at the onset of the water peak of 2 °C and calculated the amounts of water as follows:
Figure 3. Natural logarithmic plots of the attenuation (ζ∞ζt) of dried CNFs versus t/min, which is described by eq 2. Solid lines are the linear approximation. ζt is the zeta potential at time t.
calculation of Δζ by eq 3 could not be measured directly in the device; in principle, the approximate ζ0 values were then estimated using the ζ∞ and ln(ζ∞ ζ0) values, which are the ordinate intercepts of the linear approximation shown in Figure 3. Table 4 shows that the approximate ζ0 values of each sample were almost the same at ∼55 mV. Here, the ζ0 means the provisional value of the zeta potential at t = 0 when the samples are not swelling at all. The same values of the approximate ζ0 are thought to signify that the potential on fibrils before swelling is the same between CNFs dried in different solvents, and the attenuation of the ζt plot occurs mainly by the structural swelling of fibrils. Furthermore, Table 4 also shows a clear inverse correlation between the ζ∞ and Δζ values. It is thought that the different degree of aggregation occurred not only between fibrils, as shown in bulk density, but also on a single fibril surface, and it seems to affect the swelling degree shown as Δζ. We also measured the ζt plots of CNFs in 0.1 mM KCl solution. In this salt concentration, the Henry’s function f(ka) required in eq 7 was set to 2/3 by assuming the Debye length k1 = 30.4 nm34 and the major particle radius a e 30 nm, based on the TEM observation. Equation 7 with f(ka) = 2/3 is known as H€uckel’s equation, which leads the sesquialteral potentials to the Smoluchowski’s equation. The plot (see Supporting Information) showed similar reduction patterns to those in the 1 mM KCl solution. For instance, in the case of AO, which showed the most marked change of ζ in the 1 mM KCl solution, we introduced the plot to Kanamaru’s law and in turn obtained ζ0 = 117.9 mV, ζ∞ = 93.8 mV, and Δζ = 0.20. These had almost the same attenuation (ζ0 ζ∞) as that of the 1 mM solution, whereas the values of Δζ, however, showed quite small values. The validity of the results are discussed below.
Wf ¼
Q ð g 2°CÞ 3 Wt ðg=gÞ ΔH 3 m
ð8Þ
Wfb ¼
Q ð < 2°CÞ 3 Wt ðg=gÞ ΔH 3 m
ð9Þ
where Q is the peak area (mJ/mg), ΔH is the specific heat of fusion of water (334 J/g), Wt is the total amount of water (mg), and m is the mass of solid (g). The amount of Wnf was calculated according to Wnf ¼
Wt Wf Wf b ðg=gÞ m
ð10Þ
In general, Wfb is known to exist as an intermediate between Wf and Wnf, and its amount detected by thermal analysis is mainly determined by the surface hydrophilicity, the pore structure, and especially by the surface area of the outer layer of Wnf.4447 In the case of wood pulp, it is reported that the fraction of Wfb is 822
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Table 5. Contents of Freezing Bound Water and Nonfreezing Water Standardized to 1 g of CNF in the ∼10% Wet Mats of the Redispersed CNFs Wfb/g g1
Wnf/g g1
ratio Wfb/Wnf
AO
0.69
2.00
0.35
TO
0.60
2.03
0.30
WFD
0.54
1.87
0.29
ANO
0.47
1.97
0.24
EO DO
0.54 0.40
2.04 1.74
0.26 0.23
ND
0.54
1.89
0.29
maintained virtually constant at ∼0.3 g g1 and that of Wnf is ∼0.4 g g1, when the pulp contains a sufficient amount of Wf.47 The contents of absorption waters on the redispersed CNFs displayed in Table 5 indicates that the amount of Wfb differed between the redispersed CNFs, and this tendency is wellcorrelated with the Δζ shown in Table 4, whereas the contents of Wnf are nearly constant between samples at ∼2 g g1. The disintegrated CNFs, which have a larger surface area than pulp, are understood to contain a large amount of bound water. The amount of Wfb should remain constant, even though the amount of Wnf changes. The redispersed CNFs, however, represent the significant difference of the fraction of Wfb compared with Wnf. AO and TO, in particular, show a larger Wfb/Wnf ratio of 0.3 to 0.35 than that of ND, whereas DO shows 0.23. Hence, there are strong indications that there exists more surface, weakly trapping more water molecules, on AO and TO than on ND and other CNFs. Namely, the Wfb fraction and the ratios of Wfb/Wnf are thought to indicate a different degree of hydration behavior, that is, the swelling behavior of the redispersed CNFs in the order of nanofibrils. This is considered to be mainly derived from the deformation and expansion of the polymer chains of the hemicelluloses located on the fibril surface having a different degree of dry aggregation according to the dielectric constants of the substituted solvents. The ratios of Wfb/Wnf are also in good agreement with the trend in the Δζ values of redispersed CNFs shown in Table 4. Therefore, the DSC result is thought to be consistent with the discussion of the zeta potential time dependence. Dimensional Analysis. Figure 5 shows a cylindrical CNF model of the swollen fibrils based on the results so far. The hemicelluloses that locate on the fibril surface with a different degree of aggregation swell with redispersion farther from the fibrils, and the fibril bundles probably disintegrate slightly during electrophoresis, as inferred from the branched structure of the fibrils shown in Figure 1. Here, assuming that swelling occurs only in the lateral direction, the volume fraction of water in the unit length at t = ∞ based on the model then should be equal to h∞ (eq 5). This relation holds for (r∞2πl r02πl)/r∞2πl = h∞ where r0 and r∞ are the fibril radii in the dry and swollen states, respectively, and l is the unit length. Solving this equation for r∞/r0, we obtained the swelling ratio expressed by the zeta potentials as follows: 0:5 r∞ ζ0 0:5 ¼ ð1 h∞ Þ ¼ ð11Þ r0 ζ∞
Figure 5. Schematic representation of CNF swelling model. The surface hemicelluloses can be expanded, and the microfibril bundles themselves can be partially disaggregated. The slip plane, the boundary of the mobility, shifts to the outside.
Figure 6. Relation between swelling behavior and potential curve based on the GouyChapman Stern model.34 Red arrows indicate that the swelling of polymer on the fibril surface causes the decay of the potential.
the shift of the slip plane out from the bulk surface by surface polymer swelling, as shown in Figure 6. We obtained the attenuation of ζ0 to ζ∞, so the swelling radius could be calculated by the potential curve. It is known that the potential curve of the diffuse layer is a linear approximation of the PoissonBoltzmann equation, as follows:48 z ð12Þ y ¼ z exp 3 kx 2 where y = (νeψ)/(kBT), z = (νeψ0)/(kBT), ψ0 is the surface potential, ψ is the potential at the distance x, v is the valence of the ions in the water, and kB is the Boltzmann’s constant. This approximation is applicable when z . 1 and kx < 1, corresponding to our condition having large absolute values of ψ0, which is bigger than the ζ0 value of ∼55 mV, whereas the well-known DebyeH€uckel equation is only applicable to the case of ψ0 < (25.6 mV.48 The value of 1/k is ca. 30.4 nm at a salt concentration of 104 N, ca. 9.6 nm at 103 N, and ca. 3.0 nm at 102 N.34 The zeta potential was measured in 1 mM KCl solution at 25 °C in this study, so we used 1/k = 9.6 (nm) and v = 1. Additionally, the following equation is known for the calculation corresponding
The surface chemical approach was employed to examine the swelling dynamics more accurately. Kanamaru’s law is based on 823
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without the higher-order structure like pulps also swelled to 1.14 times, as shown in Table 6. Therefore, it is indicated that the origin of the dimensional behavior of wood CNF materials with a higher-order structure can be explained mainly by the swelling of amorphous hemicelluloses located on the fibril surface with moisture absorption. Interestingly, the ideal diameters 2r0 (nm) can be calculated using the obtained swelling ratio r∞/r0 and its swelling radius x at t = ∞ as follows: 2x 2r0 ¼ r∞ 1 r0 Figure 7. Swelling curves of the redispersed CNFs calculated by eq 12.
Table 6 compares the diameters 2r0 of redispersed fibrils in a nonswollen state with the diameters measured directly from the TEM images. The values of 2r0 provide good agreement with the median diameters, except those of ND. Here, the zeta potential by the electrophoresis method was calculated using the peak values of the mobility distributions detected by the Doppler shift in light scattering. The mobility is directly affected by the velocity of particles during electrophoresis in the calculation and changes proportionally with particle size.28 In the present study, the mobility distributions showed single and sharp peaks in all samples, so the peak values and obtained potentials were thought to be almost the medians. Namely, the values of 2r0 calculated by the potentials should indicate the median diameters rather than the arithmetic averages in the non-normal distributions with large standard deviations of the diameters shown in Table 3. The coincidence of the calculated 2r0 and the measured median diameters is therefore considered to demonstrate that the approximation of swelling ratios and radii of redispersed CNFs is reasonably accurate. On the other hand, ND fibrils without drying showed greatly different values of 2r0 from the measured median diameter. This is considered to be because the ζ0 for ND is not the potential of the dry condition. In addition, ND fibrils showed a slight swelling behavior, thought to be mainly induced by the electrophoresis. The DSC results showing that the content of freezing bound water of ND was almost the same as that of EO or WFD infer that the ND fibrils originally swell from the colloidal CNF surface to the extent of x = ∼1.5 nm, estimated by the radius of EO and WFD. The large swelling radii of AO and TO is presumed to be derived from the rearrangement of hemicellulose on the CNF surface during drying in the low polarity solvents in preference to aggregation, and it is partially induced by the electrophoresis to expand and contact with more water.22 It is still possible that the amorphous cellulose on the surface or inside of fibrils can contribute to the swelling dynamics, though the potential measurement is unable to discriminate it from hemicelluloses. Assuming that the amount of swelling hemicellulose is 20% on each CNF, based on the chemical composition (Table 1), the median thicknesses of hemicellulose in the nonswollen state, described as rH, were calculated as the radius ratio to r0 (Figure 5). Thus, the total normalized change of swelling layer thickness on each CNF was measured as swellability, defined as x/rH.50 In Figure 8, we plotted swellability for each sample measured in the 1 mM KCl solution against the dielectric constant of substituted solvent. Swellability of hemicelluloses exponentially increased with the decrease in dielectric constant of solvent during drying. This relationship has a margin of error expressed as a coefficient of determination R2 of 0.72 in the
Table 6. Calculated Values of Swelling Ratio, Radius, Original Fibril Diameter 2r0, and the Median Diameters Measured by TEM Observation swelling ratio
swelling radius x
2r0
median diameter
r∞/r0
(nm)
(nm)
(nm)
AO
1.28
2.77
19.52
18.76
TO
1.20
2.16
21.55
21.46
WFD
1.14
1.66
23.11
21.35
ANO
1.13
1.61
23.07
22.64
EO DO
1.10 1.04
1.25 0.58
24.43 26.51
24.70 24.75
ND
1.03
0.49
26.72
19.23
ð14Þ
to T = 298 K:48 25:6 z ð13Þ ν 3 Therefore, introducing ζ0 and ζ∞ into ψ0 and ψ, we solved the equations for x to estimate the swelling radius of the CNFs redispersed in the 1 mM KCl solution shown in Figure 7 and Table 6. Each CNF redispersed in 1 mM KCl solution swelled in the initial 60 to 100 min to reach a different equilibrium width. Both the swelling ratios and radii of the redispersed CNFs were positively correlated with the dielectric constants of the solvents and the bulk densities of the CNFs in the dry state. The degree of aggregation on a macroscopic scale and the degree of swelling at the microscopic order are thought to be coincident. Furthermore, the large swelling radii of AO and TO of 23 nm seems to have contained a large amount of freezing bound water, as indicated by the DSC measurements. Hence, it has been demonstrated that a different degree of hemicellulose aggregation occurred on the nanofibril surface according to the solvent polarity during drying, and this affected the swelling radius. The swelling curves shown in Figure 7 closely resemble the dimensional change of pulp papers with hygroexpansion that have been reported to swell in the initial ∼60 min to reach the equilibrium level.16 WFD, the most fitting condition of drying in water, also showed swelling behavior over the initial 60 min, which is also in good agreement with paper behavior. In addition, Lundberg et al.49 have reported that pulp hornification, the irreversible aggregation between fibrils in pulp with drying,25 does not affect the equilibrium moisture content, that is, the swelling degree. The present study demonstrated that WFD ψ0 ¼
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Figure 8. Swellability x/rH versus dielectric constant of substituted solvent. Solid line is the exponential approximation. The inset shows a linear relationship between Δζ obtained by Kanamaru’s law and swellability x/rH led by surface chemical calculation. The dotted line represents a linear fit to the data, with a coefficient of determination R2 = 0.99.
exponential approximation. This is attributed to the fact that the degree of dewatering during solvent substitution on the CNF surface differed between the solvent species, as observed for the inverse relation of ζ∞ in AO and TO. The solubility in water of these solvents also seemed to affect this behavior. The swellability is considered to directly reflect the aggregation forces between macromolecular chains of hemicelluloses dried in the solvent. Considering that the thickness of the hemicellulose layer rH is ∼10.6% of r0, the hemicellulose layer on the CNF surface is on the order of several nanometers. For instance, rH on a single CNF 20 nm in diameter is calculated to be ∼10.6 Å. The analysis method combining zeta potential time dependence and a surface chemical approach has a subnanometer resolution for observing the behavior at the slip plane, as shown in Figure 7. The present study demonstrates that these extremely thin layers of amorphous hemicelluloses swelled with different swellabilities of 0.4 to 2.7 and were characterized by the surrounding polarity during drying. The values of Δζ, obtained by Kanamaru’s law, indicated a linear correlation with swellability x/rH, led by the surface chemical calculation. In the 1 mM KCl solution, the values of Δζ, as the volume fraction of hydration capacity in Kanamaru’s law, proved specific for predicting the swellability of the polymer located on the bulk surface. On the other hand, in the 0.1 mM KCl solution, we also obtained the swelling radius x = 2.85 nm in AO, which is a similar radius to that in the 1 mM solution. The prediction of ζ0 by eq 2 is thought to be reasonable under this condition. However, the value 2r0 of AO calculated by eq 14 was 47.29 nm, which is significantly different from both that in the 1 mM solution and the median diameter observed by TEM. This is mainly caused by the smaller value of Δζ, which led to the smaller value of r∞/r0 than those in the 1 mM KCl solution. The main difference between the salt concentrations 1 mM and 0.1 mM is the value of ka, affecting the Henry’s function f(ka). The polydispersity of our materials makes the determination of a difficult, and we assumed a value of f(ka) depending on the relation of the Debye length and the major particle size obtained by TEM observation. By means of the surface chemical calculation, the validity of
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Kanamaru’s law in the 1 mM KCl solution, where we assumed ka > 1 and f(ka) = 1, was verified. The results in the 0.1 mM KCl solution, however, suggested that eq 5 in Kanamaru’s law, which has been developed on a measure of streaming potentials requiring ka . 1, is possibly limited to applications where ka , 1. This experimental suggestion needs to be proven theoretically. The advantage of the potential measurement is that it removes the effects of water interacting with the investigated surface, and electrochemically defines the bulk surface. The zeta potential allows tracing of the behavior at the slip plane, which can shift by the swelling dynamics of the amorphous polymer chains on the colloidal bulk surface under suitable conditions, whereas the forcedistance curves with the direct measurements contain a lot of information including the osmotic repulsion of the double layers, van der Waals force of the bulk surface, solvation force, and depletion or steric force of the polymers at the same time.51 The zeta potential, as a tool for observing the behavior on the slip plane, reveals the swelling dynamics of wood CNFs quite accurately by introducing surface chemical analysis.
’ CONCLUSIONS The swelling dynamics of individual CNFs with a onedimensional structure were revealed with considerable accuracy by employing a zeta potential time dependence measurement with the electrophoresis method in a 1 mM KCl solution. The degree of swelling of redispersed CNFs is characterized by expansion of the amorphous hemicelluloses located on the fibril surface, with a different degree of aggregation during drying, according to the dielectric constant of the substituted solvents. Furthermore, the median swelling ratio, radius, and diameter of CNFs in water were obtained from the values of ζ0 and ζ∞ by the surface chemical approach. The diameters of redispersed CNFs calculated from these zeta potential values coincided well with the median diameters measured directly by TEM observation. The swellability of hemicelluloses exponentially increased with the decrease in dielectric constant of the solvent during drying. The values of Δζ, as the volume fraction of hydration capacity in Kanamaru’s law, indicated a linear correlation with swellability of the polymer located on the bulk surface. Swelling dynamics is thought to be a fundamental property of wood CNFs containing hemicelluloses. This consequence may contribute to the explanation of many phenomena, including those involved in the production of CNFs from pulps, the dimensional stability of wood CNFs as three-dimensional materials such as pulps, papers, films, gels, and suspensions or as fillers in nanocomposites, and the surface condition for the evaluation of wetting properties or surface forces interacting with other substances. An analysis method combining the zeta potential time dependence with a surface chemical approach proved useful for specifically evaluating the swelling dynamics of polymers on the bulk surface. ’ ASSOCIATED CONTENT
bS
Supporting Information. (S1) The relative monosaccharide composition of CNFs after treatment by solvent substitution and drying, (S2) fibril diameter distribution graphs for the redispersed CNFs, and (S3) the ζ-t plot of AO in the 0.1 mM KCl solution. This material is available free of charge via the Internet at http://pubs.acs.org. 825
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Corresponding Author
*Tel: +81 774 38 3669; Fax: +81 774 38 3658; E-mail: yano@ rish.kyoto-u.ac.jp.
’ ACKNOWLEDGMENT The authors thank Dr. Y. Horikawa, Research Institute for Sustainable Humanosphere, Kyoto University, for his significant efforts in carrying out TEM observations. This research was supported by a Grant-in-Aid for Scientific Research (grant number 22-4452) from the Japan Society for the Promotion of Science (JSPS). ’ REFERENCES (1) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. Review: Current International Research into Cellulose Nanofibres and Nanocomposites. J. Mater. Sci. 2010, 45, 1–33. (2) Sakurada, I.; Nukusina, Y.; Ito, T. Experimental Determination of the Elastic Modulus of Crystalline Resions in Oriented Polymers. J. Polym. Sci. 1962, 57, 651–660. (3) Page, D. H.; El-Hosseiny, F. The Mechanical Properties of Single Wood Pulp Fibers. Part VI. Fibril Angle and the Shape of the Stressstrain Curve. J. Pulp Paper Sci. 1983, 9, 99–100. (4) Nishino, T.; Matsuda, I.; Hirao, K. All-Cellulose Composite. Macromolecules 2004, 37, 7683–7687. (5) Nogi, M.; Iwamoto, S.; Nakagaito, A. N.; Yano, H. Optically Transparent Nano Paper. Adv. Mater. 2009, 20, 1–4. (6) Capadona, J. R.; Van den Berg, O.; Capadona, L. A.; Schroeter, M.; Rowan, S. J.; Tyler, D. J.; Weder, C. A Versatile Approach for the Processing of Polymer Nanocomposites with Self-assembled Nanofibre Templates. Nat. Nanotechnol. 2007, 2, 765–769. (7) Nakagaito, A. N.; Iwamoto, S.; Yano, H. Bacterial Cellulose: the Ultimate Nano-scalar Cellulose Morphology for the Production of High-strength Composites. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 93–97. (8) Pei, A.; Malho, J.-M.; Ruokolainen, J.; Zhou, Q.; Berglund, L. A. Strong Nanocomposite Reinforcement Effects in Polyurethane Elastomer with Low Volume Fraction of Cellulose Nanocrystals. Macromolecules 2011, 44, 4422–4427. (9) Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Transparent and High Gas Barrier Films of Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation. Biomacromolecules 2009, 10, 162–165. (10) Aulin, C.; G€allstedt, M.; Lindstr€om, T. Oxygen and Oil Barrier Properties of Microfibrillated Cellulose Films and Coatings. Cellulose 2010, 17, 559–574. (11) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; MacLachlan, M. J. Free-standing Mesoporous Silica Films With Tunable Chiral Nematic Structures. Nature 2010, 468, 422–426. (12) Olsson, R. T.; Azizi Samir, M. A. S.; Salazar-Alvarez, G.; Belova, L.; Str€om, V.; Berglund, L. A.; Ikkala, O.; Nogues, J.; Gedde, U. W. Making Frexible Magnetic Aerogels and Stiff Magnetic Nanopaper Using Cellulose Nanofibrils as Templates. Nat. Nanotechnol. 2010, 5, 584–588. (13) Walther, A.; Timonen, J. V. I; Díez, I.; Laukkanen, A.; Ikkala, O. Multifunctional High-Performance Biofibers Based on Wet-Extrusion of Renewable Native Cellulose Nanofibrils. Adv. Mater. 2011, 23, 2924– 2928. (14) Terashima, N.; Kitano, K.; Kojima, M.; Yoshida, M.; Yamamoto, H.; Westermark, U. Nanostructural Assembly of Cellulose, Hemicellulose, 826
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’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on December 2, 2011, with errors in equation 2 and Figure 3. The corrected version was reposted on December 8, 2011.
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