Article pubs.acs.org/Biomac
Dispersibility in Water of Dried Nanocrystalline Cellulose Stephanie Beck,* Jean Bouchard, and Richard Berry† FPInnovations, 570 St. Jean Boulevard, Pointe-Claire, QC H9R 3J9 Canada ABSTRACT: Dispersibility is important for nanocrystalline cellulose (NCC) because recovering the unique suspension and particle properties is essential after the product has been dried for storage or transport. It is our goal to produce dried NCC that redisperses in water to yield colloidal suspensions without the use of additives or a large energy input. In contrast with the as-prepared acidic form of NCC (HNCC), suspensions of neutral sodium-form NCC (Na-NCC) dried by evaporation, lyophilization, or spray-drying are readily dispersible in water. Suspension properties and NCC particle size determined by light scattering were used as indicators of dispersion quality. The neutral counterion content, drying technique, freezing action, drying and redispersion concentrations, and moisture content in the dried NCC were all found to influence dispersibility. When a minimum of 94% of the H+ counterion is exchanged for Na+, the neutral salt form is fully dispersible in water even when fully dried. Mild sonication is generally sufficient to recover measured particle sizes identical to those in the never-dried Na-NCC sample. A threshold moisture content of 4 wt % was found, above which dried H-NCC is fully dispersible in water.
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INTRODUCTION Nanomaterials can be defined as materials consisting of particles with at least one dimension smaller than 100 nm. The current widespread interest in nanomaterials arises from the altered or enhanced properties of materials as the particle size decreases toward the atomic scale, changes that are due to the much higher specific surface area compared with the same material in larger form as well as the emergence of quantum effects. A larger specific surface area results in higher chemical reactivity and altered strength and electrical properties, whereas quantum effects alter the electrical, magnetic, and optical properties of materials. These properties can be exploited to make new value-added products such as photonic crystals or nanocomposites with greatly improved mechanical properties. A significant challenge in the handling of many nanomaterials arises when they must be dried from a suspension of welldispersed individual particles. The unique properties of nanomaterials depend on their dimensions; individual nanoparticles are essential for the full expression of those properties. Irreversible particle aggregation often occurs when the nanoparticles are dried, leading to loss of properties or functionality. Cryoprotectants, surfactants, and surface modification have been used to prevent aggregation and other detrimental effects of drying.1,2 However, the resulting dispersion often shows degraded or altered properties and can require time-consuming and expensive cleanup steps to remove the protective agent(s). Nanocrystalline cellulose (NCC) is an organic nanomaterial that has seen growing interest. Unlike many nanomaterials, NCC is not synthesized from molecular or atomic components but rather extracted from naturally occurring cellulose. NCC is typically produced by the controlled sulfuric acid hydrolysis of such cellulose sources as bleached wood pulp.3−7 The use of © 2012 American Chemical Society
sulfuric acid imparts negatively charged, acidic sulfate ester (-OSO3− H+) groups at the NCC particle surface, causing electrostatic repulsion between the rodlike colloidal particles and resulting in stable aqueous NCC suspensions.5,8−13 Undisturbed evaporation of aqueous NCC suspensions produces solid semitranslucent NCC films that retain the chiral nematic liquid crystalline order inherent to NCC suspensions above a critical concentration. Freeze-drying NCC suspensions produces a material with textures ranging from flaky lamellar to solid foams. Spray-drying produces a freeflowing white powder similar in appearance and texture to cornstarch or flour. NCC is a sustainable, biodegradable, recyclable material produced from abundant renewable resources. These factors and their unique mechanical and optical properties have generated interest in manufacturing NCC-based products on an industrial scale. Most applications will require NCC to be delivered in dried form and redispersed at the site of use to minimize shipment size, weight, and cost. Drying also inhibits bacterial and fungal growth in the NCC and is frequently a necessary step for solvent exchange prior to dispersing NCC in organic solvents for chemical modification2,14 and in polymers for nanocomposites manufacture.3 It has been found that the completely protonated acid-form of NCC (H-NCC) is not dispersible in water once it has been fully dried, even under fairly gentle conditions.15 However, when the protons are exchanged for neutral monovalent cations such as Na+, the dried salt-form NCC is completely redispersible in water to give colloidal NCC suspensions Received: February 3, 2012 Revised: March 23, 2012 Published: April 6, 2012 1486
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sonicator baths are not powerful enough to achieve full dispersion of NCC aggregates. Freeze-dried and spray-dried Na-NCC were redispersed with shaking in DI water to give various dispersed NCC concentrations. Particle Size Measurement by Light Scattering. To be considered fully redispersible, dried NCC must yield aqueous suspensions having similar particle sizes to those measured in neverdried NCC suspensions. Particle size was determined by photon correlation spectroscopy (PCS) on a Malvern Zetasizer 3000 and a Malvern Zetasizer Nano ZS. Because PCS is a light-scattering method, the measured NCC particle size values quoted are the z-average (intensity mean) hydrodynamic diameters of equivalent spheres and do not represent actual physical dimensions of the rodlike NCC particles. However, they are valid for comparison purposes. To avoid excess light scattering in the spectrometer, the samples measured on the Zetasizer 3000 were diluted to 1.0 to 1.5 wt % NCC and contain 10 mM NaCl, and those measured on the Zetasizer Nano ZS were diluted to 0.05 wt % NCC and contain 5 mM NaCl. Samples were filtered with 0.45 μm nylon or 0.7 μm GF/F Whatman syringe filters prior to measurement. No significant loss of NCC was measured by gravimetry after filtration. Measurements were performed in triplicate at 25 °C.
having similar properties to those of the original suspensions following brief ultrasonication.15 During the pioneering research on NCC conducted at McGill University’s Pulp and Paper Research Centre, Orts et al. redispersed freeze-dried NaNCC in D2O with ultrasonication to concentrations of 0.5−13 wt % NCC;16,17 however, no evidence was provided that the dispersions contained individual cellulose nanocrystals. Evaluations based on FTIR spectra of H-NCC and Na-NCC films have suggested that there are stronger and more numerous intermolecular hydrogen bonds between cellulose nanocrystals in dried H-NCC than in Na-NCC and that this difference may create the difference in observed dispersibility.15 Whereas the redispersibility of completely dried NCC has been previously studied in pure water and organic solvents,14,15,18 it is also known that freshly cast free-standing H-NCC films or thin H-NCC films on solid substrates will swell and disperse in water with slight agitation.11,18,19 The moisture content of these films was not determined. It can be speculated that the freshly cast films contain enough residual moisture to disrupt interparticle hydrogen bonding; however, there is no previous literature quantifying the effect of moisture content on the redispersibility of dried NCC. It is the goal of this Article to characterize the dispersibility in water of dried NCC and to quantify the factors needed to ensure full dispersibility and recovery of the original suspension properties.
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RESULTS AND DISCUSSION Morphology of Dried NCC. NCC suspensions yield very different products when dried by different methods. Solid NCC films produced by undisturbed evaporation at room temperature are glossy and iridescent (Figure 1a) and tough but quite brittle. Freeze-dried NCC is usually in the form of thin lamellar flakes that appear transparent or white with occasional bluish iridescence (Figure 1b). Spray-dried NCC is in the form of a free-flowing, flour-like powder (Figure 1c). Figure 1d−f shows environmental scanning electron microscopy (ESEM) images of the fracture edges of an air-dried NCC film (0.10 kg/m2 basis weight), the edge of a flake of freeze-dried NCC (dried from a 4.3 wt % NCC suspension), and rounded granules of spray-dried NCC (dried from a 5 wt % suspension), respectively. The NCC film is ∼100 ± 10 μm thick, whereas the freeze-dried NCC flake appears to be only 200 nm or so thick. Individual cellulose nanocrystals are visible at the edge of the flake (arrows). The spray-dried NCC granules vary in diameter from 5 to 30 μm. As an undisturbed NCC suspension evaporates, the NCC concentration gradually increases. Once the critical concentration for chiral nematic liquid crystal formation is exceeded, an ordered phase forms. As the NCC concentration continues to increase, the ordered phase increases in volume fraction until it occupies the entire suspension. A solid NCC film that retains the liquid crystal order in a tightly packed array of particles is obtained when all of the water has evaporated. Freeze-drying yields a different form of dried NCC because of the immobility of the particles in the frozen suspension. Owing to electrostatic repulsion, NCC nanoparticles in suspension are distributed with a fairly regular interparticle distance, which is preserved in the ice, as revealed by cryo-TEM images of cotton- and tunicin-derived NCC.20 A suspension at a concentration near the critical concentration will contain ordered domains called tactoids in which the nanoparticles are arranged with chiral nematic texture.6 This may account for the slight iridescence observed in many of the samples. The separation between the nanoparticles will depend on the total NCC concentration, higher concentrations leading to shorter separation distances. Freezing tends to cause particle aggregation,21 but ice crystal formation likely also disrupts any chiral nematic pseudolayers. During the sublimation step,
EXPERIMENTAL METHODS
Suspension Preparation and Characterization. Aqueous NCC suspensions were prepared from a commercial bleached softwood kraft pulp according to a procedure modified from the literature (e.g., ref 9). For this study, the NCC yield was ∼35% based on the dry weight of the pulp. The sulfate ester content (230 ± 10 mmol S/kg NCC) was measured by elemental analysis with inductively coupled plasma spectroscopy−atomic emission spectroscopy. Counterion Exchange. Sodium hydroxide (0.2 N, Sigma-Aldrich) was added to an acid-form NCC suspension until a pH value of 7 was obtained (Corning Pinnacle M540 pH meter with an accumet combination pH electrode). Alternatively, Dowex Marathon C sodium-form cation exchange resin (Sigma-Aldrich) was added to the acid-form NCC suspension (2.8 wt % NCC) and gently stirred for about 1 h. Different mass ratios of resin to NCC were used; conductometric titration of the resulting suspension with sodium hydroxide was performed to quantify the remaining protonated HNCC content. Unless otherwise noted, “Na-NCC” shall denote a fully neutralized NCC suspension. Drying NCC Suspensions. Aliquots of NCC suspensions were placed in polystyrene Petri dishes and allowed to evaporate under ambient conditions until solid air-dried NCC films were obtained. The moisture content of the films was determined by weighing them before and after brief (≤5 min) drying in an oven at 105 °C. Film thicknesses (60−100 μm) were measured using a digital micrometer (Fowler IP54). NCC suspensions (0.1 to 12 wt %) were frozen, typically at −65 °C, and then lyophilized (VirTis Freezemobile 12SL) at room temperature and ∼50 mTorr vacuum to give a flaky, lamellar freeze-dried product. An Na-NCC suspension (5 wt %) was pumped into the rotary atomizer of a spray dryer (V versatile P6.3, Niro Atomizer) at a feed rate of 20 L/h. Inlet and outlet temperatures were chosen appropriately. A fine white powder of spray-dried NCC was obtained. Redispersion of Dried NCC in Water. Solid air-dried H-NCC and Na-NCC films of different moisture contents were redispersed by placing portions of the films in deionized (DI) water and stirring until a uniform dispersion with no visible macroscopic particles was obtained. In some cases, the resulting mixture was sonicated (Sonics vibra-cell 130-W ultrasonic processor, 6 mm probe, 60% amplitude), typically to an energy input of 2000 J/g NCC. It should be noted that 1487
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H-NCC was not. Depending on the moisture content, air-dried H-NCC films sometimes swelled when placed in water but retained their chiral nematic structure and returned to the original state when dried again, whereas freeze-dried and spraydried H-NCC did not undergo any significant change when placed in water due to the absence of a chiral nematic structure. In contrast, air-dried Na-NCC films immediately swelled and finally dispersed to give homogeneous colloidal dispersions. Freeze-dried Na-NCC flakes also redispersed readily in water to give homogeneous colloidal dispersions. Spray-dried Na-NCC initially formed a gel in water, which dispersed with high-speed mixing. In all cases, brief ultrasound treatment produced homogeneous dispersions of Na-NCC that at equilibrium separated into isotropic and chiral nematic phases, as expected. Sodium-form cation exchange resin was applied to H-NCC at increasing mass ratios of resin to NCC, yielding suspensions containing increasing percentages of Na+ counterion, as determined by conductometric titration. The data in Table 1 Table 1. Effect of Sodium Counterion Content on the Dispersibility of Freeze-Dried NCC in Deionized Water without Sonication Treatment
Figure 1. (a) Air-dried NCC film showing its iridescent properties when viewed normal to the surface (left) and at an oblique angle to the surface (right). Scale bar = 2 cm. (b) Flakes of freeze-dried NCC. Scale bar = 1 cm. (c) Spray-dried NCC powder. Scale bar =1 cm. ESEM images: (d) Fracture edge of an air-dried NCC film with visible NCC layers. Scale bar = 20 μm. (e) Edge of freeze-dried NCC flake (arrows indicate individual nanocrystals). Scale bar = 500 nm. (f) Granules of spray-dried NCC. Scale bar = 10 μm.
a b
pH
% Na+ content
dispersibilitya
2.53 3.12 3.49 3.82 3.97 4.10 4.22 4.33 5.05 6.15 7.00
0 70.6 86.8 93.1 93.9 94.3 94.6 94.9 96.6 98.1 100
none none none very slight partial nearly complete complete complete complete complete complete; rapid
Zave particle size (nm)b
132 132 115 115 111 109 115
± ± ± ± ± ± ±
3 10 2 4 2 3 2
10 mg of freeze-dried NCC in 5 mL of DI water (0.2 wt % NCC). Measured by PCS at 1 wt % NCC in 10 mM NaCl.
show that increasing the relative amount of Na+ cations improves the dispersibility of the freeze-dried NCC. Although some dispersibility is apparent beginning around pH 3.8, there appears to be a threshold between pH 4.1 and 4.2, above which full dispersibility is reached. The proton counterions must be almost completely replaced by sodium ions (≥94%) before the interparticle hydrogen bonding network is weakened enough to allow redispersion of the dried NCC. Upon redispersion, lightscattering measurements show the particle sizes to be nanometric, even in the absence of sonication to break up residual aggregates or agglomerates. As expected, the measured particle size of the redispersed freeze-dried NCC does not decrease with pH from just above the threshold value for complete dispersibility up to pH 7 (100% Na+ counterion content). Air-dried NCC shows similar redispersion behavior. Redispersed Particle Size and Suspension Properties. Although simply redispersing dried Na-NCC in water yields a uniform colloidal suspension with no visible particles, it is clear that drying causes some aggregation of NCC particles. However, the aggregates remain stable in suspension over time without further treatment. NCC suspension properties, such as the critical concentration c* required for phase separation, are an important indication of the properties of the individual NCC particles themselves.9 Phase separation of rodlike particles is governed
further aggregation will occur mainly between particles already in close proximity due to their immobility in the ice matrix, resulting in easily separable thin lamellar flakes. When spray-drying an NCC suspension, it is passed through an atomizer, where it is broken into small droplets that upon exiting meet a stream of hot gas, losing moisture very rapidly and forming a free-flowing powder. The powder is in the form of rounded granules containing aggregated NCC particles. Heat damage of the NCC is minimal because of the very short residence times and cooling due to the evaporation of the water surrounding the particles. Neutral Counterion Content. The effect of different neutral monovalent counterions M+ on the bulk properties of aqueous M-NCC suspensions has been investigated.9 Among these properties are the critical concentration for ordered phase formation, the chiral nematic pitch of the liquid crystalline phase, and the redispersibility of solid M-NCC films. However, the redispersibility of M-NCC was only examined qualitatively; phase separation of the redispersed M-NCC suspensions was established, but quantitative data such as dispersed particle size and critical concentration for phase separation were not obtained.9 In agreement with the literature,15 it was found that fully dried Na-NCC was redispersible in water, whereas fully dried 1488
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by the particle geometry (c* = f(D/L), where D is the rod’s diameter and L is its length), as described by Onsager.22 The presence of significant numbers of NCC aggregates will therefore influence the phase separation behavior of an NCC suspension. Mechanical treatment such as sonication is a necessary step in the preparation of colloidal NCC dispersions to eliminate aggregates or agglomerates and separate the individual nanocrystals.6 Sonication of redispersed dried Na-NCC was necessary in order for a chiral nematic liquid crystalline phase to form, indicating that aggregates prevent or hinder phase separation. NCC aggregates increase the critical concentration for phase separation: A 4.5 wt % dispersion of freeze-dried NaNCC treated with 1500 J/g NCC sonication had a critical concentration that was 0.6 wt % higher than that of the original Na-NCC suspension (Figure 2). The greater critical concen-
Figure 3. Films cast from a 2.8 wt % suspension of redispersed freezedried Na-NCC. The suspensions were subjected to increasing sonication as shown. Scale bar = 1 cm.
Light scattering measurements on suspensions of redispersed dried Na-NCC indicate the presence of aggregates. The aggregation of NCC particles cannot be completely reversed by dilution; mechanical energy input such as sonication is required.9 Figure 4 shows the reduction in measured particle
Figure 2. Volume fraction of chiral nematic (CN) phase as a function of NCC concentration for never-dried Na-NCC sonicated to 667 J/g NCC (■) and redispersed freeze-dried Na-NCC sonicated to 1500 J/ g NCC (□). Freeze-dried Na-NCC was originally redispersed to 4.5 wt % NCC and diluted with DI water to prepare other samples. Lines are intended as a guide to the eye. Figure 4. Effect of sonication on the measured Zave particle size (■) and particle size polydispersity (□) for freeze-dried Na-NCC redispersed to 1.5 wt %. Measured by PCS at 1.5 wt % NCC in 10 mM NaCl. The standard deviation of the Zave particle size is smaller than the data points.
tration of redispersed dried NCC suggests that the majority of the NCC particles aggregates in “bundles” side-by-side instead of end-to-end.23 The particle diameter D will thus increase much more than the length L, resulting in a lower axial ratio and increasing the critical concentration.9,22 The relative slopes of the phase separation diagrams in Figure 2 suggest that the biphasic coexistence region is broader for dispersions of freezedried Na-NCC than for dispersions of never-dried Na-NCC at the sonication conditions used. Further sonication of redispersed NCC suspensions may result in identical phase behavior to that of never-dried NCC. Films prepared from unsonicated never-dried NCC suspensions typically show iridescence in the ultraviolet to blue range. With increasing sonication energy input, the peak reflection wavelength red shifts toward longer wavelengths.24 Solid NCC films were cast from redispersed freeze-dried NaNCC suspensions. In the absence of sonication, the films have a cloudy appearance due to light scattering from aggregates (Figure 3); they lack a chiral nematic texture and are not iridescent. After significant amounts of sonication, the freezedried Na-NCC films showed the characteristic iridescence of films cast from well-dispersed, never-dried NCC suspensions. Peak wavelengths were red-shifted with additional sonication, further indicating that the optical properties of never-dried NCC films were retrieved.
size caused by increasing sonication of suspensions of redispersed freeze-dried Na-NCC. Sonication energy input greater than ∼2000 J/g NCC appears to have only a slight effect on measured particle size, in qualitative agreement with the results of Dong et al., who detected a decrease in particle length by PCS only during the first 5 min of sonication.9 Sonication also reduces the polydispersity index of the particle size distribution. It should be noted that the value of the polydispersity (from 0 to 1) given by the Zetasizer is an indication of the broadness of the particle size distribution of the rodlike NCC measured as equivalent spheres, with larger values indicating greater polydispersity.25 Sonication applied as a pretreatment prior to lyophilization of a suspension does not have a significant effect on the measured particle size of the redispersed dried NCC. Despite the observations made in this section, sonication is not always applied to suspensions of redispersed NCC. Figure 4 shows that nanometer size particles (140−160 nm) are obtained spontaneously upon redispersion without sonication. 1489
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Drying Method. As mentioned above, the methods used to dry suspensions of NCC particles yield different forms of solid NCC with different properties. It was observed that when suspending equal masses of dried Na-NCC in the same volume of water, freeze-dried NCC invariably dispersed much more quickly than an air-dried film cast from the same suspension, producing dispersions of smaller particles requiring less sonication to obtain complete dispersion. Water penetrates more slowly into a thicker air-dried NCC film than into a thin freeze-dried NCC flake; the chiral nematic order of the NCC film structure may also facilitate the formation of a stronger or denser network of hydrogen bonds, holding the NCC layers together more tightly. The larger specific surface area of freezedried Na-NCC enhances its rapid dispersion. Spray-dried NCC also disperses slowly. Its granules are micrometric in dimension; in water, a gel layer forms at the granule surface, surrounding a relatively large core of dry NCC. The gel layer slows water penetration, increasing the mechanical energy input required to obtain a well-dispersed suspension. Table 2 lists mean Zave particle size values measured for suspensions of never-dried and redispersed air-dried, freeze-
aggregates. Spray-drying occurs in two stages. Rapid evaporation from the surface of the NCC suspension droplets occurs when they are first exposed to the hot gas, likely forming an outer shell of dried NCC. Diffusion-rate-limited evaporation then takes place as water diffuses to the surface.26 Aggregation can occur during the entire evaporation process, much like airdrying under ambient conditions, although the high rate of spray-drying can be adjusted to minimize this. In contrast, during freeze-drying, the ice surrounding the (individual and aggregated) NCC particles keeps them separate during the sublimation process, preventing the close approach required for strong hydrogen bond formation between particles. As compared with drying by evaporation, freeze-drying has an additional, inherent complication: Prior to removal of the water by sublimation, the liquid suspension of NCC must be frozen. It has been found that the rate at which aqueous particle dispersions are frozen can alter the dispersibility of the dried nanoparticles.21 Slower freezing allows water molecules to exclude the nanoparticles from the solid lattice of frozen water, pushing them into closer proximity and leading to the formation of strong agglomerates. The sublimation step following freezing is less important in this respect, as the frozen nanoparticles do not have enough mobility to form further aggregates. Freeze−thaw experiments were performed in which NaNCC suspensions (20 mL, 2.8 wt % NCC, 1350 J/g NCC sonication pretreatment) were frozen at different rates and then thawed at 25−35 °C. “Slow” freezing corresponds to freezing a room temperature sample to −20 °C (ΔT ≈ 45 °C), “medium” to freezing a sample at 4 °C to −65 °C (ΔT ≈ 69 °C), and “fast” to freezing a sample at 4 °C using liquid nitrogen (−196 °C; ΔT ≈ 200 °C). Upon thawing, the samples were vortexed for 1 min at 3000 rpm to disperse any larger, weaker agglomerates. Aggregation was measured by comparing the particle size before and after freezing. All of the frozen-thenthawed samples showed a significant increase (∼100%) in mean Zave particle size compared with the unfrozen suspension. However, this aggregation is somewhat less severe at the faster freezing rate (light gray bars, Figure 5). H-NCC suspensions showed similar freezing-induced aggregation (data not shown).
Table 2. Effect of Drying Method on the Mean Zave Particle Size for Suspensions of Never-Dried and Re-Dispersed Dried Na-NCC drying method never-dried
air-drying (evaporation)
freeze-drying
spray-drying
sonication (J/g NCC)a 0 240 950 1900 0 240 950 1900 0 240 950 1900 0 240 950 1900
Zave (nm)b 65 62 57 55 148 80 66 57 105 72 59 54 127 82 64 59
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1 1 1 1 7 1 1 1 3 1 1 1 1 2 2 1
polydispersity indexc 0.45 0.48 0.47 0.42 0.75 0.77 0.46 0.43 0.63 0.51 0.45 0.44 0.56 0.51 0.48 0.48
a
Sonication applied to 15 g of 2.8 wt % never-dried or redispersed NCC. bMeasured by PCS at 1.4 wt % NCC in 10 mM NaCl. cLarger values are indicative of a broader particle size distribution.
dried, and spray-dried Na-NCC after sonication from 0 to 1900 J/g NCC. Air-drying and spray-drying produce significantly larger aggregates than freeze-drying. However, applying 950 J/g NCC of sonication to each never-dried or redispersed sample was sufficient to render the particles identical in both size and polydispersity, regardless of the drying method. An additional sonication energy input of 950 J/g NCC (total input of 1900 J/ g NCC) did not significantly further reduce the particle size or polydispersity. Sonication effectively removes the differences caused by the drying conditions. The different drying mechanisms are likely responsible for the observed differences in aggregate sizes when identical suspensions are dried: Surface tension during the evaporation of water from aqueous suspensions will tend to pull adjacent NCC particles toward each other, facilitating the formation of
Figure 5. Mean Zave particle sizes for a 2.8 wt % 1350 J/g NCC presonicated aqueous Na-NCC suspension, which was (a) frozen at different freezing rates and then thawed without drying (light gray bars) or (b) frozen, lyophilized, and redispersed to give a 2 wt % NCC suspension (dark gray bars). No sonication was applied to the redispersed samples. Particle sizes were measured by PCS at 1 wt % NCC in 10 mM NaCl. 1490
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When the same Na-NCC suspensions were frozen at the different rates as described above and then lyophilized and redispersed, a slightly greater degree of nanoparticle aggregation was observed in the redispersed material (dark gray bars, Figure 5). The data demonstrate that the freezing process is mainly responsible for the aggregation observed in freeze-dried NCC samples. No sonication was applied to the samples to observe the particle aggregation caused by freezing and freezedrying. Initial Drying Concentration. The NCC suspension concentration at drying may control the measured particle size by controlling the extent of NCC aggregate formation through interparticle separation distances. Shorter separation distances at higher concentrations would tend to increase the intensity of the interparticle attractive forces (i.e., hydrogen bonding) during the elimination of the surrounding water. When the freeze-drying concentration increases from 0.5 to 12 wt % NCC, measured particle size for freeze-dried NCC redispersed to 1 wt % in DI water with no sonication increases by only 15 to 18% (Figure 6). As the sample volume decreases
Figure 7. Samples of redispersed spray-dried Na-NCC of increasing concentration (5, 10, 20, and 40 wt %).
0.05 wt % NCC were then prepared from the initial samples by dilution to allow particle size measurement by light scattering. Particle size measurements show a trend of increasing particle size at low redispersion concentrations, followed by decreasing particle size with increasing redispersion concentration above 2 wt % for spray-dried NCC and 5 wt % for freeze-dried NCC (Figure 8).
Figure 6. Effect of suspension concentration at drying on the measured Zave particle size for freeze-dried Na-NCC redispersed to 1 wt % NCC in DI water without sonication pretreatment to disperse further the samples. Measured by PCS at 0.05 wt % NCC in 5 mM NaCl. Figure 8. Zave particle size measured for spray-dried Na-NCC redispersed to 0.1 to 36 wt % (■) and freeze-dried Na-NCC redispersed to 0.1 to 20 wt % (□) without applying sonication to disperse further the particles. Dashed lines are linear best fit to the data. Measured by PCS after dilution to 0.05 wt % NCC containing 5 mM NaCl (0.7 μm filtered).
during water removal by evaporation, NCC concentration increases and surface tension pushes the NCC particles together. In contrast, when freeze-drying, water removal during sublimation does not force the particles closer together, and the sample volume does not change. NCC particles must approach each other closely to allow formation of the strong hydrogen bonds that cause aggregation. As the initial NCC suspension concentration is increased, the interparticle separation in suspension decreases. In the frozen state, more particles will therefore be sufficiently close to form aggregates, causing an increase in particle size, but to a much lesser extent than during evaporation from a liquid suspension of the same concentration. Redispersion Concentration. The concentration to which spray-dried and freeze-dried Na-NCC is initially redispersed affects the measured particle size. Suspensions (5 wt %) of NaNCC were spray-dried or freeze-dried, then redispersed to initial concentrations of around 0.1 to 40 wt % NCC in DI water (Figure 7). Because it is impossible to sonicate the higher concentration dispersions this produces, the dispersions were not subjected to sonication treatment; particle size differences are due only to redispersion concentration effects. Samples of
The measured particle size behavior with increasing redispersion concentration may be partially accounted for by a balance between the effects of interparticle repulsion and charge screening due to the increasing effective ionic strength in the resulting suspension caused by the Na+ counterions. At a very low redispersion concentration (0.1 wt %), the majority of the Na-NCC aggregates separate easily because the low effective ionic strength of the dilute suspension facilitates repulsion between individual particles, leading to a greater quantity of individual particles. As the concentration increases, the increasing effective ionic strength reduces the electrostatic repulsion between NCC particles in suspension such that the NCC aggregates do not tend to disperse as well, increasing the measured particle size. Above a threshold NCC redispersion concentration, the measured NCC particle size begins to decrease. Unfiltered samples show the same trend. At the 1491
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months (this work). When the proton counterion is exchanged for a neutral monovalent cation such as Na+, dried NCC is completely redispersible in water.15 However, some applications may require the use of dispersible H-NCC. Samples of H-NCC and Na-NCC suspensions of equal concentration were dried by evaporation at room temperature, and some of them were heated to 105 °C for various lengths of time to generate samples with decreasing residual moisture contents. Residual moisture contents were determined by gravimetry (heating a portion of each sample to constant mass at 105 °C). The dispersibility of the dried NCC is summarized in Table 3. Air-dried H-NCC containing more than ∼4 wt %
moment, we do not have a plausible explanation for this phenomenon; there appears to be two mechanisms of particle separation occurring during redispersion that require further investigation; however, this behavior was eliminated by sonication. Mild sonication (2500 J/g NCC) of the above-described redispersed spray-dried Na-NCC suspensions after adjusting the NCC concentration to 2 wt % resulted in significantly reduced, uniform Zave particle sizes of 82−83 nm, as illustrated in Figure 9. Samples were prepared over an initial redispersion
Table 3. Dispersibility of Air-Dried H-NCC and Na-NCC Films with Different Residual Moisture Contents dispersible?a drying time @ 105 °C (min) wt % H2O in H-NCC 0 0 0 0 2 120
10.6 4.6 3.4 0.6 ∼0 ∼0
HNCC
NaNCCb
Y Y N N N Nc
Y Y Y Y Y Y
a
Disperses completely to give colloidal suspension when undisturbed for several hours. bResidual moisture content not determined; produced under the same conditions as H-NCC films. cFilm charred.
Figure 9. Zave particle size measured for redispersed NCC suspensions to which 2500 J/g NCC sonication was applied, prepared from spraydried (■) and freeze-dried (□) Na-NCC initially redispersed to 0.1 to 20 wt % NCC and then diluted or concentrated to 2 wt % NCC prior to sonication. Measured by PCS after dilution to 0.05 wt % NCC containing 5 mM NaCl. Horizontal lines are intended as a guide to the eye.
water was found to be fully redispersible in water. After brief sonication, the resulting suspension properties are similar to those of the original suspension. At these higher water contents, there may be enough water molecules located between NCC particles to interfere with or prevent the formation of hydrogen bonds directly between the hydroxyl groups of cellulose chains on neighboring particle surfaces. Hydrogen bonds are the main force holding solid dried NCC (whether dispersible or not) together; for example, they are 100 times stronger than van der Waals forces.27 Water−water hydrogen bonds have lower binding energy than cellulose−water hydrogen bonds and will be broken first as water molecules are eliminated from between the NCC surfaces during drying. When a monomolecular layer of water remains between the cellulose surfaces, cellulose− water hydrogen bonds are cleaved and interparticle H bonds between the closely approaching cellulose hydroxyl groups are formed.27 H-NCC suspensions dried to residual moisture contents below 4 wt % were not dispersible, regardless of the drying technique used. Only the sodium form of dried NCC is fully dispersible at residual moisture contents below the threshold value of 4 wt %: For example, freeze-dried NaNCC disperses easily despite its very low moisture content after drying. Crystal-to-crystal hydrogen bonding interactions may be affected by the presence of sulfate ester groups at the C6 position, particularly when NCC is in the sodium form. However, dry H-NCC appears to have stronger interparticle bonding than Na-NCC, likely due to additional hydrogen bonds allowed by the presence of the H+ counterions: as detected by IR spectroscopy, the intermolecular hydrogen bonding generated from cellulose backbones is much stronger in dried H-NCC than in dried Na-NCC, whereas the intramolecular hydrogen bonding is similar in both forms.15 Fully dried H-NCC, in both film and freeze-dried forms, is not dispersible in 1 mM NaOH solutions even after prolonged
concentration range of 0.5 to 20 wt % and then diluted to 2 wt % NCC (the dilute 0.5 wt % sample was evaporated to 2 wt % concentration), so that sonication could be applied to all samples while avoiding any variations in the effect of sonication due to concentration differences.24 The only variable is therefore the initial redispersion concentration. Although the applied sonication energy lies in the “plateau” region of Figure 4, further sonication may lead to still smaller particle sizes for spray-dried Na-NCC. The data suggest that two separate deaggregation phenomena are taking place during redispersion and sonication; weaker “residual” aggregation (e.g., between agglomerates of smaller NCC aggregates) is broken up to different extents by dilution and mixing, whereas stronger aggregation between individual NCC particles (e.g., particles linked directly by a network of surface-to-surface hydrogen bonds) can be disrupted only by sonication. Once again, sonication effectively eliminates the differences created during drying and redispersion. Smaller particle sizes were observed in suspensions of freezedried Na-NCC as compared with spray-dried Na-NCC at all redispersion concentrations. Because the round spray-dried NCC granules have a much smaller surface-area-to-volume ratio than the thin lamellar flakes of freeze-dried NCC, water penetrates them less easily: dispersion of spray-dried NCC is a diffusion-limited process. This effect apparently outweighs the aggregation caused by the freezing step during lyophilization.21 Residual Moisture Content of Dried NCC. Acid-form NCC is not redispersible in water once it has been dried, even by gentle heat such as a vacuum oven at 35 °C for 24 h15 or evaporation at 25 °C and ≤40% relative humidity for several 1492
dx.doi.org/10.1021/bm300191k | Biomacromolecules 2012, 13, 1486−1494
Biomacromolecules
Article
immersion. H-NCC films swell in 5 M NaOH, indicating penetration of the alkaline solution into the structure and consequent neutralization of the proton counterions associated with the sulfate ester groups, but do not disperse when subsequently placed in water. Dried Na-NCC retains its dispersibility over time under mild storage conditions (23 °C, 30−60% relative humidity). Even heating for 2 to 4 h at 105 °C does not affect the dispersibility of Na-NCC. However, it begins to lose dispersibility after 8 h and is completely stabilized against dispersion after prolonged heating (40 h) at 105 °C (Table 4). The loss of dispersibility
swelling of the dried NCC structure. Water, both liquid and vapor, clearly penetrates the dried NCC structure. However, the swelling is reversible because enough hydroxyl-to-hydroxyl hydrogen bonds remain unbroken to prevent dispersion. Fully dry H-NCC films (with 0% moisture content) do not change color when placed in water and therefore do not swell, implying that the dried structure is completely inaccessible and water cannot penetrate between the NCC particles or aggregates. In addition, freeze-dried Na-NCC made nondispersible by prolonged (3 days) heating at 105 °C did not regain its dispersibility after storage at 100% relative humidity for over 24 h.
Table 4. Dispersibility of Dried Na-NCC Heated at 105 °C dispersible? time @ 105 °C (h)
air-dried filma
freeze-dried flakes
0 1 8 14 24 32 40
Y Y mostly (>90%) mostly (>90%) partially (82%)b partially (78%)b N
Y Y mostly (>90%) partially (80−90%) partially (74%)b partially (44%)b N
Film thickness 65 μm. bDried NCC was placed in water (0.5 wt %), vortexed at 3000 rpm for 1 min, and allowed to stand overnight; the supernatant was centrifuged, and the concentration of suspended material was determined by gravimetry. a
may be attributed to the removal of the tightly held bound water on the NCC surfaces, which is not removed during the brief heating using during moisture content measurements. Once the bound water is driven off, the spaces between aggregates of dried Na-NCC become inaccessible to water, possibly due to the formation of strong hydrogen bonds directly between the hydroxyl groups at the NCC particle surfaces. Solid Na-NCC films require somewhat longer heating to achieve this because they have higher initial moisture contents, and the water must diffuse through many NCC layers such that evaporation is diffusion-rate-limited. Freeze-dried NCC is composed of layers that are ∼300 times thinner than the NCC films, allowing faster removal of the bound water from the dried structure. Dried NCC adsorbs water from the atmosphere, the relative humidity to which it is exposed controlling its total moisture content. When H-NCC and Na-NCC films were placed in a controlled humidity environment and the relative humidity was increased from 30 to 80% RH, they reached total moisture contents of up to 10.8 and 11.2 wt % H2O. However, the additional water is adsorbed at the outer surfaces of the NCC; the water vapor cannot penetrate the dried NCC structure between the NCC particles or aggregates. Although the measured moisture content of a dried H-NCC sample may be greater than 4 wt %, this will not render it redispersible if it was dried to an initial residual moisture content of
