Mechanisms Behind the Stabilizing Action of Cellulose Nanofibrils in

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On The Mechanisms Behind the Stabilizing Action of Cellulose Nanofibrils in Wet-Stable Cellulose Foams Nicholas Tchang Cervin, Erik Johansson, Jan-Willem Benjamins, and Lars Wagberg Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/bm5017173 • Publication Date (Web): 30 Jan 2015 Downloaded from http://pubs.acs.org on February 9, 2015

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On The Mechanisms Behind the Stabilizing Action of Cellulose Nanofibrils in Wet-Stable Cellulose Foams Nicholas Tchang Cervin,a Erik Johansson,b Jan-Willem Benjaminsc and Lars Wågberga

a

Wallenberg Wood Science Center, Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden

b

Cellutech AB, Teknikringen 38A, SE-114 28, Stockholm, Sweden

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SP Technical Research Institute of Sweden, Brinellgatan 4, Box 857, SE-501 15, Borås, Sweden

Abstract The principal purpose of the investigation was to clarify the mechanisms behind the stabilizing action of cellulose nanofibrils (CNFs) in wet-stable cellulose foams. Following the basic theories for particle-stabilized foams, the investigation was focused on how the surface energy of the stabilizing CNF particles, their aspect ratio and charge density, and the concentration of CNF particles at the air-water interface affect the foam stability and the mechanical properties of a particle-stabilized air-liquid interface. The foam stability was evaluated from how the foam height changed over time and the mechanical properties of the interface were evaluated as the complex viscoelastic modulus of the interface using the pendant drop method. The most important results and conclusions are that CNFs can be used as stabilizing particles for aqueous foams already at a concentration as low as 5 g/L. The major reasons for this were the small dimensions of the CNF and their high aspect ratio which is important for gel-formation and the complex viscoelastic modulus of the particle-filled airwater interface. The influence of the aspect ratio was also demonstrated by a much higher foam stability of foams stabilized with CNFs than of foams stabilized by cellulose nanocrystals (CNC) with the same chemical composition. The charge density of the CNFs affects the level of liberation within larger aggregates and hence also the number of contact points at the interface and the gel formation and complex viscoelastic modulus of the airwater interface. The charges also result in a disjoining pressure related to the long-range repulsive electrostatic pressure between particle-stabilized bubbles and hence contribute to foam stability.

Keywords Cellulose nanofibrils (CNF), cellulose nanocrystals, nanocellulose, particles, foams, particle stabilized foams, Pickering foams, pendant drop

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Introduction Porous cellular materials are interesting in that they can be lightweight and permeable and have a high accessible surface area and high porosity. This is also why they are interesting for use as lightweight structural components in thermal insulation and in liquid and sound absorption applications. Polymeric foams are today used in everything from disposable coffee cups to the crash padding of an aircraft cockpit, and techniques exist for foaming not only of polymers but also metals, ceramics and glasses.1, 2 Foams can be prepared in many different ways, but a rather recent development is the use of particle-stabilized foams where tailored particles are used to create foams of high stability.3-10 Ceramic particles have earlier been used by Gonzenbach et al.2, 11-17 to prepare stable wet foams that were subsequently sintered to produce a porous cellular material. Non-spherical particles, such as organic rod-shaped particles, have also been used18, 19 to produce these ultrastable wet foams. Recently, CNFs have been used for the preparation of stable wet foams which can be dried into a low-density, porous cellulose material with good mechanical properties.20 The use of CNFs as stabilizing particles in the foam is interesting since CNFs are renewable, environmentally friendly and biocompatible.21 Their high aspect ratio, with dimensions of 560 nm in width and up to microns in length22, and gelling behavior at low solids contents have earlier been shown to result in foams with a high stability.20 Already at particle concentrations as low as 5 g/L, as shown in Figure 9, stable wet foams can be produced and it has been suggested that this is due to an intertwined network of long and slender CNFs that is formed at low solids contents.18, 23-27 However, in earlier work20 the focus was on the preparation of the porous cellulose material and there is still a lack of fundamental understanding of the stabilizing mechanism in foams made from surface-modified CNF. The aim of the present work was therefore to clarify the mechanisms behind the formation of wet-stable foams of cellulose using mainly surface-modified CNF as stabilizing particles. Since the CNFs form gels at concentrations of around 10 g/L, it was considered important to clarify the influence of the accumulation of the CNFs at the solid-liquid interface and the influence of gelling properties of the CNF in the foam lamellae on foam stability. To do this, we have employed a pendant drop technique combined with foam stability measurements to quantify the importance of the hydrophobicity of the particles, their aspect ratio, their charge density and the concentration of CNF particles at the solid-liquid interface.

Experimental Materials Preparation of Cellulose Nanofibrils A commercial sulphite softwood dissolving pulp (Domsjö Dissolving Plus; Aditya Birla, Domsjö Fabriker AB, Domsjö, Sweden) from 60 % Norwegian spruce (Picea abies) and 40 % 2 ACS Paragon Plus Environment

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Scots pine (Pinus sylvestris) with a hemicellulose content of 4.5 % and a lignin content of 0.6 % was used, as previously described.28 The pulp was used in its never-dried form. Tempo-oxidation was used to increase the charge of the fibres and to promote an easier delamination in the Microfluidizer during the preparation of CNF from the bleached fibres, as described earlier.29 A suspension of fibres (1 g) and Milli-Q water (100 mL) was prepared together with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol). Sodium hypochlorite (1-5 mmol per gram of cellulose) with an active chlorine content of 14 % was adjusted to pH 10 and then added to the suspension under constant stirring at room temperature. The pH decreased during the sodium hypochlorite addition and adjusted by adding sodium hydroxide (1 M) until the pH was constant at pH=10. Three levels of hypochlorite addition were selected and this resulted in charge densities of 500, 900 and 1400 meq/g of the fibres as determined by conductometric titration according to Scallan et al.30 Before use, the pH was adjusted so that the counter-ions of the charges on the fibres were in their sodium form according to Wågberg et al.31 The charged fibres were then defibrillated in a homogenizer using a high-pressure fluidizer (Microfluidizer M-110EH, Microfluidics Corp.) equipped with two chambers of different sizes connected in series (200 and 100 µm). Homogenization was achieved after six passages at a fibre concentration of 1.5 wt% and an operating pressure of 1650 bar. Preparation of Cellulose Nanocrystals (CNC) The CNC was prepared from TEMPO-oxidized CNF by hydrolyzing the CNF suspension (1 g of dry weight) in 300 mL of 2.5 M HCl at 105 °C for 4 hours. The reaction was stopped by dilution with deionized water. The CNC suspension was then centrifuged followed by dialysis against water for 7 days (6000-8000 MW cutoff). The charge density (500 µeq g-1) was then determined by conductometric titration Chemicals TEMPO (2,2,6,6-Tetramethyl-1-piperidinyloxy) from Sigma Aldrich, purified by sublimation 99 %, CAS: 2564-83-2. Sodium bromide from Alfa Aesar, 99+ % (dry wt.) water < 1.0 %, CAS: 7647-15-6. Sodium hypochlorite from VWR chemicals, 14 % active chlorine, CAS: 7681-52-9. Octylamine from Sigma Aldrich, 99 %, CAS: 111-86-4. Decylamine from Sigma Aldrich, 95 %, CAS: 2016-57-1. Sodium hydroxide (NaOH) from Fisher Chemicals, 98.8 %, analytical reagent grade. Hydrochloric acid (HCl) from Fisher Chemicals, 37 %, analytical reagent grade. Methods Foaming A suspension of dispersed CNFs was prepared by diluting the homogenized CNF to a final concentration of 1 g/L using an Ultra-Turrax mixer (IKA T25 digital). The surface energy of the CNF was changed by simply adsorbing cationic octylamine or decylamine on the anionic surface of the CNFs20 so that the amount of added octylamine or decylamine corresponded to 3 ACS Paragon Plus Environment

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1/3 or 1/1 of the total CNF charges. The foam was then created by shaking the sealed tube (3 cm in diameter) under ambient conditions for 10 seconds resulting in drained foam on top of a water column. In the case of the higher CNF concentration (10 g/L), the suspension was foamed with the aid of the Ultra-Turrax mixer at 10000 rpm until no further volume increase could be detected. The foam stability was determined by measuring the change in foam height as a function of time and presented as Vt/V0 where Vt is the volume at time = t and V0 is the volume at time t = 0. The measurements were conducted at 24 °C and 52 % RH and the typical uncertainties in the experiments were ± 2 %. Calculation of CNFs at the Air Bubble Interface In order to estimate the number of CNFs at the interface of one air bubble as well as the average distance between the CNFs, the total CNF area and the thickness of CNF layer at the interface of one air bubble, a wet foam was prepared from a low (1g/L) CNF concentration. The foam drained easily, ending up as a layer on top of a water column. It was assumed that all CNFs were located at the air-water interface since no more bubbles could be stabilized. By measuring the volume of the foam and after measuring the size of the air bubbles by optical light microscopy, it was possible to calculate the number of air bubbles in the foam, assuming that the bubbles packed in a 64 % random close packing and considering the foam bubbles to be spherical.32 It was also assumed that the foam consisted of close-packed, spherical air bubbles, since earlier work has shown that the CNFs stabilizes single air bubbles in the foam.20 The foams were then weighed in order to determine the amount of CNFs in the foams and, from the density of CNF and its dimensions, it was possible to estimate the concentration of CNFs surrounding a single air bubble. Based on the dimensions of the CNF, i.e. a square cross section of 4 nm, and a density of the cellulose of 1500 kg/m3 the specific surface area can easily be calculated to 667 m2/g. With a charge density of 900 µeq/g the surface charge can be calculated to 1.3 µeq/m2. It should be pointed out that a foam lamella consists of two air-water interfaces and should hence contain twice as many CNFs as a single bubble. Pendant Drop – Drop/Bubble Profile Tensiometry The drop/bubble profile tensiometry (DPT) is a well-established technique for determining the mechanical properties of a liquid-gas interface.33-35 The technique is based on the assumption that the geometry of a pendant drop can be related to the surface tension of the investigated liquid. Gravity strives to elongate the drop whereas the surface tension strives to keep the drop spherical. In the DPT equipment, the shape of the drop is constantly monitored by a computer that registers the shape as a function of area variation as the drop is oscillated by a sinusoidal change in drop volume. When surfactants are present in the liquid, they are able to be adsorbed and desorbed at the interface during the oscillation. Depending on the frequency of the oscillation, and the adsorption kinetics of the surfactant, the surface tension will change in a sinusoidal manner which is out of phase with the oscillation of the surface area. A fastadsorbing surfactant, measured at a low frequency, will show only a relatively small amplitude in the surface tension variation. However, when large molecules or small particles are adsorbed at the interface, they do not have the same tendency to desorb from the interface, due to the high adsorption energy/particle,6 and the drop shape will in this case reflect the rheological properties of the air-water interface.33, 35, 36 4 ACS Paragon Plus Environment

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The complex viscoelastic modulus determined with this procedure can be described according to the equation: 𝐸=

𝛥𝛥 ∆𝐴 𝐴0

= 𝐸0 + 𝑖2𝜋𝜋𝜋

(Eq 1)

where 𝛾 is the interfacial tension, 𝐴 is the area at a given time and 𝐴0 is the area at time = 0, ∆𝛾 = 𝛾(𝑡) − 𝛾 0 , ∆𝐴/𝐴0 = (𝐴(𝑡) − 𝐴0 )/𝐴0 , 𝐸0 = dilational surface elasticity, 𝜈 = is the perturbation frequency and 𝜂 = dilational surface viscosity. From this relationship it is obvious that the complex viscoelastic modulus 𝐸 consists of an elastic part (𝐸0 ) and a viscoelastic part (2𝜋𝜋𝜋) and will therefore be frequency dependent, and the frequency of the oscillating drop should not be higher than the requirement of mechanical equilibrium of the interface.33, 35, 36 The pendant drop experiments are performed by subjecting a droplet to 10 oscillations under ambient conditions and repeating this measurement every 10 minutes in the course of one hour. All experiments have been conducted at a concentration of 1 g/L unless otherwise. Film Thickness Measurements The film thickness was determined to complement the results of the DPT experiments in order to separate the influence of film thickness at the air-water interface on the change in complex viscoelastic modulus of the particle-covered interface. This was done using a CNF suspension of 1 g/L with different charge densities of the CNFs (900 and 1400 µeq/g). Octylamine was added corresponding to 1/3 of the total CNF charges and gently stirred without creating any bubbles. The modified CNFs were then allowed to move to the interface over a period of three days. This resulted in the formation of a film at the interface. This film was gently transferred to a plasma-treated silica wafer by drawing a silica wafer from the bottom of the beaker up to the surface. The silica wafer was then dried in an oven at 105 °C for a couple of minutes so that it was just dry. The thickness of the film assembled on the silica wafer was then evaluated from the AFM scratch height. By removing a part of the film by scratching, a sharp edge was formed which could be used to measure the height of the film. A Multimode 8 SPM with a Nanoscope V controller and equipped with a J scanner (Bruker AXS, Santa Barbara, USA) was used in the Tapping Mode to image and determine the CNF film thickness. Standard silicon cantilevers (Bruker AFM Probes, Camarillo, USA) with a nominal resonance frequency of 150-200 kHz and spring constant of 5 N/m were used. The scratch height was assessed by imaging scratches made in the CNF film, and analyzing the images using the step command in the AFM software, Nanoscope Analysis (Bruker AXS, Santa Barbara, USA). Dynamic Light Scattering (DLS) DLS measurements were made using a Zetasizer ZEN3600 particle size analyzer (Malvern Instruments Ltd., UK) on CNFs with three different charge densities 500, 900 and 1400 µeq/g and on CNC with a charge density of 500 µeq/g.37


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Results The Relationship between the Surface Energy of CNF, Foam Stability and Surface Film Properties Figure 1 a) shows the average interfacial tension (Av. IFT), from the DPT measurements, for a water drop in air (pendant drop) as a function of time with different types of surfactant particles added to the water phase. The anionic CNFs have been modified by adsorption of cationic octylamine or decylamine corresponding to 1/3 or 1/1 of the total CNF charges in the suspension in order to tailor the surface energy of the CNFs. The results show that modified CNF lowers the IFT in relation to unmodified CNF. Modified CNF also lowers the IFT much more than when octylamine is used alone. The results also show that a higher dosage of octylamine or decylamine lowers the IFT more and that CNF modified with decylamine lowers the IFT more than CNF modified with octylamine. If the change in IFT for a given change in area during oscillation of the drop is known, it is possible to calculate the complex viscoelastic modulus for the interface of the water drop. This complex viscoelastic modulus has earlier been shown to be important for film stabilization in wet foams.10 Figure 1 b) shows that the complex viscoelastic modulus increases more with modified CNF than with unmodified CNF and regular octylamine. The complex viscoelastic modulus also increases more when a higher dosage of octylamine or decylamine is used and it increases most for decylamine when added on a 1/1 charge basis. In Figure 2 the foam stability values are shown as a function of time for CNF modified with octylamine or decylamine. Foam stability is defined as the foam volume after different times normalized with respect to the initial volume (in %). It is shown that the stability of the wet foam is better when CNF modified with decylamine is used although the initial foam volume is larger when CNF modified with octylamine is used (results shown in supporting information). This indicates that there is a correlation between foam stability and the complex viscoelastic modulus achieved with CNF modified with decylamine. The dosage corresponds to 1/3 of the CNF charges in the foam stability experiments.


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CNF (900 ueq/g) Octylamine CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) CNF (900 ueq/g) modified with octylamine (1/1 charge ratio) CNF (900 ueq/g) modified with decylamine (1/3 charge ratio) CNF (900 ueq/g) modified with decylamine (1/1 charge ratio)

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Time (min) Figure 1: a) Average interfacial tension for an oscillating water drop in air, 0.1 Hz and 0.2 mm under ambient conditions, with different types of surfactant particles; CNF (900 µeq/g), octylamine, CNF (900 µeq/g) modified with octylamine (1/3 charge ratio), CNF (900 µeq/g) modified with octylamine (1/1 charge ratio), CNF (900 µeq/g) modified with decylamine (1/3 charge ratio) and CNF (900 µeq/g) modified with decylamine (1/1 charge ratio). All at a concentration of 1 g/L. b) The complex viscoelastic modulus as a function of time for the same surfactant particles as in a).


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CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) CNF (900 ueq/g) modified with decylamine (1/3 charge ratio)

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Time (h) Figure 2: Foam volume of CNF-containing foams as a function of time for CNF modified with octylamine or decylamine. The concentration is 1 g/L in both cases and the typical uncertainties in the experiments were ± 2 %.

Influence of the Aspect Ratio of CNF on Foam Stability and Surface Film Properties Figure 3 shows the complex viscoelastic modulus as a function of time for two cellulose particles with different aspect ratios. CNFs with a charge density of 500 µeq/g were measured with DLS and were found to be 1.7 µm in length and 3 nm in width. The CNCs, with the same charge density of 500 µeq/g, were found to be 300 nm in length and 4 nm in width, also measured with DLS. The aspect ratio is thus approximately 8 times higher for CNF than for CNC. The CNC were prepared by hydrolyzing the CNF suspension and the chemical composition was hence the same for the two particles. The charge density was measured with polyelectrolyte titration for both modified with octylamine (1/1 charge ratio). It is shown in Figure 3 that a higher aspect ratio of the stabilizing particle results in a higher complex viscoelastic modulus. It is furthermore shown in Figure 4 that the foam stabilized with CNF was more stable over time than the foam stabilized with CNC, although the CNC-foam had an initially greater foam volume (results shown in supporting information)


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CNF (500 ueq/g) modified with octylamine (1/1 charge ratio) CNC (500 ueq/g) modified with octylamine (1/1 charge ratio) 30 25 20 15 10 5 0

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Time (min) Figure 3: The complex viscoelastic modulus as a function of time for two different cellulose particles with different aspect ratio. CNF modified with octylamine (1/1 charge ratio) had an aspect ratio of approximately 600 while CNC modified with octylamine (1/1 charge ratio) had an aspect ratio of approximately 80 i.e. around 8 times lower. The solids concentration was 1 g/L in all experiments. CNF (500 ueq/g) modified with octylamine (1/3 charge ratio) CNC (500 ueq/g) modified with octylamine (1/3 charge ratio) 12 10

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Time (h) Figure 4: Foam volume as a function of time for CNF and CNC with different aspect ratios but otherwise similar properties. CNF has an aspect ratio approximately 8 times higher than CNC. The solids concentration was 1 g/L in all experiments and the typical uncertainties in the experiments were ± 2 %. 9 ACS Paragon Plus Environment

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Influence of the Number of Charged Groups of the CNF and Electrolyte Concentration on Foam Stability and Surface Film Properties Figure 5 shows the complex viscoelastic modulus as a function of time for CNFs with different charge densities. As in the previous experiments, the CNFs were all modified with octylamine adsorption corresponding to a charge ratio of 1/3. The result shows that when the charge density was increased from 500 to 900 µeq/g, the complex viscoelastic modulus increased, but that a further increase in the charge density to 1400 µeq/g reduced the complex viscoelastic modulus from approximately 40 to 30 mN/m. Apart from the different charge, these latter particles also had a difference in length, due to the oxidation, the length being approximately 750 and 580 nm for the 900 and 1400 meq/g respectively. The CNF with a charge density of 1400 µeq/g was also tested at different salt concentrations (Figure 6) to investigate how a screening of the electrostatic repulsion between the particles affects the complex viscoelastic modulus. The CNFs were modified with octylamine corresponding to a charge ratio of 1/3. The results show that there is a trend towards an increase in the complex viscoelastic modulus with increasing salt concentration, even though the scatter in the data is larger than for the results without salt addition. To further investigate how the charges affect the accumulation of CNFs at the interface, films of CNF were prepared (see experimental). In this procedure, CNFs pretreated with octylamine (1/3 charge ratio) with different charge densities were allowed to accumulate at the air-water interface in a beaker. The films formed were extracted by transfer to a silica wafer and further investigated with AFM for thickness measurements. The results are presented in Figure 10 and show that the films formed by the accumulated CNFs at the air-water interface became thinner with increasing charge density. Interestingly the thinnest film i.e. the one made from 1400 µeq/g became thicker when salt was added.

CNF (500 ueq/g) modified with octylamine (1/3 charge ratio) CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) CNF (1400 ueq/g) modified with octylamine (1/3 charge ratio)


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Figure 5: The complex viscoelastic modulus as a function of time for CNFs with different charge densities. The different CNFs have all been modified with octylamine corresponding to a charge ratio of 1/3 and the solids concentration was 1 g/L in all experiments.

CNF (1400 ueq/g) modified with octylamine (1/3 charge ratio) CNF (1400 ueq/g) modified with octylamine (1/3 charge ratio) + 1 mM NaCl CNF (1400 ueq/g) modified with octylamine (1/3 charge ratio) + 5 mM NaCl CNF (1400 ueq/g) modified with octylamine (1/3 charge ratio) + 10 mM NaCl 70


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Figure 6: Complex viscoelastic modulus as a function of time for CNFs with different concentrations of salt. The different CNFs were all modified with octylamine corresponding to a charge ratio of 1/3 and 1, 5 or 10 mM NaCl was added to the suspensions before the experiments were started in the pendant drop method. All the experiments were performed at a solids concentration of 1g/L. The reference contained no added salt.

Figure 7 shows the foam stability at different salt concentrations. CNF (900 µeq/g) was modified with octylamine corresponding to a charge ratio of 1/3 and salt was added up to 8 mM to clarify how the salt affected the foam stability. The results show that adding salt reduced the foam stability, indicating that the electrostatic repulsion between the CNF particles is important for good foam stability. There was no real difference in the beginning but it was shown after 4 hours that 5 and 8 mM did indeed affect the foam stability.


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CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) + 1 mM NaCl CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) + 5 mM NaCl CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) + 8 mM NaCl

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Time (h) Figure 7: Foam volume as a function of time for CNF (900 µeq/g) modified with octylamine (1/3 charge ratio) at different salt concentrations (0, 1, 5 and 8 mM). The solids concentration was 1 g/L. Typical uncertainties in the experiments were ± 2 %.

Influence of the Concentration of CNF on Foam Stability and Surface Film Properties Figure 8 shows the complex viscoelastic modulus as a function of time for CNF suspensions with different concentrations. The complex viscoelastic modulus increased with increasing concentration although there seems to be a maximum at 5 g/L where the complex viscoelastic modulus started to decrease with time. In Figure 9 the foam stability is shown as a function of time for different concentrations of CNFs. There was a significant increase in foam stability when the CNF concentration was increased from 1 to 5 g/L. At that concentration, 90 % of the initial foam volume was preserved after 5 days. When the concentration was increased further to 10 g/L there was only a smaller increase in foam stability, but the volume of the foam was significantly higher at this higher concentration. It should be stressed that at this higher CNF concentration the solution could not be foamed by shaking the tube, so it was foamed with the aid of an UltraTurrax mixer.


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CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) 1 g/L CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) 3 g/L CNF (900 ueq/g) modified with octylamine (1/3 charge ratio) 5 g/L 80 70 60 50 40 30 20 10

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Time (h) Figure 9: Foam volume as a function of time for five different CNF concentrations (0.1, 0.5, 1, 5 and 10 g/L). The CNFs were all modified with octylamine corresponding to a charge ratio of 1/3 and the typical uncertainties in the experiments were ± 2 %. 13 ACS Paragon Plus Environment

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Discussion Properties of the CNF-Stabilized Air-Water Interface The ability of the particles in general to adhere to the air-liquid interface can be related to the work needed to remove the particles from the interface and is given by: 𝑊 = 𝜋𝑅 2 γ𝑊 (1 ± 𝑐𝑐𝑐𝑐)2

(Eq 2)

where 𝑅 is the particle radius, γ𝑊 is the surface tension of water, 𝜃 is the contact angle between the particle and the liquid, “+” means moving the particle into the gas phase and “-“ means moving the particle into the liquid phase, which means that the probability that a particle is stabilized at the interface is 𝜀 = (1 ± cos 𝜃)2 . The contact angle should thus be between 30 and 150° to give a reasonable probability for the particles to reside at the interface.38 It is hence necessary to modify the surface energy of cellulose to enhance the driving force for their accumulation at the air-water interface in order to use CNF as stabilizing particle. A simple way of doing this is to adsorb a surfactant onto the CNF.20 It is also clear that the surface tension should not be decreased too much in order to keep the work needed to move the particles from the interface as high as possible. It should be pointed out that the above mentioned equation has been derived for spherical particles. We have only used the principles described by the equation to explain the results achieved in our experiments where rod shaped CNFs have been used as stabilizing particles. As can be seen in Figure 1 a), the interfacial tension of an oscillating water drop in air decreases when the surface energy of the CNFs is surface modified with surfactants. There is a decrease in interfacial tension both when more surfactants are adsorbed onto the CNFs and when a surfactant with a longer carbon chain is used. Adsorbing more surfactant to the CNFs makes them more hydrophobic, which is probably the major explanation of the decrease in interfacial tension. The hypothesis that the CNFs become more hydrophobic and lower the IFT more is also supported by the results with decylamine with a longer carbon chain than octylamine. Also, when more surfactants are adsorbed to the CNFs, the repulsion between the CNFs decreases and more CNFs can accumulate at the interface and contribute to the lower interfacial tension. In Figure 1 b) the complex viscoelastic modulus is shown as a function of time for the same experiments as in Figure 1 a). The complex viscoelastic modulus was calculated from the change in interfacial tension upon controlled area variations and was based on the measurements presented in Figure 1 a). As for the interfacial tension, it is obvious that decreasing the surface energy of the CNFs results in a higher complex viscoelastic modulus, clearly indicating that the surface-modified CNF is accumulated at the interface and probably locally increases the CNF concentration above its gel-forming concentration. In addition to this, Binks et al. have earlier suggested, based on experimental results, that the value for the complex viscoelastic modulus should exceed half the surface tension of the liquid in order to create a wet-stable foam.10 When using water as the solvent, this means that the complex viscoelastic modulus needs to be around 36 mN/m and this is not achieved either when untreated CNF is used or when octylamine is used alone. However, when tailoring the CNF 14 ACS Paragon Plus Environment

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through the adsorption of octylamine corresponding to 1/3 of the total CNF charges, the complex viscoelastic modulus increases to approximately 40 mN/m indicating that the modified CNFs should be able to create a stable foam, and this is indeed shown in Figure 2 . By increasing the amount of added surfactant or by using decylamine as modifying surfactant, an even higher complex viscoelastic modulus is achieved and, as shown in Figure 2 , this gives a more stable foam. All these results indicate that it is indeed an accumulation of the modified CNF at the solid liquid interface that increases the complex viscoelastic module of the interface, which is needed to create particle-stabilized foams of CNF. It is also important to stress that these extremely dilute CNF suspensions are able to create foams with a very good stability. There may be several reasons for this, but one obvious factor is the very high aspect ratio of the CNFs and it was therefore important to clarify the effect of shortening the CNFs. This was achieved by converting the CNF to CNC which has an aspect ratio about 8 times lower than that of CNF, as shown by the DLS measurements. From the results in Figure 3, it is obvious that the complex viscoelastic modulus of the interface when CNCs were used as stabilizing particles, were not even close to the values achieved with CNFs at the same concentration of 1 g/L. This shows that the CNFs have superior properties than the other particles and this can most probably be related to the gelforming properties of the CNF at very low solids concentrations as described by Fall et al. (2011). In their work, it was reported that the CNFs in a 1 g/L CNF suspension with a charge density ranging from 400-600 µeq/g form a continuous network throughout the solution volume already at these low concentrations when the electrostatic repulsion between the particles is reduced by salt addition or by lowering the pH below pH = 4. It has also been shown that an increase in CNF concentration above a certain limit induces gel formation of the CNF suspensions.39, 40 All these results indicate that, as the surface-modified CNF particles accumulate at the air-liquid interface, they will entangle to form a gel that is able to stabilize the foam over an extended period of time and also allow the foam to dry without major collapse.20 In order to test this hypothesis of gel formation at the air-liquid interface, a simple calculation was made to estimate the number of CNFs at the air-liquid interface. The results of these estimates, are shown in Table 1, where it is clear that there will actually be much more than a single layer of CNFs at the interface. Calculating the surface area of the stabilizing CNFs and comparing it to the interfacial area of the bubble reveals that the total surface area of the CNFs is approximately 60-120 times greater than the total bubble surface area. This shows that the CNFs form a multilayer at the interface. The layers are calculated to be approximately 600 nm and 1200 nm thick at CNF concentrations of 5 g/L and 10 g/L respectively. These values correspond very well with unpublished data of Cervin and Wågberg showing that the thickness of the cell wall in a dry foam, as measured from SEM images, is in this size range. The importance of the aspect ratio of CNF for creating a high complex viscoelastic modulus of the interface can also be demonstrated by comparing the present results (Figure 8) with earlier published results using 0.3 wt% silica particles. At a 0.3 wt % silica concentration, a complex viscoelastic modulus of 17 mN/m, after 167 min., was detected which may be compared to the CNFs which have a modulus of 70 mN/m after 60 min. at the same concentration. 15 ACS Paragon Plus Environment

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As demonstrated by Fall et al39 the charge density is an important factor for the gel formation of the CNF and should hence also be an important property for creating a stable air-water interface provided, that the gel formation is important for the elastic properties of the interface. The effect of the charge is actually twofold, since a higher charge will enhance the liberation of the CNFs from CNF aggregates39 and hence increase the number of free CNFs per unit volume at a given solids concentration. The second effect of the charge is that the CNFs will have a higher repulsion between each other and this will lead to a higher repulsion between the CNFs at the air-liquid interface and prevent the formation of a thick gel layer at a given solids concentration in solution. There should thus be a balance between the driving force for accumulation at the interface and the repulsion between the CNFs as they approach each other. To test this hypothesis, an experiment was conducted where the thickness of spontaneously formed films at the air-water interface was evaluated at different charge densities of the CNF and at different salt concentrations. Figure 10 shows that the thickness of the film decreased as the charge of the CNFs increased, indicating that the repulsion starts to hinder the accumulation of the CNF at the air-water interface. This was supported by the increase in thickness of the films when NaCl was added, which will decrease the repulsion between the CNF. These results are also in accordance with the data in Figure 6 showing that the complex viscoelastic modulus increases when salt is added, even though there is considerable scatter in the data when salt is added which may be due to a slight aggregation of the CNF suspension. However, when these data are compared with the data for the complex viscoelastic modulus shown in Figure 5, it is found that as the charge of the CNFs increases there seems to be a maximum in the complex viscoelastic modulus at a charge of 900 meq/g. The increase in complex viscoelastic modulus, despite the formation of a thinner film, may be due to a more efficient liberation of the CNFs as the charge is increased,41 leading to a larger number of contacts per CNF at a given concentration at the interface. The decrease in complex viscoelastic modulus when the charge was increased from 900 to 1400 meq/g is probably due to the repulsion between the particles and a thinner film at the air-liquid interface. However, a significant decrease in length of the CNFs from 750 nm to 580 nm measured with DLS when the charge was increased from 900 to 1400 µeq/g, could also have an influence on the properties of the interface and on the foam stability. A possible explanation to why CNF forms such thick films at the air-water interface is that as the surface modified CNFs will start to move towards the air-water interface their concentration close to the bubbles will be so high that they will start to assemble in a loose structure where the CNF is in an arrested state. This in turn means that the CNF will move as a loosely intertwined network towards the air water interface, and not as single CNF particles, forming rather thick layers at the interface. A higher charge density on the CNFs results in thinner films at the air-water interface and this is most probably a result of a more welldispersed system with lower degree of intertwined fibrillar networks due to the higher electrostatic repulsion between the CNFs in dispersion.


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Table 1: Properties of wet foams stabilized by CNFs at two different solids concentrations 5 g/L and 10 g/L. Number of bubbles in the foam is calculated assuming 64 % random close packing and details mentioned under experimental.32 The bubble diameter was measured with optical light microscopy. CNF concentration (g/L)

5

10

Foam volume (cm3)

26

46

Foam weight (mg)

39

186

Measured bubble diameter (µm)

959

708

Number of CNFs at the bubble air-water interface

6,0e10 6,5e10

Average distance between CNFs surrounding one bubble (nm)

7,0

4,9

CNF area/bubble surface area

62

124

Thickness of CNF layer at bubble interface (nm)

600

1200

1200 1 mM NaCl 10 mM NaCl

1100 1000

Film thickness (nm)

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900 800 700 600 500 400 300 200 100 0 200

400

600

800

1000

1200

1400

CNF (meq/g) Figure 10: Film thickness as a function of the charge density of the CNFs and the salt concentration. All the CNFs were modified with octylamine corresponding to a charge ratio of 1/3 and the solids concentration was 1 g/L in all the experiments. Factors Controlling Foam Stability As clearly pointed out by Kaptay,38 the ability of the particles to assemble at the air-water interface is a necessary but not sufficient prerequisite for foam stability. The stabilizing shell must also be able to prevent Ostwald ripening and coalescence when the bubbles are brought together after the initial foam formation. In order to describe the stabilizing effect of the particles, the maximum capillary pressure between the bubbles was introduced. According to 17 ACS Paragon Plus Environment

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Kaptay,38 the maximum capillary pressure between two approaching bubbles carrying particles at the air-water interface is defined as: 𝑃𝑐𝑚𝑚𝑚 = 𝑃𝑐∗ 2𝛾𝑙𝑙 /𝑅

(Eq 3)

where 𝑃𝑐∗ is a parameter with a positive value and a function of the interface coverage of the particles, 𝛾𝑙𝑙 is the liquid-gas interfacial energy and 𝑅 is the radius of stabilizing particle. For double layers of particles i.e. when both bubbles are covered with particles this entity can be written as: 𝑃𝑐∗ = 𝑏(𝐶𝐶𝐶𝐶 + 𝑐)(1 ± 𝐶𝐶𝐶𝐶)2

(Eq 4)

where b and c are constants that were assigned values of 𝑏 = 1.4𝑓 + 4.4𝑓 4 and 𝑐 = 0.405 when 𝜃 < 90° and 𝑓 is the theoretical coverage of a flat surface by a closely packed layer of equal spheres. This means that the maximum capillary pressure that the foams can withstand can be written as: 𝑃𝑐𝑚𝑚𝑚 =

2𝑏𝛾𝑙𝑙 𝑅

(𝐶𝐶𝐶𝐶 + 𝑐)(1 ± 𝐶𝐶𝐶𝐶)2

(Eq 5)

and “+” represents the case when 𝜃 > 90° and “-“ represents the case when 𝜃 < 90°. This means that the foam stability will be higher when a larger fraction of the bubbles are covered by particles, when the particles are smaller and the surface tension of the liquid is higher, and when the contact angle is between 70 and 86°. When this general description of particlestabilized foams is compared with our foam stability data, it is clear that the smaller radius of the particles, the higher is the foam stability. A contact angle between 70 and 86 would be ideal and the octylamine adsorption, used in the present work, leads to a lower contact angle, 40° However, the surface tension is also important and the addition of more surfactant would both lower the repulsion between the CNFs (see further discussion in the following section) and lower the surface tension of the liquid. It is also clear that a higher surface coverage of the bubbles is beneficial and this can explain why a higher particle concentration is better (Figure 9) and, as shown in Table 1, this higher particle concentration in the suspension also leads to a higher concentration of particles at the air-liquid interface. Another pressure that acts perpendicular to the bubbles in the foam and prevents them from coalescence is the disjoining pressure. Since the bubbles are charged due to the charged CNFs (reduced to 1/3 due to the addition of surfactant) there will be a disjoining pressure related to the long-range repulsive electrostatic pressure (Π𝑒𝑒𝑒𝑒 ) between the bubbles.42 This is demonstrated in Figure 7 where the foam stability decreases with increasing salt concentration. The disjoining pressure is also the reason why the results in Figure 7 differ from the results in Figure 6. Figure 6 shows that the complex viscoelastic modulus is increased by a higher salt concentration and it is also shown in Figure 1 and Figure 2 that a higher complex viscoelastic modulus gives better foam stability. Thus the disjoining pressure is very important for foam stability and even more important than an increase in the complex viscoelastic modulus. A higher disjoining pressure is also the reason why the foam stability is better for systems where octylamine has been adsorbed corresponding to 1/3 of the total CNF charges than with a 1/1 ratio.20 18 ACS Paragon Plus Environment

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As is well known the foam drainage is also important for foam stability. A slow drainage rate will increase the foam stability and prevent the foam lamellae to dewater.18 In this respect it is necessary to separate the drainage rate of free water between the stabilized air bubbles and the water immobilized between the CNFs at the air-water interface where the densely packed CNF is forming a weak gel with immobilized water. The reported foam stability tests were all conducted at 1 g/L and with particles that were made partly hydrophobic. It can therefore be assumed that the particle concentration in between the air bubbles is less than 1 g/L. This means that the CNF is below the gel-forming concentration40 where they spontaneously form a weak gel, or more correctly an arrested state, even though the particle concentration, 1g/l, is above the overlap concentration according to Fall et al.39 This means that drainage rate of the free liquid phase between the bubbles can be assumed to be comparably much higher than the drainage of the water immobilized in the CNF-phase close to the air-water interface. The water in the CNF phase can most accurately be described as gel-water where the counter-ions to the CNF-phase at the air-water interface induces an osmotic pressure that will keep the water in the CNF-phase. In turn this also means that a more highly charged CNF will result in a higher amount of associated water in the CNF phase and an increased salt concentration will dewater this phase, most probably leading to poorer foam stability. All this taken together means that the results in the present investigation using different types of CNF are in good agreement with the general understanding of how particle-stabilized foams should be created. However, even though the focus in this work has been on the properties of the air-liquid interface which is crucial when using cationic surfactants, it must be stressed that the free concentration of CNF in the lamellae between the air bubbles will also be of importance. When starting to remove the water from the lamellae, these free CNF particles will gel in the lamellae and impart a significant stability to these lamellae during drying and can be of significant importance for the foam stability during the drying process. This was however beyond the scope of the present investigation.

Conclusions In this work, the mechanisms behind the stabilizing action of CNFs in wet-stable cellulose foams have been investigated and it can be concluded that: Wet-stable cellulose foams can be made already at a CNF concentration as low as 5 g/L. This is due to the high aspect ratio that leads to intertwined networks and a gel-formation at the particle-filled air-water interface that increases the complex viscoelastic modulus and the foam stability over time. The complex viscoelastic modulus of the CNF-stabilized air-water interface is important for good foam stability. It is increased by adsorbing octylamine to the CNFs and is increased even more if decylamine is adsorbed to the CNFs. The CNF concentration was important for the complex viscoelastic modulus of the air-water interface, and the complex viscoelastic modulus increased with increasing CNF concentration up to approximately 3 g/L. It was also shown that the foam stability increased with CNF concentration and that at a CNF concentration of 5 g/L the foam stability could remain at 90 19 ACS Paragon Plus Environment

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% of the original height after five days. The CNFs accumulated in multilayers and the thickness of the multilayer was estimated to be around 600 nm. An increase in charge density of the stabilizing CNFs led to an increase in the complex viscoelastic modulus of the air-water interface up to approximately 900 µeq/g. The addition of salt made it possible to adsorb more highly charged CNFs at the interface than if no salt was added. On the other hand, it was also shown that it was more important to not screen the charges then to increase the particle concentration at the interface for foam stability.

Supporting Information Supporting information provides graphs over the foam stability i.e. foam volume with respect to the initial foam volume (V0). The foam stability over time is shown as a function of CNF modified with octylamine or decylamine and the aspect ratio. This material is available free of charge via the Internet at http://pubs.acs.org.

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35. Wulff-Perez, M.; Torcello-Gomez, A.; Martin-Rodriguez, A.; Galvez-Ruiz, M. J.; de Vicente, J., Bulk and interfacial viscoelasticity in concentrated emulsions: The role of the surfactant. Food Hydrocolloids 2011, 25, 677-686. 36. Mezdour, S.; Desplanques, S.; Relkin, P., Effects of residual phospholipids on surface properties of a soft-refined sunflower oil: Application to stabilization of sauce-types' emulsions. Food Hydrocolloids 2011, 25, 613-619. 37. Boluk, Y.; Danumah, C., Analysis of cellulose nanocrystal rod lengths by dynamic light scattering and electron microscopy. Journal of Nanoparticle Research 2013, 16. 38. Kaptay, G.; Babcsán, N., Foam Engineering: Fundamentals and Applications. First edition ed. by P.Stevenson; John Wiley & Sons, Ltd.: 2012 pp.121-143. 39. Fall, A. B.; Lindström, S. B.; Sundman, O.; Ödberg, L.; Wågberg, L., Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions. Langmuir 2011, 27, 1133211338. 40. Fall, A. B.; Lindstrom, S. B.; Sprakel, J.; Wagberg, L., A physical cross-linking process of cellulose nanofibril gels with shear-controlled fibril orientation. Soft Matter 2013, 9, 1852-1863. 41. Fall, A. B.; Wagberg, L.; Burman, A., Liberation of nanofibrils from different types of wood. Abstracts of Papers of the American Chemical Society 2013, 245. 42. Stubenrauch, C.; von Klitzing, R., Disjoining pressure in thin liquid foam and emulsion films - new concepts and perspectives. J. Phys.-Condes. Matter 2003, 15, R1197R1232.


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Wet foam stabilized with surface modified cellulose nanofibrils


Mechanisms Behind the Stabilizing Action of Cellulose Nanofibrils in

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