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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Structural Analysis of Cellulose-Coated Oil-inWater Emulsions Fabricated from Molecular Solution Sofia Napso, Dmitry M. Rein, Zhendong Fu, Aurel Radulescu, and Yachin Cohen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01325 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018
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Structural Analysis of Cellulose-Coated Oil-inWater
Emulsions
Fabricated
from
Molecular
Solution Sofia Napso1,*, Dmitry M. Rein1, Zhendong Fu2,†, Aurel Radulescu2, Yachin Cohen1 1
Department of Chemical Engineering, Technion – Israel Institute of Technology, Technion City,
Haifa 3200003, Israel 2
Jülich Center for Neutron Science, Forschungszentrum Jülich GmbH Outstation at MLZ, 85747
Garching, Germany
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ABSTRACT
Natural cellulose has been used as a coating to stabilize oil-in-water (o/w) emulsions by exploiting the amphiphilic character of the cellulose chains molecularly dissolved in the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIMAc). Its cellulose coating exhibits a continuous amorphous structure which differs significantly from cellulose particle stabilization used in Pickering emulsions. The structure of these cellulose-coated o/w emulsion particles, in particular the cellulose coating shell characteristics (thickness, porosity and composition), is studied by using a combination of direct imaging methods as cryogenic electron microscopy (cryo-EM) and fluorescence microscopy with small-angle neutron scattering (SANS) measurements. This work suggests a unique multi-compartment structure of the emulsion particles: an oil core, surrounded by an inner shell composed of a porous cellulose gel, encapsulated by a dense outer cellulose shell, a few nanometers in thickness. The thickness of the inner cellulose shell varies significantly. Nano-scale emulsion droplets exhibit thickness of 10 ± 3 nm, whereas the larger micron-sized droplets exhibit a thicker inner cellulose shell of 500-750 nm. It is also inferred that the cellulose shells contain water rather than oil.
KEYWORDS: Amorphous cellulose; Amphiphile ; Ionic liquid; Emulsion stabilization; Shell structure; Small-angle neutron scattering (SANS); Cryogenic transmission electron microscopy (cryo-TEM); Cryogenic scanning electron microscopy (cryo-SEM)
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INTRODUCTION Cellulose is the most abundant and renewable biopolymer on earth. Its derivatives, obtained by chemical modifications, are utilized extensively in food and healthcare applications serving several functions, including as surface-active agents for stabilizing emulsions. [1-3] Most emulsions are stabilized by low molecular-weight surfactants. Pickering emulsions are stabilized by the adsorption of solid particles at the oil/water interface due to their partial wettability to each phase. [4,5] Extensive research on cellulose particles as stabilizers in emulsion systems over the years show that microcrystalline or microfibrillar cellulose particles [6-9], and more recently nanocellulose particles [10-16] in the form of cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs), can serve to form stable Pickering emulsions. The cellulose crystals present hydrophobic and hydrophilic surfaces, facilitating their adsorption at the oil/water interface. [1719] Surface modifications of these particles can further affect their interfacial functionality. [2022] Until recently, the native cellulose chain was not considered as capable for stabilizing emulsion systems. The amphiphilic character of the cellulose chain and its relevance for cellulose dissolution in different solvent systems have been re-discussed. [23-28] Cellulose form strong intermolecular hydrogen bonds between its chains and is considered hydrophilic in nature. Recently the role of hydrophobic interactions between the cellulose chains were also considered important to the recalcitrance of cellulose crystals towards dissolution in water and many other solvents, suggesting that cellulose is significantly amphiphilic. [23-28]
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It was recently reported that natural cellulose can molecularly stabilize o/w emulsions, using cellulose solutions or coagulated hydrogels. [29,30] This unique emulsion system is stabilized by exploiting the amphiphilic character of the cellulose chains molecularly dissolved in the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIMAc), [31,32] or of the amorphous cellulose hydrogel regenerated from such solution. The cellulose chains interact with both water and oil molecules at the oil/water interface on the oil droplet stabilizing the emulsion without using any additional surfactants. A recent study on the behavior of cellulose chains in such o/w emulsion systems by molecular dynamics simulations, [33] showed that the interactions are between CH groups of the cellulose chain glucose rings and the oil molecules, whereas the hydroxyl groups of the cellulose chain are adjacent to water molecules forming hydrogen bonds. The cellulose chains organize to form a thin amorphous coating at the oil/water interface. Other works [34,35] also showed that amorphous cellulose prepared by dissolution and regeneration from phosphoric-acid solutions, can serve as emulsifier for stabilizing o/w emulsions, forming micron-sized emulsion droplets. The ability to form a continuous coating of amorphous cellulose on oil droplets of sub-micron dimensions may offer potential benefits in numerous applications due to its unique properties. Although the cost of ILs are considered prohibitive for large-scale applications, they can serve as a good model system to investigate this new phenomenon, since so far they are the only solvents in which true molecular dissolutions has been observed. Insight gained from their study may be relevant for other more readily available solvents such as protic ionic liquids, [36,37] phosphoric acid, [34,35] etc. The structure of the cellulose coating shell is relevant for specific applications such as controlled release of functional hydrophobic compounds in pharmaceutics and food applications, encapsulated phase-change materials for thermal control in textiles or insulation, and cellulose
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enzymatic hydrolysis for alternative fuel production from biomass. However, information on the cellulose shell structure in such emulsions is still lacking. This work studies the structure and morphology of cellulose-coated o/w emulsions of micron- and nano-sized droplets fabricated from molecularly dissolved cellulose solutions. Structural information on the emulsion particles focusing on the cellulose coating itself, its thickness, porosity and composition, is achieved by cryogenic electron microscopy (cryo-EM) and fluorescence microscopy imaging, along with small-angle neutron scattering (SANS) measurements that provide a more quantitative evaluation. EXPERIMENTAL Materials Microcrystalline cellulose powder with particle size in the range of 70-250 µm [degree of polymerization (DP) ~295 as given by the supplier, ~308 by light scattering [31], was obtained from Sigma-Aldrich. The IL EMIMAc >95% purity was purchased from Iolitec, ILs Technologies, Germany. Cellulose and EMIMAc were dried thoroughly in a vacuum oven at 60 °C at 0.26 kPa for at least 24 hours before use. n-hexadecane and toluene were supplied by Merck, Germany. dichloromethane (DCM), n-eicosane, bromododecane, deuterium oxide (D2O) and fluorescent dyes, Calcofluor-White (CFW) and Nile-Red (NR), were purchased from SigmaAldrich. Deuterated oil, n-hexadecane-d34 was purchased from Cambridge Isotope Laboratories, US. These chemicals were used without additional treatment. Unless otherwise stated, the weight percentage of the components were calculated based on the total composition of the final mixture.
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Cellulose solution Microcrystalline cellulose powder was first wetted by DCM, using a magnetic stirrer, while keeping the dispersion vial in cold ice-water bath in order to reduce DCM evaporation. Next, EMIMAc was added and the solution was subsequently heated to 70 °C and stirred (150 rpm) for additional 2 hours. The final solution composition were 4 %wt. cellulose and 3-3.5 %wt. DCM. At this stage, the solution appeared visually clear and homogenous, without noticeable aggregates. SAXS patterns from these solutions did not show any signs of aggregation, and were similar to previously studies cellulose solutions in EMIMAc, [31,32] that were interpreted as molecular dissolution of the polymer chains. The effect of DCM on cellulose dissolved in EMIMAc is currently under study. Cellulose-coated emulsions Cellulose-coated o/w emulsions were prepared with several oils: n-hexadecane, nhexadecane/toluene mixture (1/1), bromododecane, n-eicosane and n-hexadecane-d34 which was used for SANS measurements. The oil was first dispersed in the 4 %wt. cellulose solution in EMIMAc using mechanical homogenization. Next, water was added to this dispersion, which was further homogenized mechanically at 24,000 rpm for 10 minutes, followed by high-pressure homogenization (HPH) at 20 kpsi for 4 minutes in order to achieve smaller nano-emulsion droplets, and at 2 kpsi for 2 minutes for larger, micron-sized droplets. The mechanical homogenization process was carried out at room temperature and during HPH the temperature was kept in the range of 37-41 °C. For n-eicosane emulsion, the whole process was carried out above its phase transition at temperature of ~40 °C (melting temperature Tm ~ 36°C). The final emulsions were kept at room temperature and were stable for several days before any noticeable
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phase separation. Depending on the oil/cellulose (o/c) weight ratio, it ranged from 6 days at 8/1 ratio to several months at 4/1 and 3/2. Ultra-Small and Small-Angle Neutron Scattering (USANS/SANS) Samples for ultra-small and small-angle neutron scattering (USANS/SANS) experiments with contrast variation were prepared at the same process conditions and compositions. Deuterium oxide and water were used as the aqueous medium, and deuterated or hydrogenated oils were used for three different contrasts: full contrast (hydrogenated cellulose and oil in D2O), shell contrast (mixture of deuterated and hydrogenated oil matched to D2O), and core contrast (D2O/H2O mixture matched to hydrogenated cellulose, contrasting the deuterated oil). The neutron scattering length density (SLD) and the final compositions of the emulsion components, for cellulose-coated n-hexadecane nano-emulsion are presented in Supporting Information Table S1. USANS and SANS measurements from the samples were conducted using the very small angle scattering diffractometer with focusing mirror (KWS-3) [38] and small angle scattering diffractometer (KWS-2) [39] beamlines (respectively), operated by the Jülich Center for Neutron Science at the Heinz Maier-Leibnitz Zentrum, Garching, Germany. In the KWS-2 beamline the wavelength (λ) was 5 Å with a wavelength spread ∆λ/λ=20%, and a sample aperture of 8 mm X 8 mm was used. The experiments were carried out at sample to detector distances of 20, 8 and 2 m and the measurable scattering vector range was 0.003
