Isabelle Capron

New Pickering Emulsions Stabilized by Bacterial Cellulose Nanocrystals

We studied oil in water Pickering emulsions stabilized by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of bacterial cellulose. The resulting solid particles, called bacterial cellulose nanocrystals (BCNs), present an elongated shape and low surface charge density, forming a colloidal suspension in water. The BCNs produced proved to stabilize the hexadecane/water interface, promoting monodispersed oil in water droplets around 4 μm in diameter stable for several months. We characterized the emulsion and visualized the particles at the surface of the droplets by scanning electron microscopy (SEM) and calculated the droplet coverage by varying the BCN concentration in the aqueous phase. A 60% coverage limit has been defined, above which very stable, deformable droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption associated with the formation of a 2D network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions open opportunities for the development of environmentally friendly new materials.

Modulation of Cellulose Nanocrystals Amphiphilic Properties to Stabilize Oil/Water Interface

Neutral cellulose nanocrystals dispersed in water were shown in a previous work to stabilize oil/water interfaces and produce Pickering emulsions with outstanding stability, whereas sulfated nanocrystals obtained from cotton did not show interfacial properties. To develop a better understanding of the stabilization mechanism, amphiphilic properties of the nanocrystals were modulated by tuning the surface charge density to investigate emulsifying capability on two sources of cellulose: cotton linters (CCN) and bacterial cellulose (BCN). This charge adjustment made it possible to determine the conditions where a low surface charge density, below 0.03 e/nm(2), remains compatible with emulsification, as well as when assisted by charge screening regardless of the source. This study discusses this ability to stabilize oil-in-water emulsions for cellulose nanocrystals varying in crystalline allomorph, morphology, and hydrolysis processes related to the amphiphilic character of nonhydrophobized cellulose nanocrystal.

Cellulosic nanorods of various aspect ratios for oil in water Pickering emulsions

Cellulosic colloidal nanorods of different origins were used in order to investigate the effect of various elongated shapes adsorbed at the oil–water interface for Pickering emulsion characteristics. Nanocrystals of length ranging from 185 nm to 4 μm were obtained from the hydrolysis of cellulose microfibrils of three different biological origins: cotton (CCN), bacterial cellulose (BCN) and Cladophora (ClaCN) leading to aspect ratios ranging from 13 to 160. These nanocrystals are irreversibly adsorbed at the oil–water interface and form ultrastable emulsions. Individual droplets of similar diameter were obtained under diluted conditions, illustrating both similar wetting properties and nanocrystal flexibility for the three different types of nanocrystals. However, it was shown that the aspect ratio directly influences the coverage ratio giving rise, on the one hand to a dense organisation (coverage >80%) with short nanocrystals and on the other hand to an interconnected network of low covered droplets (40%) when longer nanocrystals are used. An estimation is made showing that for the longer nanocrystals, 55% of the nanocrystals introduced are involved in the network of the material. The capillary force that promotes attractive interactions between nanocrystals was also addressed. These results lead to a better understanding of the adsorption process for rod-like particles of various aspect ratios for the elaboration of a controlled surface architecture, from a homogeneous monolayer to interconnected porous multilayered interfaces.

Surfactant-free high internal phase emulsions stabilized by cellulose nanocrystals.

Cellulose nanocrystals (CNCs) are rod-like colloidal particles that irreversibly adsorb at the oil-water interface to produce ultrastable emulsions. When the internal phase fraction is increased, these CNCs can produce gel-like oil-in-water high internal phase emulsions (HIPEs) in which more than 90% of the hydrophobic phase is stabilized by less than 0.1% wt. of CNCs. However, a one-step preparation of HIPEs is not possible, and incorporation of the high internal phase fraction requires the prior preparation of Pickering emulsions. We propose that this two-step process to create CNC HIPEs relies on a swelling process of the droplets that does not desorb the CNCs from the interface, decreasing the coverage ratio of the droplets and leading to coalescence. As a result, this process leads to a drops deformation and a new interfacial networking organization as revealed by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images.

Preparation of Double Pickering Emulsions Stabilized by Chemically Tailored Nanocelluloses

Nanocelluloses are bio-based nanoparticles of interest as stabilizers for oil-in-water (o/w) Pickering emulsions. In this work, the surface chemistry of nanocelluloses of different length, nanofibrillated cellulose (NFC, long) and cellulose nanocrystals (CNC, short), was successfully tailored by chemical modification with lauroyl chloride (C12). The resulting nanofibers were less hydrophilic than the original and able to stabilize water-in-oil (w/o) emulsions. The combination of the two types of nanocelluloses (C12-modified and native) led to new surfactant-free oil-in-water-in-oil (o/w/o) double emulsions stabilized by nanocellulose at both interfaces. Characterization was performed with respect to droplet size distribution, droplet stability over time, and stability after centrifugation. Nanocellulose-based Pickering emulsions can be designed with a substantial degree of control, as demonstrated by the stability of the chemically tailored NFC double emulsions. Furthermore, it was demonstrated that increased nanofiber length leads to increased stability.

Structural Description of the Interface of Pickering Emulsions Stabilized by Cellulose Nanocrystals.

The cotton cellulose nanocrystals (CNCs) used in this study are rod-like particles with dimensions in the nanoscale (195 nm long, 23 nm width and 6 nm thick) able to stabilize Pickering emulsions. The adsorption of CNCs at an oil-water interface has been investigated by small angle neutron scattering (SANS) with and without surface charge, and varying CNC concentration from 2 to 5 g/L. Average thicknesses of the interfacial CNC layer around the emulsion droplets of 7 and 18 nm were determined for charged and uncharged CNC, respectively, regardless of their concentration in suspension. This suggests that CNC particles lie as a monolayer varying in surface density. Using several phase contrast variations, the neutron wave vector (Q) dependence with the intensity showed that CNCs are in contact with the oil phase only via the surface of the CNC and not immersed in oil since the Porod behavior is observed over the whole Q-range revealing no deformation of the oil surface at a nanometer scale. This result promotes the postulate that the (2 0 0) crystalline plane of the CNC directly interacts with the interface.

Influence of Charge Density and Ionic Strength on the Aggregation Process of Cellulose Nanocrystals in Aqueous Suspension, as Revealed by Small-Angle Neutron Scattering

Aggregation of rodlike colloidal particles is investigated here through the aggregation process by either increasing ionic strength or decreasing surface charge density of cellulose nanocrystals (CNCs). The form factor of the nanoparticles is characterized up to the Guinier plateau using small-angle neutron scattering (SANS) extended to very small scattering vector Q. Ionic strength, above the threshold of screening charges, brings aggregative conditions that induced fractal organizations for both charged and uncharged CNCs. These two structures display respective fractal dimensions of 2.1 for charged CNCs at high ionic strength and 2.3 for desulfated CNCs over more than a decade of the scattering vector Q, irrespective of salinity, revealing a denser structuration for neutral particles. This is discussed in the framework of aggregation of rodlike particles with an aspect ratio higher than 8. Furthermore, dilution of the rod gel led to disentanglement of the network of fractal aggregates with a subsequent macroscopic sedimentation of the suspensions, with a characteristic time that depends upon the ionic strength and surface charge density. It revealed a threshold independent of salt content around 2.5 g/L and the metastable out-of-equilibrium character of CNC suspensions.

Versatile cellular foams derived from CNC-stabilized Pickering emulsions

Lightweight cellular foams were prepared by freeze-drying oil-in-water emulsions stabilized with cellulose nanocrystals (CNCs). Oil and water were removed while preserving the cellular emulsion template without any shrinkage or deformation both at the internal cell and bulk foam scales. This versatile process proved to be relevant for controlling the pore size of the foam with respect to emulsion droplets. Moreover, the composite foams could easily be obtained as shown here by the addition of polyelectrolytes. Mechanical properties of the foam were investigated and they were found to be reinforced by chitosan addition. Thus, our finding opens new routes to a broad range of cellulose-based porous materials.

Chitin Nanocrystals for Pickering High Internal Phase Emulsions

Chitin is a natural polymer of glucosamine bearing N-acetyl groups. Chitin nanocrystals (ChiNCs), resulting from the acid hydrolysis of amorphous regions of chitin, are crystalline positively charged rod-like particles. ChiNCs show some interfacial properties and very efficiently stabilize oil/water interfaces, leading to the so-called Pickering emulsions. In accordance with the irreversible adsorption of particles, these Pickering emulsions proved stable over time, with constant emulsion volume for several months, even though natural creaming may occur. The emulsions produced are not clearly susceptible to ionic strength or pH in terms of average droplet diameter. However, when mixed with a large amount of oil, high internal phase emulsions (HIPE) containing up to 96% of internal phase are formed as a gel with a texture that can be modulated from soft to solid gel by adjusting concentration, pH, and ionic strength.

Some modification of cellulose nanocrystals for functional Pickering emulsions.

Cellulose nanocrystals (CNCs) are negatively charged colloidal particles well known to form highly stable surfactant-free Pickering emulsions. These particles can vary in surface charge density depending on their preparation by acid hydrolysis or applying post-treatments. CNCs with three different surface charge densities were prepared corresponding to 0.08, 0.16 and 0.64 e nm−2, respectively. Post-treatment might also increase the surface charge density. The well-known TEMPO-mediated oxidation substitutes C6-hydroxyl groups by C6-carboxyl groups on the surface. We report that these different modified CNCs lead to stable oil-in-water emulsions. TEMPO-oxidized CNC might be the basis of further modifications. It is shown that they can, for example, lead to hydrophobic CNCs with a simple method using quaternary ammonium salts that allow producing inverse water-in-oil emulsions. Different from CNC modification before emulsification, modification can be carried out on the droplets after emulsification. This way allows preparing functional capsules according to the layer-by-layer process. As a result, it is demonstrated here the large range of use of these biobased rod-like nanoparticles, extending therefore their potential use to highly sophisticated formulations. This article is part of the themed issue ‘Soft interfacial materials: from fundamentals to formulation’.