Subscriber access provided by UNIV OF NEW ENGLAND
Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Amphiphilic cellulose nanocrystals for enhanced Pickering emulsion stabilization Chunxia Tang, Stewart Blair Spinney, Zengqian Shi, Juntao Tang, Baoliang Peng, Jianhui Luo, and Kam (Michael) Chiu Tam Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02437 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Amphiphilic cellulose nanocrystals for enhanced Pickering emulsion stabilization
Chunxia Tang1, Stewart Spinney1, Zengqian Shi1, Juntao Tang1,2*, Baoliang Peng,3 Jianhui Luo3 Kam C. Tam1*
1Department
of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University
Avenue, Waterloo, ON N2L 3G1, Canada 2College
of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China
3Research
Institute of Petroleum Exploration & Development (RIPED), PetroChina, Key Laboratory of Nano
Chemistry, Key Laboratory of Oilfield Chemistry, CNPC, Beijing 100083, P. R. China
Author Information Corresponding authors: Juntao Tang:
[email protected] Kam C. Tam:
[email protected] ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract
Sulfated cellulose nanocrystals (CNC) with high surface charge density are inadequate for stabilizing oil-water emulsions, which limits their applications as interfacial stabilizers. We performed end-group modification by introducing hydrophobic chains (polystyrene) to CNC. Results showed that the modified CNC are more effective in emulsifying toluene and hexadecane than pristine CNC. Various parameters were investigated, such as concentration of particles, electrolytes and polarity of solvents on the characteristics of the emulsions. This study provides strategies for the modification of cellulose nanocrystals to yield amphiphilic nanoparticles that enhance the stability of emulsions. Such systems, bearing biocompatible and environmentally friendly characteristics, are attractive for use in a wide range of industries spanning food, biomedicine, pharmaceuticals, cosmetics, petroleum etc.
Keywords: cellulose nanocrystals; Pickering emulsions; surfactant; reducing end
ACS Paragon Plus Environment
Page 2 of 24
Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Introduction Emulsions stabilized by solid particles, known as Pickering emulsions, have attracted increasing attention as alternatives to surfactant stabilized emulsions.1–3 They require less emulsifier, benefit from increased stability, and possess other advantages when compared to emulsions stabilized by surfactants.4–8 Pickering emulsions perform better over classical emulsions because the solid particles are irreversibly adsorbed onto the oil-water interface, and subsequently form an effective electro-steric protective shield for the emulsified droplets.9–13 While in surfactant-based emulsions, thermodynamic equilibrium is usually observed, which means that the stabilizer molecules are exchanging between oil-water interface and bulk continuous phase.4 Currently, there is an increasing interest in sustainable systems that are natural, green and abundant, where the present need is achieved without compromising the resource of future generation. Thus, the production and extraction of sustainable nanoparticles for use in the formulation of Pickering emulsions have been actively pursued over the last 10 years.14,15 Cellulose nanocrystal (CNC), derived via the acid hydrolysis of biomass, are one type of nanoparticles that have received increasing attention.3,9–11,16–18 Due to their renewable and “green” characteristics19–21, and functional groups on their surface that can be readily modified22,23, new systems have been developed. Hence, this sustainable material has been used in a wide range of applications, such as catalyst and drug carrier24,25, composite nanofiller26, as well as Pickering emulsifier.27,28 Their capabilities to stabilize various types of emulsions, such as oil-in-water, water-in-oil, water-in-water and complex emulsions have been reported.14,18,29,30 All these studies have shown that surfactant-free emulsions for pharmaceutical, food, and cosmetic applications can be formulated using nanomaterials from nature.31 As a result of the anhydrous glucose units, CNC possesses abundant functional moieties, such as aldehyde and hydroxyl groups, which can be modified, enabling the control over their physical or chemical properties.32,33 Anisotropy in the modified CNC can be enhanced by taking advantage of decorated aldehyde groups at the end of the CNC nanorods, allowing for the selective modification of the reducing ends of CNC. Although such study has not been widely reported, several groups have taken advantage of the reactive reducing ends to prepare functional cellulose nanocrystal with special characteristics.34–37 Lokanathan et al. introduced thiol groups at the reducing aldehyde ends of cellulose nanocrystals using either a one-step method via direct
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 24
reductive amination or a two-step procedure consisting of oxidation and carbodiimide-mediated reaction.36 Very recently, Zoppe et al. reported on a 3-step process to introduce initiators at the reducing end of cellulose nanocrystals, where the end-group can be used to initiate a controlled radical polymerization reaction.37 However, such modification protocol is complicated and some degree of functionalization occurred along the principal CNC axis (surface), which is unavoidable. Acid hydrolysis by sulfuric acid is one of the common methods to produce negatively charged CNC. However, sulfated CNC is not effective in stabilizing oil droplets due to the strong repulsion between the nanoparticles that inhibit their adsorption at the oil-water interface.15,38 Surface modifications of the CNC are commonly performed to improve the stability of the emulsions,
and
most
of
these
studies
focused
on
the
surface
hydroxyl
group
modifications.13,27,28,39,40 However, a possible modification of CNC can be conducted at the reducing end to prepare amphiphilic nanoparticles or surfactant-like particle, where such modification is rare. In this work, we seek to prepare an amphiphilic nanoparticle, which is akin to the concept of a “particle surfactant”. With a hydrophilic CNC surface and a hydrophobic end group, the amphiphilic character of the modified CNC will improve the wetting characteristics that promotes the partitioning of the nanoparticle to the oil-water interface, thereby enhancing the emulsion stability. This approach consists of a one-step modification protocol, is simple and straight forward, where polystyrene is grafted to the reducing end of sulfated CNC via the Schiff base reaction. Additionally, the imine linkage could be further reduced to amine groups by the addition of reducing agents (Scheme 1). Different reaction conditions and parameters, such as concentration of particles, electrolytes and polarity of solvents on the properties of the emulsions were examined. This study provides some perspectives on the modification of CNC, which could be utilized to prepare alternative amphiphilic nanoparticles to improve the stability of emulsions. Materials Sulfated cellulose nanocrystals were kindly provided by Celluforce Inc. Amine terminated polystyrene (average Mn = 5000 Da, PDI
