Cellulose-Based Surfactants: Designing Particle-Based Surfactants for Sustainable Applications
Surfactants are versatile chemicals used in a wide variety of everyday goods, ranging from personal care products to pharmaceuticals. Their usefulness lies in their amphiphilic nature, which makes them essential for any product where oil and water are required to mix and form a stable mixture. In 2023, their market size was valued at USD 30 billion, with expected growth of 10% by 2033. To date, the majority of the market is dominated by molecular surfactants obtained partially or wholly from the petrochemical industry. Current efforts are focused on replacing traditional surfactants with greener alternatives, especially from renewable sources, without compromising on performance. In recent decades, there has been a surge of interest in the use of solid particles as emulsifiers. The resulting oil/water mixtures are known as Pickering emulsions, and are particularly stable due to the localisation of the particles at the oil/water interface, which forms a physical barrier that prevents coalescence. The Pickering emulsifiers currently in commercial use are all particles with homogeneous wettability. However, it has been demonstrated that particles with dual wettability have the potential to decrease the interfacial energy further and act as even more effective emulsifiers. Research in this area has predominantly focused on metallic, silica, plastic, and inorganic materials. This thesis set out to investigate the synthesis of cellulose Janus nanospheres with dual wettability by the partial modification of the particle surface via the use of Pickering emulsions. Chapter 2 investigates the key parameters for the formation of hydrophilic spherical nanocellulose particles (SNC) and their partial surface functionalization with hydrophobic alkyl chains. Partial modification relied on the ability of SNCs to form oil-in-water Pickering emulsions, with the particles sitting at the oil/water interface, with one face wetted by water and the other wetted by oil. A lipophilic modifier was added to the oil phase, resulting chemical modification on a single hemisphere, allowing the synthesis of amphiphilic Janus cellulose nanospheres. One of the more significant findings to emerge from this study is that the amphiphilicity of the particles could be tuned by changing the lipophilic modifier and changing the reaction time to modify the degree of chemical modification. Furthermore, our modified particles showed enhanced surface activity compared to hydrophilic nanospheres, likely attributable to the particles’ dual affinity. Following optimisation of the reaction parameters, we demonstrated that this chemical modification improved the emulsification properties of the particles, as oil-in-water emulsions obtained with our nanospheres showed superior performance than the commercial surfactant polyoxyethylene (10) oleyl ether (Brij O10). Chapter 3 presents an alternative method of producing amphiphilic cellulose nanoparticles that involves an initial complete esterification of the cellulose surface, followed by cleavage of the esters from a single hemisphere to produce Janus particles. We found that the completely esterified particles had a high degree of substitution (higher than for the etherified particles) and were able to efficiently stabilise water-in-oil (inverse) emulsions. Prior to this study, few examples have described the use of bio-derived nanoparticles as water-in-oil Pickering emulsifiers. The partial cleavage of the esters from these nanospheres was achieved by the addition of NaOH to the water phase to achieve hydrolysis on the single hemisphere wetted by water. The degree of cleavage could be controlled by the reaction time, resulting in changes in the amphiphilic nature of the particles and their emulsification properties. Although this work was limited to the application of our particles as bio-based emulsifiers, these results highlight the potential of this partial modification methodology as a simple way to synthesise cellulose nanospheres with adjustable hydrophilic-hydrophobic balance that mimics the action of surfactants with different hydrophobic-hydrophilic balance (HLB). In addition, the synthesis of Janus particles by using Pickering emulsions to partially mask particles on the nanoscale is methodology that can be extended for the synthesis of anisotropic nanoparticles from a range of other materials.
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