Regulatory Mechanism Of Nanocellulose-stabilized Emulsions

Emulsions are belonged to multiphase system,which thermodynamically unstable but achieve kinetic stability by the action of surface-active substances or colloidal particles.Some surfactants used in the stabilization of multi-phase systems have raised environmental concerns and this is one of the reasons why colloidal particles have been considered as suitable stabilizer.For such purpose,cellulose-based(nano)materials are appealing given their sustainability,biodegradability,and nontoxicity.In fact,these materials have unique and useful properties and have been shown to assemble at oil-water interfaces or inside the one phase and impart new functionality to emulsion systems.Herein,the stability mechanism of nanocellulose in both Pickering emulsions and bicontinuous emulsions is studied,include(ⅰ)nanocellulose modified via heteroaggregation for stable emulsion,(ⅱ)exploring the effect of nanocellulose components(lignin)on emulsion stability,(ⅲ)emulsion stabilization mechanism(depletion effect and network structure stabilization),(ⅳ)and stabilization mechanism of nanocellulose stabilizing liquid crystal emulsion with bicontinuous structure,etc.In addition,nanocellulose can tune emulsion properties,such as stability,droplets size,and morphology of droplet or networks of interpenetration domains.The role of nanocellulose is discussed,and perspective for future direction is presented.The main contents are as follows:(ⅰ)Nanochitin(NCh),another polysaccharide nanoparticle with opposite charge to cellulose nanocrystals(CNC),was used to modify CNC by electrostatic interaction to improve the stability of Pickering emulsions stabilized by CNC.The colloidal behavior and heteroaggregation in aqueous media of the two particle types was first studied,especially in relation to complex formation and identification of net charge balance.All CNC/NCh complexes can stabilize oil-in-water(O/W)Pickering emulsions.However,when the system is close to charge electroneutrality(CNC/NCh～5),large heteroaggregates formed in the aqueous phase and the emulsions were highly unstable.The complexes were remarkably effective in stabilizing O/W emulsions under conditions of slightly net positive or net negative electrostatic charges,as determined by the CNC/NCh mass ratio(net anionic at CNC/NCh>5 and net cationic at CNC/NCh<5).Hence,it was possible to gain control on coalescence and stability in a broad range of compositions.Under net cationic condition(relatively high NCh loading),in the region 1<CNC/NCh<2.5,interfacial arresting of the nanopolysaccharides led to nondeformable droplets and high emulsion stability(at least for 9 months)with no signs of creaming.By increasing the oil fraction at a given CNC/NCh concentration,we note that stable emulsions formed up to 50%(w/w)oil fraction,also producing larger drop sizes.In sum,the changes in the stability of the emulsions as a function of CNC/NCh ratios are shown to be particularly useful in controlling emulsion properties.The results provide important guidance for regulating the properties of emulsions stabilized by natural nanoparticles,CNC and NCh.(ⅱ)Lignocellulose nanofibrils(LCNF)were used to prepare oil-in-water Pickering emulsions and to assess the role residual lignin in interfacial stabilization.Two LCNF fractions of similar morphology and structure were obtained by microfluidization of fibers obtained by hydrothermal treatment of wood with an organic acid.However,the lignin contents are different in the two kinds of LCNF.The LCNF with higher residual lignin was less hydrophilic and,correspondingly,performed better as Pickering stabilizer,producing emulsions of smaller droplet size and higher resistance to creaming.Long-term emulsion stabilization(over 40 days)was achieved with LCNF at concentrations as low as 0.24(w/v)% based on emulsion volume.We conclude that LCNF-stabilized Pickering emulsions can be tuned by varying the residual lignin content,including droplets size and emulsion stability.(ⅲ)Non-adsorbing cellulose nanofibrils(CNF)are shown to induce stabilization of dilute oil-in-water Pickering emulsions formed by interfacial adsorption of TEMPO-oxidized CNF(TOCNF).The correlative interactions that occur upon sequential addition of the two types of nanofibrils(TOCNF and CNF)afford control of the properties of the obtained emulsions.For instance,the emulsions undergo(1)creaming of non-flocculated droplets at low CNF concentrations;(2)destabilization by depletion flocculation at intermediate concentrations of the non-adsorbing CNF and,(3)stabilization by formation of a fibrillar network and gelation above a critical CNF concentration(～0.15 wt%).The observed phenomena are reproduced in TOCNF-stabilized emulsions of varying droplet sizes(3.6,6.9,and 11 μm).(ⅳ)Here we describe the all-aqueous bicontinuous emulsions with cholesteric liquid crystal domains and hierarchical colloidal self-assembly through osmotic-driven segregation of nanoparticles.Bicontinuous emulsion usually originates from demixing of binary fluid mixtures,displaying tortuous,interconnected liquid structure due to phase incompatibility and stabilization of nanoparticles.This is achieved by homogenization of rodlike CNC with two immiscible,phase separating polyethylene glycol(PEG)and dextran polymer solutions.The dispersed CNCs exhibit unequal affinity for the nonionic,hydrophilic binary polymer mixtures that depending on the osmotic potential.Once upon the critical concentration,excess CNC particles are constrained within one component of the polymer phases,further aggregate,and self-assemble into a fingerprint cholesteric organization,yielding a dynamic compartmented liquid crystal bicontinuous emulsion with a characteristic length scale of several micrometres.The obtained liquid crystal emulsion demonstrates a confined three-dimensional percolating bicontinuous network with nanoscopic helical assembly of CNC within the PEG phase,meanwhile the nanoparticles in dextran phase keep in isotropic instead.Our results provide an alternative way to arrest bicontinuous structures through intraphase trapping and assembling of nanoparticles,which is a viable and flexible route to extend for a wide range of colloidal systems.In this study,nanocellulose was used as the main colloidal particle,and the controllability of the emulsion morphology was achieved by regulating the self-assembly of the colloidal particles.Among them,colloidal particles can be adsorbed on the two phases interface to construct oil-in-water Pickering emulsions;or construct bicontinuous phase emulsions through intraphase trapping.In Pickering emulsions,the properties of the emulsion can be further adjusted by tuning the concentration of NCh,lignin or CNF;while in bicontinuous emulsions,the difference in osmotic pressure between the two phases drives the aggregation of CNCs and exhibits cholesteric liquid crystal structure.The role of nanocellulose in emulsion systems was systematically studied,providing a research basis for the application of nanocellulose in emulsions,especially in areas with high requirements for the biocompatibility of colloidal particles,such as food,medicine,medicine,and bioengineering.