| The application of complexes via electrostatic complexation between biopolymers to stabilize the oil-water interface and construct Pickering emulsion delivery systems has gained popularity in the field of delivery of bioactive compounds in recent years.Octenyl succinate anhydride starch(OSA starch,OSAS)is a widely used polysaccharide emulsifier.However,its emulsification efficiency is limited because of the strong rigidity and large size.Through the electrostatic interaction between carboxyl groups of OSAS and amino groups of chitosan(CS),the structure and interfacial properties of their complexes can be tuned thus improving the stability of Pickering emulsions.So far,there has been no systematic study on OSAS/CS complex system,and there is a lack of in-depth understanding of the relationship between the structure of the complexes and their adsorption and stabilization at the oil-water interface.Therefore,based on the electrostatic interaction between OSAS and CS,the formation rules of OSAS/CS complexes with different structures were systematically investigated.The interfacial adsorption behavior of the complexes and the influence of interfacial structure on the stability of emulsions were explored.Finally,the sustained release and transdermal transport mechanism of the complex-stabilized Pickering emulsions were deeply investigated with resveratrol as a model bioactive compound,which may provide new ideas to design transdermal delivery systems.The main contents and results are as follows:Firstly,the complexation mechanism of OSAS and CS and the influencing factors of complex formation were investigated.The structure and properties of OSAS/CS complexes were characterized.The results of turbidity curve and Zeta-potential showed that p H induced the phase transition of complex system by affecting the dissociation of ionic groups of OSAS and CS,forming soluble or insoluble complexes.The turbidity of complex system significantly increased with the increase in substitution degree of OSAS from 0.0162 to 0.0460,indicating the enhanced interactions between OSAS and CS.The complexes had a wide range of phase separation at OSAS/CS ratio of 3:1-9:1 for the same substitution degree of OSAS,implying that the complex system could be stable over a wide p H range.Combining the turbidity,micromorphology and thermodynamic analysis,the formation of OSAS/CS complexes was mainly related to the electrostatic interactions between the carboxyl groups of OSAS and the amino groups of CS.The hydrogen bonding and hydrophobic effects may also be involved.The electrostatic complexation of OSAS with CS resulted in significant changes in structures and properties of the complexes.All complexes showed compact lamellar networks after lyophilization,with CS attaching to the OSAS surface by electrostatic interactions.The crystal structure of the complexes was destroyed to some degree and the complexes were susceptible to thermal decomposition.Through adjusting the mixing ratio and p H,the electrostatic interactions between OSAS and CS could be enhanced,thus increasing apparent viscosity and thermal stability of the complexes.Secondly,OSAS/CS complexes formed at a certain mixing ratio and p H were selected as the research objects,and the adsorption and stabilization of complexes at the oil-water interface were investigated using interfacial adsorption kinetics,quartz crystal microbalance with dissipation,interfacial shear rheology and molecular simulation coupled with atomic force microscopy.The results showed that OSAS/CS complexes exhibited significantly improved partial wettability and wettability stability.The contact angle reached 91.97° at a mixing ratio of 6:1 and p H 6.0,with a near-neutral wettability.The complexes significantly reduced the interfacial tension,with faster diffusion and adsorption rate towards the oil-water interface.The diffusion followed a modified Ward-Tordai model.OSAS and CS formed thin weakly elastic and fluid-like monolayers,respectively,while the complexes concentrated at the interface to form highly viscoelastic multilayer adsorbed films that could resist shear deformation.When the mixing ratio was increased from 3:1 to 6:1 or p H was increased from 5.5 to 6.0,the complexation strengths between OSAS and CS were enhanced,accelerating the adsorption of the complexes to the interface and increasing the thickness of adsorbed films.However,when the p H was increased to 6.5,the self-aggregation of CS occurred,which destroyed the complex structure and affected the adsorption.The analysis of molecular dynamics simulations theoretically confirmed that the complexes adsorbed at the interface effectively hindered the migration and diffusion of oil and water.Thirdly,emulsions with different types of interfacial assembly of OSAS and CS,including OSAS/CS complex emulsions,OSAS-CS bilayer emulsions and CS-OSAS bilayer emulsions were prepared,and the droplet size,microstructure and physicochemical stability of the emulsions at different p H values were systematically investigated.The results showed that the droplet size of all emulsions significantly increased with increasing p H,and the droplet size of OSAS/CS complex emulsions was smaller than that of OSAS-CS and CS-OSAS bilayer emulsions at the same p H value.Confocal laser scanning microscopy observations clearly showed that OSAS and CS covered the surface of oil droplets,with the adjacent droplets bridging through the interfacial layer to form partial flocculation structures.The thick interfacial layers(～2 μm)and tight gel networks of OSAS/CS complex-stabilized emulsions were more resistant to coalescence,creaming,and lipid oxidation.In comparison,bilayer emulsions were prone to destabilization owing to their poor homogeneity of interfacial structures.When the p H was increased from 5.5 to 6.0,the stability of all emulsions increased,but when it continued to increase to 6.5,the self-aggregation of CS around its p Ka attenuated the interfacial strength and bridging network,resulting in decreased stability and aggravated extent of lipid oxidation of emulsions.The analysis confirmed that emulsions with proper bridging flocculation facilitated the formation of elastic network structures and therefore decreased the coalescence and creaming of emulsions during storage.Next,the effects of oil fraction(Φ)and complex concentration(c)on the formation,microstructure,gel structure strength and rheological properties of OSAS/CS complexstabilized Pickering emulsions were systematically studied,and the storage and environmental stability of emulsions were also explored.The results showed that emulsions were stable with homogeneous gel-like structures at Φ = 50%-85%.When Φ was increased from 20% to 85%,the droplet size of emulsions significantly increased from 12.48 μm to 26.57 μm,while when c was increased from 0.5 wt% to 2.5 wt%,the droplet size was reduced by 2.8-fold.Optical microscopy combined with low-field nuclear magnetic resonance and rheological tests showed that the increase in Φ or c led to tighter droplet arrangement,extrusion deformation and enhanced elastic gel network of emulsions.Confocal laser scanning microscopy observations further revealed that a typical structure with small droplets embedded in the interstices of large droplets was formed,and all droplets were bridged by sharing complex layers to form threedimensional cross-linked network with free OSAS and CS in the continuous phase.The physical and storage stability of emulsions increased with the increase of Φ or c,and exhibited high stability to ionic strength and heat treatment.However,emulsions were susceptible to p H changes.In a p H range from 3.0 to 7.5,all emulsions were gel-like with high stability,but when the p H was increased to 9.0,droplet aggregation and demulsification quickly occurred due to the deprotonation of CS that reduced the electrostatic repulsions between droplets.Finally,the potential of OSAS/CS complex-stabilized high internal phase Pickering emulsions as an encapsulation vehicle for transdermal delivery was explored using resveratrol as a model bioactive compound.The results showed that the droplet size and microstructure of emulsions were greatly influenced by the complex concentrations,but less affected by the loading amounts of resveratrol.With the increase in complex concentrations or loading amounts of resveratrol,emulsions formed stronger viscoelastic networks,which effectively enhanced the stability of resveratrol when subjected to light,temperature and UV radiation.Antioxidant results showed that compared with resveratrol bulk oil,the DPPH scavenging rate of emulsions increased by 4.6%-23.5%,indicating that resveratrol was effectively encapsulated in emulsions.The in vitro release assay showed that the release of resveratrol was 51.20%-68.01% within 24 h,and the release followed a combined mechanism of Fickian diffusion and Case-II transport.Compared to bulk oil,emulsions greatly improved the skin permeation of resveratrol,with an approximately 5.24-fold increase in total skin uptake,and the permeation depth was also significantly increased.Attenuated total reflectance Fourier transform infrared spectroscopy results confirmed that the performance of emulsions was related to an increase in the random coil structure of keratin.Further analysis on skin irritation and cell cytotoxicity(cell viability >88%)further confirmed the application safety of emulsions. |
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